摘要:

4369 4377 4387 4397 4407 4417 4427 4439 445 1 4457 447 1 4475 4487 4495 450 1 4509 Con tents A New Form of the High-temperature Isopiestic Technique and its Applica- tion to Mercury-Bismuth, Mercury-Cadmium, Mercury-Gallium, Mercury- Indium and Mercury-Tin Binary Amalgams Z-C. Wang, X-H. Zhang, Y-Z. He and Y-H. Bao The Derivation of Chemical-diffusion Coefficients of Oxygen in UO,,, over the range 180-300 "C. Spectroscopic Procedure and Preliminary Results T. R. Griffiths, H. V. St. Aubyn Hubbard, G. C. Allen and P. A. Tempest Pho tophysics at Solid Surfaces. Evidence of Dimer Formation and Polarization of Monomer and Excimer Fluorescences of Pyrene in the Adsorbed State on Silica-gel Surfaces T. Fujii, E. Shimizu and S. Suzuki Ordering in Monodispersed Polymer Latices induced by a Temperature Gradient K.Furusawa, N. Tobori and S. Hachisu X-Ray Diffraction Study of Molten Eutectic LiF-NaF-KF Mixture K. Igarashi, Y. Okamoto, J. Mochinaga and H. Ohno Viscosity Measurements of Some Tetra butylammonium, Copper( I), Silver( I) and Thallium( 1) Salts in Acetonitrile-Pyridine Mixtures at 15, 25 and 35 "C D. S. Gill and B. Singh The Ethane- 1,2-diol-Water Solvent System. The Dependence of the Dis- sociation Constant of Picric Acid on the Temperature and Composition of the Solvent Mixture Silver(1) Complexation with Tertiary Amines in Toluene M. Soledade Santos, E. F. G. Barbosa and M. Spiro Enhanced Oxygen Evolution through Electrochemical Water Oxidation mediated by Polynuclear Complexes embedded in a Polymer Film G. J. Yao, A. Kira and M. Kaneko Nature of Acid Sites in SAP05 Molecular Sieves.Part 1.-Effects of the Concentration of Incorporated Silicon C. Halik, J. A. Lercher and H. Mayer Hemimicelle Formation of Cationic Surfactants at the Silica Gel-Water Interface T. Gu, Y. Gao and L. He Nuclear Magnetic Resonance Relaxation in Micelles. Deuterium Relaxation at Three Field Strengths of Three Positions on the Alkyl Chain of Sodium Dodecyl Sulphate Studies of the Temperature Dependence of Retention in Supercritical Fluid Chromatography K. D. Bartle, A. A. Clifford, J. P. Kithinji and G. F. Shilstone Hydrogen and Muonium Atom Adducts of Trimethylsilyl Derivatives of Ethyne The Radical Cation of Formaldehyde in a Freon Matrix. An Electron Spin Resonance Study Phase Transition of the Water confined in Porous Glass studied by the Spin- probe Method H.Yoshioka G. C. Franchini, A. Marchetti, L. Tassi and G. Tosi 0. Soderman, G. Carlstrom, U. Olsson and T. C. Wong C. J. Rhodes and M. C. R. Symons C. J. Rhodes and M. C. R. Symons4369 4377 4387 4397 4407 4417 4427 4439 445 1 4457 447 1 4475 4487 4495 450 1 4509 Con tents A New Form of the High-temperature Isopiestic Technique and its Applica- tion to Mercury-Bismuth, Mercury-Cadmium, Mercury-Gallium, Mercury- Indium and Mercury-Tin Binary Amalgams Z-C. Wang, X-H. Zhang, Y-Z. He and Y-H. Bao The Derivation of Chemical-diffusion Coefficients of Oxygen in UO,,, over the range 180-300 "C. Spectroscopic Procedure and Preliminary Results T. R. Griffiths, H. V. St. Aubyn Hubbard, G. C. Allen and P. A. Tempest Pho tophysics at Solid Surfaces.Evidence of Dimer Formation and Polarization of Monomer and Excimer Fluorescences of Pyrene in the Adsorbed State on Silica-gel Surfaces T. Fujii, E. Shimizu and S. Suzuki Ordering in Monodispersed Polymer Latices induced by a Temperature Gradient K. Furusawa, N. Tobori and S. Hachisu X-Ray Diffraction Study of Molten Eutectic LiF-NaF-KF Mixture K. Igarashi, Y. Okamoto, J. Mochinaga and H. Ohno Viscosity Measurements of Some Tetra butylammonium, Copper( I), Silver( I) and Thallium( 1) Salts in Acetonitrile-Pyridine Mixtures at 15, 25 and 35 "C D. S. Gill and B. Singh The Ethane- 1,2-diol-Water Solvent System. The Dependence of the Dis- sociation Constant of Picric Acid on the Temperature and Composition of the Solvent Mixture Silver(1) Complexation with Tertiary Amines in Toluene M.Soledade Santos, E. F. G. Barbosa and M. Spiro Enhanced Oxygen Evolution through Electrochemical Water Oxidation mediated by Polynuclear Complexes embedded in a Polymer Film G. J. Yao, A. Kira and M. Kaneko Nature of Acid Sites in SAP05 Molecular Sieves. Part 1.-Effects of the Concentration of Incorporated Silicon C. Halik, J. A. Lercher and H. Mayer Hemimicelle Formation of Cationic Surfactants at the Silica Gel-Water Interface T. Gu, Y. Gao and L. He Nuclear Magnetic Resonance Relaxation in Micelles. Deuterium Relaxation at Three Field Strengths of Three Positions on the Alkyl Chain of Sodium Dodecyl Sulphate Studies of the Temperature Dependence of Retention in Supercritical Fluid Chromatography K. D. Bartle, A. A. Clifford, J. P. Kithinji and G. F. Shilstone Hydrogen and Muonium Atom Adducts of Trimethylsilyl Derivatives of Ethyne The Radical Cation of Formaldehyde in a Freon Matrix. An Electron Spin Resonance Study Phase Transition of the Water confined in Porous Glass studied by the Spin- probe Method H. Yoshioka G. C. Franchini, A. Marchetti, L. Tassi and G. Tosi 0. Soderman, G. Carlstrom, U. Olsson and T. C. Wong C. J. Rhodes and M. C. R. Symons C. J. Rhodes and M. C. R. Symons

ISSN:0300-9599    

DOI:10.1039/F198884FX029

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet 'Quantities, Units, and Symbols' (1 975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W1V OBN). These recommendations are applied by The Royal Society of Cemistry in all its publications. Their basis is the 'Systeme International d'Unit6s' (9). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers.In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn). Nomenclature of Inorganic Chemistry (Butterworths, London, 1971 , now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Compendium of Chemical Terminology: IUPAC Recommendations (Blackwells, Oxford, 1987).A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society's editorial staff. (xiv)NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet 'Quantities, Units, and Symbols' (1 975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W1V OBN). These recommendations are applied by The Royal Society of Cemistry in all its publications.Their basis is the 'Systeme International d'Unit6s' (9). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn). Nomenclature of Inorganic Chemistry (Butterworths, London, 1971 , now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Compendium of Chemical Terminology: IUPAC Recommendations (Blackwells, Oxford, 1987). A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society's editorial staff. (xiv)

ISSN:0300-9599    

DOI:10.1039/F198884BX031

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

ISSN 0300-9238 JCFTAR 84(8) 251 9-291 4 (1 988) 2519 2537 2545 2553 2567 2573 258 5 2595 2603 2609 2619 2635 264 1 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases The first 13 papers of this issue are on the Physical Chemistry of Small Carbohydrates, part of the 8th International Solute-Solute-Solvent Interaction Symposium held at Regensberg University in August 1987. CONTENTS Derivation of Parameters for Conformational Calculations on Carbohydrate Systems, including Bacterial Cell Wall Peptidoglycan A. Marsden, B. Robson and J. S. Thompson Correlation between the Hydrophobic Nature of Monosaccharides and Cholates, .and their Hydrophobic Indices K. Miyajima, K. Machida, T. Taga, H. Komatsu and M. Nakagaki Homotactic and Heterotactic Interactions in Aqueous Solutions containing some Saccharides.Experimental Results and an Empirical Relationship between Saccharide Solvation and Solute-Solute Interactions S. H. Gaffney, E. Haslam, T. H. Lilley and T. R. Ward Excess Enthalpy and Excess Volume in Ternary Aqueous Solutions with Sucrose-Glucose, Sucrose-Glycerol and Glucose-Glycerol at 298.1 K N. Daldrup and H. Schonert Interactions between Cations and Sugars. Part 4.-Free Energy of Interaction of the Calcium Ion with Some Aldopentoses and Aldohexoses in Water at 298.15 K J-P. Morel, C. Lhermet and N. Morel-Desrosiers Interaction of Divalent Cations with Polyuronates A. Cesaro, F. Delben and S. Paoletti Fourier-transform Infrared Vibrational Circular Dichroism of Simple Carbo- hydrates C.M. Tummalapalli, D. M. Back and P. L. Polavarapu A Nuclear Magnetic Resonance Study of Isomeric Pentitols in Aqueous and Non-aqueous Solutions F. Franks, R. L. Kay and J. Dadok Conformaton of Polyols in Water. Molecular-dynamics Simulation of Mannitol and Sorbitol J. R. Grigera Influence of Water on Pure Sorbitol Polymorphism S. Quinquenet, C. Gr abielle-Madelmont, M. Ollivon and M. Serpelloni Thermomechanical Properties of Small-carbohydrate-Water Glasses and ' Rubbers '. Kinetically Metastable Systems at Sub-zero Temperatures H. Levine and L. Slade Structure, Sweetness and Solution Properties of Small Carbohydrate Molecules G. G. Birch and S. Shamil Solution Properties and the Sweet Taste of Small Carbohydrates M. Mathlouthi and A-M. Seuvre 2651 Thermodynamic Studies of Transfer of Some Amino Acids and Peptides from Water to Aqueous Glucose and Sucrose Solutions at 298.15 K R.Bhat, N. Kishore and J. C. Ahluwalia 83 FAR 12667 2677 2683 2697 2703 2723 2735 2755 277 1 2783 2795 2807 282 1 283 1 2843 2855 2875 2885 2895 Effect of Micelle Formation on the Absorption Spectra of a Functionalized Detergent with the Anthraquinone Moiety K. Hoshino, T. Saji, K. Suga and M. Fujihira Synthesis of a Viologen-Tetratitanate lntercalation Compound and its Photochemical Behaviour H. Miyata, Y. Sugahara, K. Kuroda and C. Kato Salt Solutions in Supercritical Water. Some Preliminary Studies on the Influence of Gravity Thermodynamics of Transfer of Glycine, Diglycine, p-Nitroaniline and Benzoic Acid from Water to Aqueous Solutions of Polyhydroxy Compounds J.P. Chatterjee and I. N. Basumallick Transfer Chemical Potentials for Ions, Solubilities of Salts and Kinetics of Reactions involving Inorganic Complex Ions at Ambient Pressure and 298.2 K in Binary Aqueous Mixtures containing Ethanol and Propan-2-01 M. J. Blandamer, B. Briggs, J. Burgess, D. Elvidge, P. Guardado, A. W. Hakin, S. Radulovic and C. D. Hubbard Crystallisation Kinetics of Calcite in the Presence of Magnesium Ions W. A. House, M. R. Howson and A. D. Pethybridge Nuclear Magnetic Resonance Relaxation Study of I- and Na' Solvation Structure in N-Methylformamide (NMF) and Preferential Solvation of these Ions in the Mixture NMF-H,O Multicomponent Ion Exchange in Zeolites. Part 4.-The Exchange of Magnesium Ions in Zeolites X and Y Dynamic Studies of the Photoinduced Metathesis of C,H, and Photoreduction of Mo with CO on Anchored and Impregnated Mo/SiO,Catalysts M.Anpo, M. Kondo, Y. Kubokawa, C. Louis and M. Che Investigation of Structural Changes in the Nickel-Uranium Oxide Catalyst System by Uranium &-Edge and Nickel K-Edge EXAFS and XANES F. J. Berry, A. Murray and A. T. Steel Metal Oxides as Heterogeneous Catalysts for Oxygen Evolution under Photochemical Conditions A. Harriman, I. J. Pickering, J. M. Thomas and P. A. Christensen Kinetics of the Interacton of Acridine Dyes with Nucleic Acids. An Iodine-laser Temperature-jump Investigation B. Marcandalli, G. Stange and J. F. Holz- warth Iridium Oxide Hydrosols as Catalysts for the Decay of Zinc Porphyrin Radical Cations in Water Thermal Decomposition of KIO, and NaIO, in Relation to Solid-state Isotopic Exchange Reactions CO Adsorption on MgO and CaO.Spectroscopic Investigations of Stages .prior to Cyclic Anion Cluster Formation E. Garrone, A. Zecchina and F. S. Stone Successive Addition of Electrons to Sodium Quinizarin-2- and -6-Sulphonate in Aqueous Solution. A Pulse and y-Radiolysis Study T. Mukherjee, E. J. Land, A. J. Swallow, P. M. Guyan and J. M. Bruce Electron Spin Resonance and Electron Nuclear Double Resonance Study of Mixed Crystals of Benzophenone and Diphenylnitroxide A. L. Maniero and M. Brustolon Correlation of the Solvation Properties of Water and Dimethyl Sulphoxide in Metal Nitrate-Water and Metal Nitrate-Dimethyl Sulphoxide Melts with the Molecular Properties of the Constituents of the Melts G. A. Sacchetto and Z. Kodejd Measurements of Tracer Diffusion Coefficients of Sulphate Ions in Aqueous Solutions of Ammonium Sulphate and Sodium Sulphate, and of Water in Aqueous Sodium Sulphate Solutions D. J. Turner C. K. Finter and H. G. Hertz K. R. Franklin and R. P. Townsend A. Harriman, G. S. Nahor, S. Mosseri and P. Neta S. Takriti and G. DuplQtre K. Tanaka2899 Review of Books A. D. Yoffe; G. J. T. Tiddy; R. N. Perutz; J. Klinowski; S. K. Scott; A. Keller; K. Stead; M. J. Blandamer; G. Saville; S. K. Scott; S. M. Richardson; N. V. Richardson; M. R. Mackley; L. V. C. Rees; C. J. Wormald; N. Sheppard; A. R. Jones 83-22899 Review of Books A. D. Yoffe; G. J. T. Tiddy; R. N. Perutz; J. Klinowski; S. K. Scott; A. Keller; K. Stead; M. J. Blandamer; G. Saville; S. K. Scott; S. M. Richardson; N. V. Richardson; M. R. Mackley; L. V. C. Rees; C. J. Wormald; N. Sheppard; A. R. Jones 83-2

ISSN:0300-9599    

DOI:10.1039/F198884FP105

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

96 1 987 997 1015 1023 1041 1053 1067 1083 1095 1109 1115 1129 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions II, lssue8,1988 Molecular and Chemical Physics Professor Geoffrey Luckhurst of Southampton University was invited to contribute a Keynote Paper on the general theme of the Behaviour of Liquid Crystals at the Molecular Level. He was supported by a group of research workers who submitted original papers on cognate subjects. All these papers have now been refereed, and are collected at the beginning of this month’s issue of Faraday Transactions II. For the benefit of readers of Faraday Transactions I , the contents list is reproduced below. Pretransitional Behaviour in Liquid Crystals. The Roles of Nuclear Magnetic Resonance Spectroscopy and Molecular Field Theory Pretransitional Isotropic Kerr Effect in Eutectic Mixtures El 20, El 30 and El40 Higher-order Director Fluctuations First- and Second-order Smectic-A to Nematic Phase Transitions in p,p- Dialkylazoxybenzenes studied by Birefringence E.F. Gramsbergen and W. H. de Jeu Nematic-Isotropic Transition in Bounded Thin Films An Investigation of the Potential Governing Rotation about the 0-CH, Bond in 4-Chloroethoxybenzene by comparing Observed and Calculated Dipolar Couplings obtained for a Sample dissolved in a Nematic Solvent G. Celebre, M. Longeri and J. W. Emsley Molecular Dynamics in a Nematogen studied by Deuterium Nuclear Magnetic Resonance Spectroscopy Molecular Dynamics of a Siloxane Liquid-crystalline Polymer as studied by Dielectric Relaxation Spectroscopy K. Araki, G.S. Attard, A. Kozak, G. Williams, G. W. Gray, D. Lacey and G. Nestor Chain Ordering in Cyanobiphenyls and Cyanophenylcyclohexanes P. Forster and B. M. Fung Molecular Hydrogen in Nematic Liquid Crystals. Unusual Isotope Effects A. J. van der Est, E. E. Burnell and J. Lounila Light-scattering Study of Nematogenic Molecules with a Flexible Core D. A. Dunmur and M. R. Wilson Phase Behaviour and Lytropic Liquid Crystals in Cationic Surfactant-Water Systems Naphthalene Character and Absence of the C=O Stretching Vibration in the Resonance Raman Spectrum of 2-Naphthaldehyde in its Lowest m* Triplet State A. M. J. van Eijk, G. B. Ekelmans, R. van der Steen, A. H. Huizer and C. A. G. 0. Varma G. R. Luckhurst H. J. Coles and S. V. Kershaw R. L. Vold, R. R.Vold and M. Warner H. Yokoyama R. Y. Dong and G. M. Richards E. S. Blackmore and G. J. T. Tiddy I 139 Adsorption Theory of Volume Filling of Micropores for Structurally Heterogeneous Solids M. Jaroniec and R. Madey (i)1149 I163 1177 1185 1191 1199 121 3 1229 Definition of Surface Tension at a Non-spherical Interface V. S. Markin, M. M. Kozlav and S. L. Leikin Solute--Solvent Interaction in the Photoinduced Tautomerization of 7- Azaindole in Various Alcohols and in Mixtures of Cyclohexane and Ethanol J. Konijnenberg, A. H. Huizer and C. A. G. 0. Varma Theoretical Correlations of Stretching Force Constants and Lengths of Bonds in Ion-Molecule Clusters and Transition States of Methyl Transfer Reactions 1. Lee, J. K. Cho and C. H. Song Conduction-electron Spin Resonance of a Tetrathiafulvalene-Copper Bromide Complex, (TTF),CuBr (x = 6; y = 4) M.Inoue, M. B. Tnoue, T. Asaji, L. S. Prabhumirashi and D. Nakamura 35Cl Nuclear Quadrupole Resonance and Molecular-orbi tal Studies of DDT- Type lnsecticides J. Komasa, J. Rychlewskj and B. Nogai The Microwave Spectrum and Potential Function of Propanal J. Randell, J. A. Hardy and A. P. Cox Energy Pooling in an Equilibrium-coupled System between Ca[4s3d(lD2)] and Mg[3~3p(~P,)] Atoms following Pulsed Dye-laser Excitation D. Husain and G. Roberts Ab Znitio Calculations of the Potential-energy Curves of the Low-lying States of NeC2&, Arc2+, Ne02+ and Ar02+ M. A. Vincent and I. H. Hillier The following papers were accepted for publication in Faraday Transactions 1 during May 1988.A Method for the Determination of Hyperfine Coupling Matrices from the ENDOR Spectra of Polycrystalline Samples Baker, G. J. and Raynor, J. B. A Study on the Dielectric Spectra of the Doped AgBr Emulsion Systems. Effect of Hg2+, I and Br Ions 7/1514 7/1715 Cheng, H . and Tang, Y. 8/000671(/ F 1 P 8/00220G/F 1 P 8/00226F/FlP 8/00227D/FlP 8/00228B/FlP 8/00230D/F 1 P Mechanism of Chemical Vapour Deposition of Silicon Alkoxide on Mordenites Niwa, Miki, Kawashima, Yoshimi, Hirino, Takashi and Murakami, Yuichi Separation of Viscosity B Coefficients into Ionic Contributions. Part 5.-Acetonitrile Lawrence, Kenneth G., Sacco, Antonio, de Giglio, Angelo and dell'Atti, Angelo The Direct Determination of Chemical Diffusion Coefficients of Oxygen in UO,,, over the Range 180-399 "C.Spectroscopic Procedure and Preliminary Results Griffiths, T. R., St Aubyn Hubbard, H. V., Allen, Geoffrey, C. and Tempest, P. A. Photophysics on Solid Surfaces. Evidence of Dimer Formation and Polarization of Monomer and Excimer Fluorescences of Pyrene in the Adsorbed State on a Silica-gel Surface Fujii, Tsuneo, Shimizu, Etsuro and Suzuki, Satoshi Ordering in Monodispersed Polymer Lattices induced by Temperature Gradients Furusawa, Kunio, Torori, Morio and Hachisu, Sfi Temperature-programmed Desorption of Ethanol from ZSM-5, ZSM- 1 1 and Theta-1 Zeolites Chen, Li-Feng, Wacker, T. and Rees, L. V. C. (ii)8/00329G/FlP 8/00334C/F 1P 8/00335A/F 1 P 8 /00364E/ F 1 P 8/00367J/F1 P 8/0047 1 D/ F 1 P 8/0050 1 J / F 1 P 8/00559A/FIP 8/00562A/FlP 8/00720I/F 1 P 8/00722E/Fl P 8/00768C/FlP 8/008 12D/FlP 8/008 13B/FlP 8/008 14K/F 1 P S/OOS 16G/ F 1 P 8/008 18C/F I P X-Ray Diffraction Study of a Molten Eutectic LiF-NaF-KF Mixture Igarashi, Kazuo, Okamoto, Yoshihiro, Mochinaga, Junichi and Ohno, Hideo Infrared Study of the Adsorption of Ammonia, Pyridine and Hydrogen Chloride on Barium Sulphate Neagle, William and Rochester, Colin H.Infrared and Gravimetric Studies of the Adsorption of Water on Barium Sulphate Neagle, William and Rochester, Colin H. Viscosity Measurements of Some Tetrabutylammonium, Copper(I), Silver(1) and Thallium(1) Salts in Acetonitrile-Pyridine Mixtures at 15, 25 and 35 "C The Properties of Boralites studied by Infrared Spectroscopy Datka, Jerzy and Piwowarska, Zofia Investigations into the Effect of Spinel Oxide Composition on the Rate of Carbon Deposition The Ethane- 1,2-diol-Water Solvent System.The Dependence of the Dissociation Constant of Picric Acid on the Temperature and Com- position of the Solvent Mixture Franchini, G. C., Marchetti, A., Tassi, I. and Tosi, G. Double Channel Electrodes. Beyond the Leveque Approximation Compton, Richard G. and Stearn, Geoffrey M. Effect of Metal-Support Interaction on the Heat of Hydrogen Absorption Foo, Chung Hsian, Hong, Cheng Tsung and Yeh, Chuin-Tih Silver( I) Complexation with Tertiary Amines in Toluene Soledade Santos, M., Barbosa, Ester F. G. and Spiro, Michael Chromia Silica-Titania Cogel Catalysts for Ethene Polymerisation. Polymerisation Kinetics Conway, Steven J., Falconer, John W. and Rochester, Colin H.Zeolites H-[GaIZSM-5 and H-ZSM-5. Comparative Study of Intro- duction of Transition Metal Cations by a Solid-state Reaction Kucherov, A. V., Slinkin, A. A., Beyer, G. K. and Borbely, G. Some New Developments in Solid-state Nuclear Magnetic Resonance Spectroscopic Studies of Lipids and Biological Membranes, including the Effects of Cholssterol in Model and Natural Systems Forbes, Jeffrey, Bowers, John, Shan, Xi, Moran, Liam, Oldfield, Eric and Moscarello, Mario A. Deuterium Quadrupole-echo Studies of Molecular Motion in a Bi- phenyl-P-Cyclodextrin Clathrate Ronemus, Alan D., Vold, Regitze R. and Vold, Robert L. Enhanced Oxygen Evolution through Electrochemical Water Oxidation mediated by Polynuclear Complexes embedded in a Polymer Film Yao, Guang Jun, Kira, Akira and Kaneko, Masao Crystal-like Structures of Deionized Polystyrene Spheres at Interfaces with Quartz, Air, n-Hexane, Polystyrene and Poly(methy1 methacrylate) Okubo, Tsueno Wetting Characteristics of Hydrophobic Minerals in relation to the Singh, B.and Gill, D. S. Allen, Geoffrey C. and Jutson, Josephine A. (iii)8/008 19A/F1 P 8/00861 B/FlP 8/00873 F/ F 1 P 8/00921 J/FlP 8/00969D/FlP 8/01 162A/FlP 8/01217B/FlP 8/01290C/FlP 8/01 327F/FlP 8/01 337C/F1 P 8/0 1460D/F 1 P 8/01941 J / F 1 P 8/02123F/FlP Surface Tension of Aqueous Methanol Solutions Kelebek, S., Smith, G. W., Finch, J. A. and Donini, J. C. Stepwise Adsorption at the Same Site; a Thermodynamic Treatment Garrone, Edoardo and Ugliengo, Piero Hemimicelle Formation of Cationic Surfactants at the Silica Gel-Water Interface Tin Dioxide Gas Sensors.Part 3.-Solid-state Electrochemical Investi- gations of Reactions taking place at the Oxide Surface McAleer, J. F., Maignan, A., Museley, P. T. and Williams, D. E. Photocatalytic Oxidation and Adsorption of Methylene Blue on Thin Films of Near-ultraviolet Illuminated TiO, Matthews, Ralph W. The Adsorption of Methylene Blue from Aqueous Solutions by A1-ZSM- 5-Type Zeolites and Related Silica Polymorphs Handreck, G. Paul and Smith, Thomas D. Photoinduced Electron-transfer Reactions of Micelle-forming Surfactant Ruthenium(r1) Bipyridyl Derivatives El Torki, Farin, M., Reed, Wayne F. and Schmehl, Russell H. Nuclear Magnetic Resonance Study of Pt-Rh Bimetallic Clusters Slitcher, Charles P., Wang, Z., Ansermet, J-Bh. and Sinfeit, J.€3. Recognition of Ions by Non-steady-state Analysis in their Permeation in Membranes Higuchi, Akon, Katoh, Takeynki and Nakagawa, Tsutomu Adsorption and Oligomerization of. Isobutene on Oxide Catalyst Surfaces. A Fourier-transform Infrared Study Busca, Guido, Ramis, Giancuido and Lorenzelli, Vincenzo Studies of the Temperature Dependence of Retention in Supercritical Fluid Chromatography Bartle, Keith D., Clifford, Anthony A., Kithinji, Jacob P. and Shilstone, Gavin F. The Radical Cation of Formaldehyde in A Freon Matrix. An Electron Spin Resonance Study Rhodes, J. Christopher and Symons, Martyn C. R. lH Nuclear Magnetic Resonance Spin-diffusion and Spin-Lattice Relaxation in Solid, High-density Polyethylene Packer, K. J., Poplett, I. J. F. and Taylor, M.J. An Electron Spin Resonance and ENDOR Study of the Photochemical Decomposition of Substituted Quinoxaline Di-N-oxides Christidis, T. C. and Heineken, F. W. Gu, Tiren, Gao, Yueying and He, LinshuCumulative Author Index 1988 Abdel-Kader, M. H., 2241 Abe, H., 511 Abraham, M. H., 175, 865, 1985 Abraham, R. J., 1911 Adachi, H., 1091 Ahluwalia, J. C., 2651 Aicart, E., 1603 Allen, G. C., 165, 355 Amorelli, A., 1723 Anazawa, I., 275 Andersson, S. L. T., 1897 Anpo, M., 751, 2771 Antonini, A. C. R., 1889 Aoi, H., 2421 Aoyama, T., 2209 Aracil, J., 539 Archer, G. P., 2499 Arora, K. S., 1729 Asakura, K., 1329, 2445, 2457 Aveyard, R., 675 Ayyoob, M., 2377 Baba, K., 459 Back, D. M., 2585 Bagchi, S., 1501 Baglioni, P., 467 Baldini, G., 979 Barna, T., 229 Barone, G., 1919 Basumallick, I.N., 2697 Baulch, D. L., 1575 Bazsa, G., 215, 229 Benmouna, M., 1563 Benoit, H., 1563 Berei, K., 367 Berroa de Ponce, H., 255, 1671 Berry, F. J., 2783 Bertoldi, M., 1405 Beyer, H. K., 1447 Bhat, R., 2651 Binks, B. P., 675 Birch, G. G., 2635 Blandamer, A. H., 1889 Blandamer, M. J., 1243, 1889, 2703, 2906 Blesa, M. A., 9 Blinov, N. N., 1075 Bloor, D. M., 2087 Bonnefoy, J., 941 Borbely, G., 1447 Borckmans, P., 1013 Borgarello, E., 261 Borowko, M., 1961 Bourdillon, C., 941 Brandreth, B. J., 1741 Breen, J., 293 Briggs, B., 1243, 2703 Brown, M. E., 57, 1349 Bruce, J. M., 2855 Brustolon, M., 2875 Brydson, R., 617, 631 Bulow, M., 2247 Burgess, J., 1243, 1889, 2703 Burget, U., 885 Busca, G., 237, 1405, 1423 Buxton, G. V., 1101, 1113 Caballero, A., 2369 Caceres, M., 539 Caceres-Alonso, M., 1603 Carbone, A.I., 207 Caro, J., 2347 Carr, N. J., 1357 Castronuovo, G., 1919 Cavani, F., 237 Cavasino, F. P., 207 Celik, F., 2305 Centi, G., 237 Cesiro, A., 2573 Chagas, A. P., 1065 Chandra, H., 609 Chatterjee, J. P., 2697 Che, M., 751, 2771 Cheek, P. J., 1927 Cheng, V. K. W., 899 Chien, J. C. W., 1123 Chinchen, G. C., 2135 Chirico, G., 979 Christensen, P. A., 2795 Chudek, J. A., 1145, 1737 Clarke, J. K. A., 251 1 Clarke, R. J., 365 Clint, J. H., 675 Coates, J. H., 365 Coles, B. A., 2357 Coller, B. A. W., 899 Coluccia, S., 751 Compton, R. G., 473, 483, 2013, 2057, 2155, 2357 Contarini, S., 2335 Cook, A., 1691 Costas, M., 1603 Covington, A. K., 1393 Crowther, N. J., 1211 Dadok, J., 2595 Daldrup, N., 2553 Danil de Namor, A.F., 255, 1671, 2441 Das, S., I057 Dash, A. C., 75, 2387 Dash, N., 75 Davydov, A., 37 Dawber, J. C., 41 Dawber, J. G., 41, 713 Day, M. J., 2013 de Bleijser, J., 293 Delben, F., 2573 Del Vecchio, P., 1919 Diaz Peiia, M., 539 Dickinson, E., 871 Disdier, J., 261 Domen, K., 511 Dougal, J. C., 657 Duarte, M. Y., 97, 367 Duce, P. P., 865 Duckworth, R. M., 1223 Dupliitre, G., 2831 Dyster, S., 1113 Eagland, D., 1211 Eaton, G., 2181 Egawa, C., 321 Einfeldt, J., 93 1 Ekechukwu, A. D., 1871 Eley, D. D., 2069 Elia, V., 1919 Elliot, A. J., 1101 Elvidge, D., 2703 Engel, W., 617, 631 Eszterle, M., 575 Evans, J. C., 1723 Everett, D. H., 1455 Eyears, J. M., I437 Fernandez, A., 1543 Fernandez-Pineda, C., 647 Finter, C. K., 2735 Flanagan, T. B., 459 Fletcher, P. D. I., 113 1 Foresti, E., 237 Foresti, M.L., 97 Forni, L., 2397, 2477 Forster, H., 491 Foster, R., 1145, 1737 Fraenkel, D., 1817, 1835 Franklin, K. R., 687, 2755 Franks, F., 2595 Fubini, B., 1405 Fujihira, M., 2667 Furedi-Milhofer, H., 130 1 Gal, D., 1075 Gabrail, S., 41 Gaffney, S. H., 2545 Galwey, A. K., 57, 729, 1349, Gans, P., 657 Gardner, P. J., 1879 Garrone, E., 2843 Geblewicz, G., 561 Geertsen, S., 1101 Georges, V., 1531 Giamello, E., 1405 1357Gill, D. S., 1729 Gill, J. B., 657 Gilot, B., 801 Girdult, H. H., 2147 Giuliacci, M. E., 2311 Goldfarb, D., 2335 Gopalakrishnan, R., 365 Grabielle-Madelmont, C., 2609 Grampp, G., 366 Gratzel, M., 197, 1703 Gray, A. C., 1509 Gray, P., 993 Green, P., 2109 Green, S. I. E., 41 Green, W. A., 2109 Grepstad, J. K., 1863 Griffiths, J.F., 1575 Grigera, J. R., 2603 Grigo, M., 931 Grimson, M. J., 1563 Gritzner, G., 1047 Grzybkowski, W., 1551 Guardado, P., 1243, 2703 Guarini, G. G. T., 331 Guarino, G., 2279 Guglielminotti, E., 2195 Guidelli, R., 97, 367 Gupta. D. Das, 1057 Guyan, P. M., 2855 Hadjiivanov, K., 37 Hakin, A. W., 1889, 2703 Hall, D. G., 773, 2087, 2215, Hall, N. D., 1889 Halle, B., 1033 Hamada, K., 1267 Hanawa, T., 1587 Handreck, G. P., 1847 Hanson, G. R., 1475 Harrer, W., 366 Harriman, A., 2109, 2795, 2821 Hasebe, T., 187 Hashimoto, K., 87 Haslam, E., 2545 Hatayama, F., 2465 Hayashi, K., 2209 Hazra, D. K., 1057 Heatley, F., 343 Hegarty, B. F., 251 1 Hegde, M. S., 2377 Herley, P. J., 729 Herrmann, J-M., 261 Hertz, H. G., 2735 Hey, M. J., 2069 Heyward, M. P., 815 Hidalgo, M. del V., 9 Hill, A., 255 Holzwarth, J.F., 2807 Hoshino, K., 2667 House, W. A., 2723 Howson, M. R., 2723 Hubbard, C. D., 1243, 2703 Hudson, B. D., 1911 Huis, D., 293 Hunter, R., 131 1 2227 AUTHOR INDEX Hutchings, G. J., 1311 Ige, J., 1 Ikeda, S., 151 Imai, H., 923 Imamura, H., 765 Imanaka, T., 851, 2173 Inoue, A., 1195 Irinyi, G., 1075 Ishiguro, S., 2409 Ishikawa, T., 1941 Isobe, T., 1199 Ito, D., 1375 Ittah, B., 1835 Iwamoto, E., 1679 Iwasawa, Y., 321, 1329, 2445, Iyer, R. M., 2047 Jackson, S. D., 1741 Jaeger, N. I., 1751 Jaenicke, W., 366 Jeminet, G., 951 Jens, K-J., 1863 Johnson, G. R. A., 501 Johnson, I., 551 Johnston, C., 309, 2001 Jonasson, R. G., 231 1 Jones, A. R., 2914 Jonson, B., 1897 Jorge, R. A., 1065 Jorgensen. N., 309, 2001 Joiwiak, M., 2077 Juillard, J., 951, 959, 969, 1713 Kaizu, Y., 1517 Kakei, K., 1795 Kane, H., 851 Kaneko, K., 1795 Kanno, T., 281, 2099 Kasahara, S., 765 Kato, C., 2677 Kato, S., 151 Katz, N.E., 9 Kawasaki, Y., 1083 Kay, R. L., 2595 Keeble, D. J., 609 Keller, A., 2904 Kemp, T. J., 2027 Kermode, M. W., 1911 Kevan, L., 467, 2335 Kimura, T., 2099 Kinnaird, S., 2135 Kirby, C., 355 Kiricsi, I., 491 Kishore, N., 2651 Kiss, I., 367 Kiwi, J., 1703 Klinowski, J., 2902 Klinszporn, L., 1551 Klissurski, D., 37 Kobayashi, A., 1795 Kobayashi, H., 1517 Kobayashi, M., 281, 2099 KoEiiik, M., 2247 Koda, S., 1267 KodejS, Z., 2885 Koksal, F., 2305 2457 (vi) Komatsu, H., 2537 Kondo, J., 51 1 Kondo, M., 2771 Kondo, S., 1941 Kondo, Y., 111 Konishi, Y., 281 Kordulis, C., 1593 Kornhauser, I., 785, 801 Kosugi, N., 1795 Kowalak, S., 2035 Kraehenbuehl, F., 1973 Krausz, E., 827 Krebs, P., 2241 Kristyan, S., 917 Kubelkova, L., 1447 Kubokawa, Y., 751, 2129, 2771 Kumamaru, T., 1679 Kurimura, Y., 841, 1025 Kuroda, H., 1329, 1795 Kuroda, K., 2677 Kuroda, Y., 2421 Kusabayashi, S., 11 1 Kuwabata, S., 1587, 2317 Lahy, N., 1475 Laing, M.E., 2013 Lajtar, L., 19 Larnbi, J. N., 1 Land, E. J., 2855 Larsson, R., 1897 Laubry, P., 969 Laval, J-M., 941 Lawrence, K. G., 175 Lea, J. S., 1181 Leaist, D. G., 581 Lefever, R., 1013 Lefferts, L., 1491 Lengyel, I., 229 Levine, H., 2619 Levy, A., 1817 Levy, M., 1835 Lewis, T. J., 1531 Leyendekkers, J. V., 397, 1653 Leyte, J. C., 293 Lhermet, C., 2567 Lilley, T. H., 1927, 2545 Lincoln, S. F., 365 Lindner, Th., 631 Lips, A., 1223 Llewellyn, J. P., 153 I Logan, S.R., 1259 Louis, C., 2771 Lycourghiotis, A., 1593 Machida, K., 2537 MacKay, R. L., 1145, 1737 Mackley, M., 2910 Maezawa, A., 851 Malanga, C., 97 Malet, P., 2369 Mandel, M., 2483 Maniero, A. L., 2875 Marcandalli, B., 2807 Marcus, Y., 175, 1465 Markovid, M., 1301 Maroto, A. J. G., 9 Marsden, A., 2519AUTHOR INDEX Martin, R. R., 231 1 Martins, L. J. A., 2027 Maruya, K., 511 Mason, D., 473, 483, 2057 Mathlouthi, M., 2641 Matsumoto, T., 1375 Matsumura, Y., 87 Matsuoka, K., 1277 Matteoli, E., 1985 Mayagoitia, V., 785, 801 McAleer. J. F., 441 McMurray, N., 379 Mead, J., 675 Medda, K., 1501 Mehta, G., 2297 Mensch, C. T. J., 65 Merkin, J. H., 993 Meunier, F., 1973 Mills, A., 379, 1691 Mines, J. R., 1911 Mintchev, L., 1423 Mirti, P., 29 Mitsushima, I., 851 Miura, K., 2421 Miyagawa, S..2129 Miyajima, K., 2537 Miyakawa, K., 1517 Miyanaga, T., 2173 Miyata. H., 2129, 2465, 2677 Mohamed, M. A-A., 57, 729, Moiroux, J., 941 Moller, K., 1751 Morel, J-P., 2567 Morel-Desrosiers, N., 2567 Morimoto, T., 2421 Morris, J. J., 865 Morterra, C., 1617 Morton, J. R., 413 Moseley, P. T., 441 Mosseri, S., 2821 Mousset, G., 969 Muhler, M., 631 Mukai, T., 2465 Mukherjee, T., 2855 Murray, A., 2783 Murray, B. S., 871 Nagao, M., 1277 Nahor. G. S., 2821 Nakagaki, M., 2537 Nakagawa, Y., 2129 Nakarnura, T., 1287 Nakamura, Y., 1 1 1 Nakao, N.. 665 Nakayama, N., 665 Nandan, D., 2047 Narayanan, S., 521 NaLhat. N. B., 501 Neta, P., 2109 Newman, K. E., 1387, 1393 Nicolis, G., 1013 Nishihara, C., 433 Nishikawa, S., 665 Nishio, E., 1639 Nisi, M., 2279 I349 Nomura, H., 151, 1267 Norris, J.0. W., 441 Northing, R. J., 2013 Noszticzius, Z., 575 Nucci, L., 97 Ohno, T., 2465 Ohshima, K., 1639 Ohtaki, H., 2409 Ohtani, S., 187 Okabayashi, H., 1639 Okamoto, K., 2317 Okamoto, Y., 851 Okubo. T., 703, 1163, 1171, Oliva, C., 2397, 2477 Oliver. S. W., 1475 Ollivon, M., 2609 Olofsson, G., 551 Ommen. J. G. van, 1491 Onishi, T., 511 Ono, T., 2465 Ono, Y., 1091 Oosawa, Y., 197 Ozeki. S., 1795 Ozutsumi, K., 2409 Page, F. M., 1145 Painter, D. M., 773, 2087 Pal, M., 1501 Pan. C.-f., 1341 Pandey, J. D., 1853 Pandey, P. C., 2259 Pang, P.. 1879 Paoletti, S., 2573 Pappin, A. J., 1575 Parrott, D., 1 13 I Passelaigue, E., 17 13 Patil, K., 2297 Patterson, D., 1603 Pedatsur, N., 2821 Pelizzetti, E.. 261 Pena-Nuiiez, A. S., 2181 Penar, J., 739 Penman, J.I., 2013 Perutz, R. N., 2901 Pethybridge, A. D., 2723 Pezzatini, G., 367 Pfeifer, H., 2347 Piccini, S., 331 Pichat, P., 261 Pickering. I. J., 2795 Pickl, W., 131 I Piekarski, H., 529, 591 Pilarczyk, M.. 1551 Pilbrow, J. R., 1475 Pilkington, M. B. G., 2155 Plath, F. J., 1751 Pointud, Y.. 959, 1713 Polavarapu, P. L., 2585 Pota, G.. 215 Pradhan, J., 2387 Preston, K. F., 413 Price, W. E., 2431 Prior, D. V., 865 Pushpa, K. K,, 2047 Quinquenet, S., 2609 1949 (vii) Quist, P-O., 1033 Radulovic, S., 1243, 2703 Rai, R. D., 1853 Rajam, S., 1349 Rajaram, R. R., 391 Rao, B. G., 1773, 1779 Rao, K. J., 1773, 1779 Rao, K. M., 2195 Rebenstorf, B., 1897 Rebuscini, C., 2397 Rees, L. V. C., 2911 Reller, A., 2327 Renuncio, J. A. R., 539 Rhodes, C. J., 1187 Richardson, N.V., 2909 Richardson, S. N., 2909 Richoux, M-C., 2109 Riis, T., 1863 Riva, A., 1423 Robson, B., 2519 Rochester, C. H., 309, 2001 Rojas, F., 785, 801, 1455 Rooney, J. J., 2511 Ross, J. R. H., 1491 Rowlands, C. C., 1723 Rubio, R. G., 539 Saadalla-Nazhat, R. A., 501 Sacchetto, G. A., 2885 Saito, M., 1025 Saito, Y., 275 Saji, T., 2667 Sakaiya, H., 1941 Sakamoto. Y., 459 Sakata, Y., 511 Salvagno, S., 1531 Sarkany, A., 2267 Sartorio, R., 2279 Sato, T., 275 Sauer, H., 617 Savilc, G., 2907 Sawabe. K., 321 Sayari, A., 413 Sbriziolo, C., 207 Scarano, D.. 2327 Schelly, Z. A,, 575 Schiffrin, D. J., 561 Schiller, R. L., 365 Schlenoff, J. B., I123 Schlogl, R., 631 Schmelzer, N., 931 Schonert, H., 2553 Schulz, R. A., 865 Schwarz, W.. 1703 Scott, S . K., 993, 2904, 2908 Seidl, V., 1447 Sellers, R.M., 355 Senna, M., 1199 Senoda, Y., 1091 Sermon, P. A., 391 Serpelloni, M., 2609 Serpone, N.. 261 Seuvre. A-M., 2641 Shamil, S., 2635 Sheppard. N., 29 I3 Shindo, H., 433AUTHOR INDEX Shukla, A. K., 1853 Shukla, R. K., 1853 Sidahmed, I. M., 1153 Simmons, R. F., 1871 Sinclair, G. R., 1475 Singh, B., 1729 Singh, P. P., 1807 s’Jacob, K. J., 1509 Slade, L., 2619 Smith, E. R., 899 Smith, T. D., 1475, 1847 Sokolowski, S., 19, 739 Somsen, G., 529 Soriyan, 0. O., 1 Speight, J. M., 2069 Spoto, G., 2195 Stainsby, G., 871 Stange, G., 2807 Stead, K., 2905 Stearn, G. M., 2155, 2357 Steel, A. T., 2783 Stevens, J. C. H., 165 Stirling, C. J. M., 1531 Stocker, M., 1863 Stoeckli, F., 1973 Stone, F. S., 2843 Stone, W. E. E., 117 Stramel, R. D., 1287 Struve, P., 2247 Subba Rao, M., 1703 Suga, K., 2667 Sugahara, Y., 2677 Sun, L-M., 1973 Suzuki, T., 1795 Swallow, A. J., 2855 Sykes, A.F., 1575 Symons, M. C. R., 609, 1 181, Szamosi, J., 917 Taga, K., 1639 Taga, T., 2537 Tagawa, T., 923 Takada, T., 765 1187, 2181, 2499 Takagi, Y., 1025 Takato, K., 841 Takisawa, N., 2087 Takriti, S., 2831 Tamaki, J., 2173 Tanaka, F., 1083 Tanaka, K., 601, 2895 Tanaka, K-i., 601 Taniewska-Osinska, S., 2077 Tardajos, G., 1603 Taylor, D. M., 1531 Taylor, P. J., 865 Tazaki, K., 231 1 Tewari, J., 1729 Thampi, K. R., 1703 Theocharis, C. R., 1509 Thomas, J. K., 1287 Thomas, J. M., 617, 631, 2795 Thompson, J. S., 2519 Tiddy, G. J. T., 2900 Tissier, C., 951, 969 Tofield, B. C., 441 Torres-Sanchez, R-M., 117 Townsend, R. P., 687, 2755 Tra, H. V.. 1603 Trifiro, F., 237, 1405, 1423 Tschirch, G., 2247 Tsuchiya, S., 765 Tsukamoto, K., 1639 Tummalapalli, C.M., 2585 Turner, D. J., 2683 Twiselton, D. R., 1145 Uematsu, R., 111 Uma, K., 521 Unwin, P. R., 473, 483, 2057 Vaccari, A., 1405, 1423 van Rensburg, L. J., 13 1 1 van Veen, J. A. R., 65 van Wingerden, R., 65 Varani, G., 979 Vasaros, L., 367 Vazquez-Gonzalez, M. I., 647 Vid6czy, T., 1075 Viguria, E. C., 255 Vink, H., 133 Viswanathan, B., 365 Vogel, V., 1531 Vordonis, L., 1593 Walker, R. A. C., 255 Waller, A. M., 2013, 2357 Wang, E., 2289 Ward, J., 713 Ward, T. R., 2545 Webb, G., 2135 Wells, C. F., 815, 1153 Welsh, M. R., 1259 Wijmenga, S. S., 2483 Williams, B. G., 617, 631 Williams, D. E., 441 Williams, R. A., 713 Winstanley, D., 1741 Wong, J., 1773, 1779 Wood, N.D., 1113 Wormald, C. J., 1437, 2912 Wurzburger, S., 2279 Wyn-Jones, E., 773, 2087 Yamada, M., 2457 Yamada, Y., 751 Yamamoto, Y., 2209 Yamane, T., 2173 Yamasaki, S., 1679 Yamashita, S., 1083 Yao, S., 1375 Yasugi, E., 2421 Yoffe, A. D., 2899 Yoneyama, H., 1587, 2317 Yoshida, S., 87 Yuqing, L., 2289 Zecchina, A., 751, 2195, 2327, Zeitler, E., 617, 631 Zelano, V., 29 Zibrowius, B., 2347 Zielinski, R., 151 Zundel, G., 885 2843 (viii)JOURNAL OF CHEMICAL RESEARCH Papers dealing with physical chemistry or chemical physics which appear currently in J. Chem. Research, The Royal Society of Chemistry’s synopsis + microform journal, include the following: Evaluation of the Broyden-Fletcher-GoldfarbShanno (BFGS) Variable Metric Method in Geometry Optimisation using Semi-empirical SCF-MO Procedures Rzepa (1988, Issue 3) I.Brookes, Charles Kemball and H. Frank Leach (1988, Issue 4) Lars Carlsen and Helge Egsgaard (1 988, Issue 4) Podmore and Martyn C. R. Symons (1988, Issue 4) Dimitris K. Agrafiotis and Henry S. The Measurement of Exchangeable Hydrogen associated with Trtanium Dioxide (Rutile) Beverley The Reaction between lmidogen and Elemental Carbon. An Alternative Route to Interstellar HCN ? Radical Cations of N,N-Dimethyluracil and N,N-Dimethylthymine Christopher J. Rhodes, Ian D. Inhibition or Acceleration of the lsomerisation of But-1 -ene on Titanium Dioxide (Rutile) by Adsorbed Molecules Beverley 1. Brookes, Charles Kemball and H. Frank Leach (1 988, Issue 5) Methods Henry S. Rzepa (1 988, Issue 7) Cheletropic Elimination of CO and N2.A Comparison of the MNDO, AM1 and ab initio SCF-MO The Cyclopropenyl Anion: an ab initio Molecular Orbital Study Wai-Kee Li (1 988, Issue 7) The Influence of Environmental Effects on the Geometry of Molecules in Crystals Slawomir J. Grabowski (1 988, Issue 8)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY WITH THE ASSOCIAZIONE ITALjANA DI CHIMICA FISICA, DIVISION DE CHlMlE PHYSIQUE OF THE SOCIETE FRANCAISE DE CHlMlE AND DEUTSCHE BUNSEN GESELLSCHEFT FUR PHYSIKALISCHE CHEMlE JOINT MEETING Structure and Reactivity of Surfaces Centro Congressi, Trieste, Italy, 13-16 September 1988 Organising Commit fe e: M. Che V. Ponec F. S. Stone G. Ertl R. Rosei A. Zecchina The conference will cover surface reactivity and characterization by physical methods: (i) (ii) (iii) The meeting aims to stimulate the comparison between the surface properties of dispersed and supported solids and the properties of single crystals, as well as the comparison and the joint use of chemical and physical methods.Further information may be obtained from: Professor C. Morterra, lstituto di Chimica Fisica, Corso Massimo D’Azeglio 48,10125 Torino, Italy Metals (both in single crystal and dispersed form) insulators and semiconductors (oxides, sulphides, halides, both in single crystal and dispersed forms) Mixed systems (with special emphasis on metal-support interaction) THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No. 8 6 Spectroscopy at Low Temperatures University of Exeter, 13-1 5 September 1988 Orga nising Committee : Professor A.C. Legon (Chairman) Dr P. B. Davies Dr B. J. Howard Dr P. R. R. Langridge-Smith Dr R. N. Perutz Dr M. Poliakoff The Discussion will focus on recent developments in spectroscopy of transient species (ions, radi- cals, clusters and complexes) in matrices or free jet expansions. The aim of the meeting is to bring together scientists interested in similar problems but viewed from the perspective of different envi- ronments. Speakers include: L. Andrews, K. H. Bowen, B. J. Howard, L. B. Knight Jr, E. Knozinger, D. H. Levy, J. P. Maier, J. Michl, M. Moskovits, A. J. Stace, M. Takami, M. Poliakoff, A. J. Barnes, J. M. Hollas, M. C. R. Symons and P. Suppan. The final programme and application form may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBNTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM Orientation and Polarization Effects in Reactive Collisions To be held at the Physikzentrum, Bad Honnef, West Germany, 12-14 December 1988 Organising Committee: Dr S.Stolte Professor R.A. Levine Dr K. Burnett Professor R.N. Dixon Professor J.P. Simons Dr H. Loesch The Symposium will focus on the study of vector properties in reaction dynamics and photodissoci- ation rather than the more traditional scalar quantities such as energy disposal, integral cross-sec- tions and branching ratios. Experimental and theoretical advances have now reached the stage where studies of Dynamical Stereochemistry can begin to map the anisotropy of chemical interac- tions.The Symposium will provide an impetus to the development of 3-0 theories of reaction dyna- mics and assess the quality and scope of the experiments that are providing this impetus. The following areas will be covered: (A) Collisions of oriented or rotationally aligned molecular reagents (B) Collisions of orbitally aligned atomic reagents (C) Photoinitiated 'collisions' in van der Waals complexes (D) Polarisation of the products of full- and half-collisional complexes. The preliminary programme may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London WIV OBN. THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No. 87 Catalysis by Well Characterised Materials University of Liverpool, 11-13 April 1989 Organising Committee: Professor R.W. Joyner (Chairman) Professor A. K. Cheetham Professor F. S. Stone The understanding of heterogeneous catalysis is Dr K. C. Waugh Professor P. B. Wells n important academic activity, which complements industry's continuing search for novel and more efficient catalytic processes. The emergence of rele- vant, in particular in sifu techniques and new developments of well established experimental ap- proaches to catalyst characterisation are making a very significant impact on our knowledge of catalyst composition, structure, morphology and their inter-relationships. Well characterised cata- lysts, which will be the subject of the Faraday Discussion, include single-crystal surfaces, whether of metals, oxides or sulphides; crystalline microporous solids, such as zeolites and clays, and ap- propriate industrial catalysts.The elucidation of structure/function relationships and catalytic mech- anism will be important aspects of the scientific programme. Contributions describing novel methods for synthesising well characterised catalysts and also reporting important advances in characterisa- tion techniques will also be included. The preliminary programme may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN.FARADAY DIVISION INFORMAL AND GROUP MEETINGS Electrochemistry Group with the Electroanalytical Group and The Society of Chemical Industry Elect roc hemical Dynamics To be held at the University of Strathclyde on 5-10 September 1988 Further information from Dr S.P. Tyfield, CEGB, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB ~~ ~ Statistical Mechanics and Thermodynamics Group Dense Fluids To be held at the University of Cambridge on 14-16 September 1988 Further information from Dr P. Francis, Department of Chemistry, University of Hull, Hull HU6 7RX ~~ Carbon Group with the Carbon and Graphite Group of The Society of Chemical Industry Carbon 88 To be held at the University of Newcastle upon Tyne on 18-23 September 1988 Further information from The Conference Secretariat, Carbon 88, Society of Chemical Industry, 1411 5 Belgrave Square, London SW1X 8PS Polar Solids Group with Applied Solid State Chemistry Group Ceramic Superconductors Conference To be held at Imperial College, London on 19-20 September 1988 Further information from Dr A.R. West, Department of Chemistry, University of Aberdeen, Meston Walk, Aberdeen AB9 2UE ~ ~~ ~~~~~ Surface Reactivity and Catalysis Group Interfaces and Catalysis To be held at the University of Glasgow on 19-21 September 1988 Further information from Dr G. Webb, SCRG Meeting, Department of Chemistry, University of Glasgow, Glasgow G12 8QQ Division Autumn Meeting: Polymerisation and Polymer Behaviour To be held at the University of Birmingham on 20-22 September 1988 Further information from Professor I. W. M. Smith, Department of Chemistry, University of Birmingham, PO Box 363, Birmingham B15 2TT Colloid and Interface Science Group Structure in Colloidal Systems and its Characterisation To be held at the University of Bath on 21-23 September 1988 Further information from Dr R.Buscall, ICI plc, Corporate Colloid Science Group, PO Box 11, The Heath, Runcorn, Cheshire WA7 4QE Division jointly with Dalton Division Inorganic Solids and their Surfaces (including the Nyholm Lecture by R. Hoffmann) To be held at the Scientific Societies’ Lecture Theatre, London on 22 November 1988 Further information from Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN Polymer Physics Group jointly with Physical Crystallography Group Diffraction from Polymers To be held at the Geological Society, London on 30 November 1988 Further information from Dr M. Richardson, National Physical Laboratory, Teddington, Middlesex TWll OLW (x ii)Polar Solids Group with the Applied Solid State Chemistty Group Computer Modelling of Inorganic Solid Structures To be held at the Scientific Societies' Lecture Theatre, London on 2 December 1988 Further information from Dr A.E. Comyns, R & D Department, Laporte lndusties Ltd., Moorfield Road, Widnes WA8 OQJ Theoretical Chemistry Group Beyond the Born-Oppenheimer Approximation To be held at Trent Polytechnic, Nottingham on 14 December 1988 Further information from Dr R. G. 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Ashfold, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 ITS ~~~ ~ Neutron Scattering Group Muon Spectroscopy To be held at the University of Nottingham on 20-22 December 1988 Further information from Dr S. Cox, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX1 1 OQX ~~ Electrochemistry Group with the Electrotechnology Group of the SCI Battery Workshop To be held at the University of Oxford on 3-4 January 1989 Further information from Dr S.P. Tyfield, CEGB, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB Electrochemistry Group with Organic Reactions Mechanisms Group Electron Transfer Reactions To be held in London on 5 January 1989 Further information from Dr S. P. Tyfield CEGB, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB ~ Gas Kinetics Group Reactions of ions and Free Radicals To be held at the University of Warwick on 6 January 1989 Further information from Professor R. G. Donovan, Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ Division with the Institute of Physics Sensors and their Applications To be held at the University of Kent at Canterbury on 19-22 September 1989 Further information from The Meetings Officer, Institute of Physics, 47 Belgrave Square, London SWlX 8QX (xiii)Polar Solids Group with the Applied Solid State Chemistty Group Computer Modelling of Inorganic Solid Structures To be held at the Scientific Societies' Lecture Theatre, London on 2 December 1988 Further information from Dr A.E. Comyns, R & D Department, Laporte lndusties Ltd., Moorfield Road, Widnes WA8 OQJ Theoretical Chemistry Group Beyond the Born-Oppenheimer Approximation To be held at Trent Polytechnic, Nottingham on 14 December 1988 Further information from Dr R. G. Woolley, Department of Physical Sciences, Trent Polytechnic, Clifton Lane, Nottingham NGll 8NS He ctrochemis tv Group New Ideas in Electrochemistry To be held at the University of Cambridge on 15-16 December 1988 Further information from Dr S. P. Tyfield, CEGB, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB Colloid and Interface Science Group Aggregation in Colloidal Systems To be held at the Scientific Societies' Lecture Theatre, London on 16 December 1988 Further information from Dr R. Buscall, ICI plc, Corporate Colloid Science Group, PO Box 11, The Heath, Runcorn, Cheshire WA7 4QE High Resolution Spectroscopy Group High Resolution Molecular Spectroscopy To be held at the University of Birmingham on 19-20 December 1988 Further information from Dr M. N. R. Ashfold, School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 ITS ~~~ ~ Neutron Scattering Group Muon Spectroscopy To be held at the University of Nottingham on 20-22 December 1988 Further information from Dr S. Cox, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX1 1 OQX ~~ Electrochemistry Group with the Electrotechnology Group of the SCI Battery Workshop To be held at the University of Oxford on 3-4 January 1989 Further information from Dr S. P. Tyfield, CEGB, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB Electrochemistry Group with Organic Reactions Mechanisms Group Electron Transfer Reactions To be held in London on 5 January 1989 Further information from Dr S. P. Tyfield CEGB, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB ~ Gas Kinetics Group Reactions of ions and Free Radicals To be held at the University of Warwick on 6 January 1989 Further information from Professor R. G. Donovan, Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ Division with the Institute of Physics Sensors and their Applications To be held at the University of Kent at Canterbury on 19-22 September 1989 Further information from The Meetings Officer, Institute of Physics, 47 Belgrave Square, London SWlX 8QX (xiii)

ISSN:0300-9599    

DOI:10.1039/F198884BP109

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1988, 84(8), 2519-2536 Derivation of Parameters for Conformational Calculations on Carbohydrate Systems, including Bacterial Cell Wall Pep tidoglycan Alan Marsden, Barry Robson and J. Stuart Thompson .Epsitron Peptide and Protein Engineering Research Unit, Department of Biochemistry and Molecular Biology, School of Biological Sciences, University of Manchester, Manchester M13 9PT and Proteus Biotechnology Ltd, 48 Stockport Rd, Marple SK6 6AB The use of software designed for the prediction of minimum-energy conformations of peptides and proteins has been extended to include a number of glycoconjugates. For this purpose it was necessary to derive empirical interatomic parameters for sugars consistent with the existing amino-acid parameter set.The principle source is high-grade ab initio quantum-mechanical calculations. Parameters describing the anomeric effect were derived from the ab initio surface of dimethoxymethane. Parameters describing the pyranose ring were optimised against the energy profiles of 2-ethyltetrahydropyran and further refined against the profiles of 2-methoxytetrahydropyran to take account of the anomeric and exo- anomeric effects. Optimisation against the ab initio surface of meth- oxymethanol was used to study the anomeric hydroxyl. The use of arbitrary rotation potentials to model the anomeric and em-anomeric effects was avoided by extension of an orbital force field method through specific parametrisation of lone-pair molecular orbitals. Assumptions and limita- tions inherent in the derivation and use of potentials are discussed.An important goal of theoretical chemists is to serve the experimentalist by developing tools to predict the shapes and biological properties of molecules from their structural formulae alone. The advantages of computer programs as tools include (a) the ability to design molecules for specific purposes, objectively, from first principles and (b) to reduce or eliminate much of the laborious and expensive synthesis and testing involved in the identification of new drug molecules. Our interest in conformational calculations of pyranose structures has evolved from studies of oligosaccharide chains of the bacterial cell wall polymer, peptidoglycan,' and its derivative, the immunostimulant, muramyl dipeptide (MDP).2 Here we attempt to derive more general parameters for use with all types of pyranosides based upon their underlying conformational properties.An im- portant goal in producing these parameters has been their compatibility with the peptide parameters used in the LUCIFER suite of programs (logical use of conformational information in fast energy routine^).^ This peptide data base is in any event important for molecules such as peptidoglycan which contain both sugars and amino acids in polymeric form. Also, in view of the increasing importance of peptides as potential drugs and vaccines, whose conformations and potencies can often be there is a need to extend computer-prediction methods to peptidesarbohydrate conjugates. The lifetime and targetting of both endogenous and exogenous peptides and proteins can be greatly influenced by the presence of sugar residues. While it is relatively easy to devise a set of parameters which can be used on an ad hoe basis to describe a limited range of closely related molecular structures, the problem is to find a set of parameters which allow molecules of different chemical types, (i.e.with the same functional groups in different chemical environinents) to be compared and their 25 192520 Conformational Calculations on Carbohydrates energies calculated. Irrespective of the desirability of devising parameters which are widely applicable, the fundamental differences in general properties of peptides and oligosaccharides influence the detailed strategies for theoretical modelling in each case.For example, in contrast to the flexibility of peptide side-chains, oligosaccharides are considerably constrained by the relative rigidity of the pyranose ring. Moreover, the frequent occurrence of repeat units sometimes justifies a periodicity constraint which can greatly reduce the number of variables to be considered. On the other hand, whereas considerable insight into the three-dimensional structures of proteins can now be gained by combining energy-minimisation programs with the intelligent use of molecular graphics and data bases' relating sequence to secondary structure, there are no comparable data bases available for carbohydrates, even though the general principles underlying the shapes adopted have been known for some time.' The relation between the ' exact ' and time-consuming ab initio quantum-mechanical methods and the relatively rapid and approximate methods based on parameter sets has been discussed elsewhere.' Semiempirical methods, e.g.perturbative configurational interactions using localised orbitals (PCILO), have been applied to mono- or di- saccharide~,~ but larger molecules require the use of faster empirical potential function methods. The latter, when coupled with fast minimisation and possibly specially adapted molecular-dynamics techniques, remain, in the light of our extensive experience with peptides, the most feasible method for the calculation of macromolecular conformations at the present time. Empirical parameters can, however, be derived from ab initio or semiempirical quantum-mechanical methods and from suitable experimental data, e.g.from X-ray crystallography or n.m.r. spectroscopy when available. The experimental techniques are also used to supply angular or interatomic distance constraints [from coupling constants and nuclear Overhauser enhancement (NOE) data, respectively] both for empirical simulations and for checking the results of such simulations.2. lo Any satisfactory method for calculating preferred pyranose conformations must take account of the anomeric and exo-anomeric effects.l' The former refers to the tendency for an equatorial OH group on the anomeric carbon to be less stable than it would be in other ring positions. This modulates ring puckering by influencing orientation about the 0 5 x 1 bond. Similarly, the exo-anomeric effect depends on orientation about C1-0 1.To account for this, the acetal segment (C5-05-C 1-0 1-C) requires special treatment in conformational energy calculations to reproduce the preference for the synclinal-synclinal ( + sc, + sc) conformation characteristic of the anomeric effect.12 Both the HSEA (hard-sphere exo-anomeric) method13 and NBEA (non-bonded exo- anomeric) method14 use separate internal rotation potentials for a- and a-anomers which are then added to the other terms in the total energy function. As internal rotation potentials are often merely correction factors compensating for lack of understanding, we regard this as the last resort when non-bonded parameters alone are unable to satisfy the observed energy surface. Also, intrinsic potentials are inappropriate when the effect is extrinsic to the bond in question.The use of rotation potentials was successfully circumvented in calculations on nucleic acids and s~lphonamidesl~ by the inclusion of localised molecular orbitals (lmos) with parameters derived from high-grade ab initio quantum-mechanical calculations.16 This orbital force field (OFF) method is particularly suited to our potential-energy calculations using 9-6- 1 Lennard-Jones-type potential functions of the type Eii = A, I--' - Bij r-' + Ci Cj r-l where Eij is the energy between two atom or lone-pair centres, i and j , so that the total energy of a conformation is the sum of such pairwise interactions and parameters A and B relate to the van der Waals repulsion and attraction terms, respectively.Ci and Ci represent the partial charges on atoms or orbital centroids. It is worth noting thatA. Marsden, B. Robson and J. S. Thompson 2521 Table 1. Scheme for the derivation of empirical parameters I. . 2. 3. 4. 5;. 6 . _ _ starting point Robson and Platt all-atom peptide parameter Ether parameters derived from methyl-0-D-lactylamide (A. M., unpublished). anomeric effect Parameters from 1 optimised for dimethoxymethane (DMM) against ab initio energy surface.lg ring atoms Parameters from 1 optimised for 2-ethyltetrahydropyran (ETHP) against PCILO energy profiles12 where consistent with ab initio studies on fragments to give ring-atom parameters. exo-anomeric effect Parameters from 2 and 3 further refined for 2-methoxy- tetrahydropyran (MTHP) against PCILO energy profiles12 where consistent with ab initio studies on fragments to include the anomeric and exo-anomeric effects.anomeric OH Parameters from 1 and 2 optimised for methoxymethanol (MOM) against ab initio energy surface.2o D-glucopy ranose Parameters from 4 and 5 transferred to D-glucopyranose and checked, by energy minimisation, against experimental data. - _ ~ _ _ _ _ ~ occasional difficulties reported verbally by some workers using this approach can be overcome by inclusion of weak van der Waals repulsion terms for orbitals, for which insufficient data for calibration were available at the time. Any difficulties so overcome were technical rather than physical : as in any method neglecting a repulsion term on an interaction centre, there may be problems at short contact distances.Hence the choice of degree of steric repulsion is not too critical. Rees and Smith” demonstrated that the anomeric effect was adequately described by conformational energy calculations on pyranoses using simple non-bonded functions without the need to account for the lone-pair orbitals on 0 5 . Our calculations support this observation and the OFF method is not required to give correct pyranose ring puckering. As with peptides, inclusion of an OFF makes little difference here since the stereochemistry is dominated by interactions between core orbitals. However, we found that lone-pair orbitals on both 0 5 and 0 1 were necessary to reproduce the properties of the exo-anomeric effect. Derivation of Parameters The strategy for deriving a set of parameters that are suitable for exploring the likely minimum-energy conformations of a wide variety of glycoconjugates is outlined in table 1.Parameters derived for simple, oxygen-containing molecules were progressively transferred to new chemical contexts in more complex molecules and eventually to sugars. We employed a conservative approach to the investigation of parameters. We changed only the parameters of the atom types within the functional group of immediate interest and then only by the minimum amount necessary to achieve agreement with the reference data. Derivation of parameters for sugar acetamide groups and methyl-0-D-lactylamide (MODL) and the details of their application to studies of peptidoglycan will be reported in detail elsewhere (Marsden, Robson and Thompson, in preparation).The latter compound is an analogue of the N-acetylmuramyl-L-alanine residues which join the glycan chains via ether-linked D-lactyl branches to the cross-linked peptide structure in the bacterial cell wall. Hence we are now potentially able to study a wide variety of glycoconjugate materials using these newly derived sugar parameters in conjunction with our well established peptide parameters. The extent to which these empirical2522 Conformational Calculations on Carbohydrates parameters can be transferred to more complex molecules is not yet certain and requires further work on a wider variety of molecules. However, our experience of simulations on these larger molecules is so far encouraging and has yielded results which are consistent with their chemical and biological properties.Parameter Optimisation Program The computer program P P M ~ (E. Platt and A. Marsden) was used to derive empirical parameters from various types of reference data. All computations were undertaken on a CDC 7600 computer at the University of Manchester Regional Computer Centre. van der Waals A and B parameters are normally derived from conformational energy surfaces obtained from experimental sources. In addition, or alternatively, depending on the availability of experimental data, they were calculated from ab initio quantum- mechanical results and published semiempirical quantum-mechanical methods for larger systems, providing PCILO results for the latter are consistent with behaviour extrapolated from ab initio studies on fragments.By means of a SIMPLEX minimisation procedure, the program refines the initial parameters assigned to the atoms in the test molecule to give an optimum fit between the empirical and reference conformational energy surfaces. Partial charges are optimised against the total dipole moment of the test molecule or the individual dipole moments of the functional groups. These charges are distributed over the atoms and orbitals of the molecule such that the total charge on the molecule or individual functional groups remains constant. The root-mean-square deviation of the final parameter set is nominally required to be < ca. 1 kcal? mol-1 for each conformation of the test molecule. This is roughly the level of error in ab initio calculations, although for certain interactions it may be higher." In practice, it is only those reference conformers with an energy less than 10 kcal mol-l above the global minimum that are of interest.Reference conformers within 3 kcal mol-' of this minimum are therefore thermodynamically very significant. In these regions, closer agreement of the empirical and reference energies is normally required. Calculation of ab initio potential-energy surfaces employed the ATMOL3 suite of programs and an extended basis set with (7s3,3p2) orbitals for carbon and oxygen. Dimethox ymethane Methanediol (MDO) and its mono- and di-0-methyl derivatives (MOM and DMM) have been extensively studied as models for carbohydrates, either to derive parameters for molecular mechanics from ab initio surfaces" or to study the nature of the anomeric effect by semiempirical quantum mechanics.21 Since in other application areas we have found some semiempirical quantum-mechanical methods questionable (disagreeing with ab initio calculations, parameters calibrated at crystals and experimental solution data) careful monitoring against previous ab initio studies on HOCH20H was considered important.15* 22 We used the geometry and ab initio energy surface of DMM1' to modify the ether parameters derived for MODL to suit the -0-C-0- environment.Only conformations resulting from rotation about the q5 and 8 bonds, with rigid standard geometry (i.e. fixed bond lengths and valence angles) were used in the parametrisation (fig. 1). The rigid-geometry approximation is an important factor in achieving rapid and efficient computation.The range of conformations considered was restricted to those of relatively low energy and then further limited by considering only one half of the symmetric energy surface. Partial charges were optimised to give a dipole moment equal t 1 cal = 4.184 J.A . Marsden, B. Robson and J. S. Thompson 2523 Fig. 1. Diagram of dimethoxymethane showing its preferred conformation. The approximate positions of the lmos are marked on the oxygen atoms in black. OE = ether oxygen. Table 2. Summary of the parametrisation results for dimethoxymethane conformation __- ab initio calculated dipole e energy energya AE moment no. # 60 180 180 180 - 120 - 120 60 20 60 180 120 60 120 60 120 120 0.0 7.68 8.05 2.59 10.39 5.64 3.28 8.49 0.0 6.0 1 7.74 2.36 10.17 4.59 3.77 9.97 - 0.502 - 1.67 3.777 - 0.3 1 3.453 - 0.23 2.690 - 0.22 3.831 - 1.05 3.318 0.49 1.373 1.48 2.351 The energies are given in kcal mol-' relative to the ab initio minimum-energy conformer.The ab initio energy surface was obtained from Jeffrey et d . 1 9 The root-mean-square fit to the ab initio energy surface is 0.93 kcal mol-'. The calculated dipole moments are given in D with the statistical average = 0.67 D. The fit represents more conformers than are shown because of the molecular symmetry. a Obtained in the present work using the program SPECIAL^. to the experimental value of 0.67 DT at 298 K.23 The van der Waals potentials could not be optimised unless lmos were explicitly included on the oxygen atoms of DMM to reproduce the energy profiles for the rotations about the variable bonds.The orbital centroids were assumed to possess ether-like geometry (as in the ab initio calculations on MODL) and to carry partial negative charges as in normal OFF calculations. However, specific van der Waals potentials were also needed between the lmos on the separate oxygen atoms. This therefore represents an extension of the OFF method in which the anomeric and exo-anomeric effects are at least partially described by the lmo-specific van der Waals potentials. This is not to say that in reality the lone-pair orbitals alone are responsible for these effects.'l The results give a reasonable fit to the ab initio energy surface of DMM as a model for the anomeric effect (table 2).7 1 D = 3.33564 x C m.2524 Conformational Calculations on Carbohydrates Fig. 2. Diagram of axial 2-methoxytetrahydropyran (a) and equatorial 2-methoxytetrahydropyran (b) showing their preferred conformations. The approximate positions of the lmos are marked on the oxygen atoms in black. 2-Methoxytetrahydrop yran The direct transferability of DMM parameters to the anomeric and exo-anomeric positions of carbohydrates is complicated by the need to consider both a relaxation of the geometry and a change in the chemical environment of the anomeric carbon atom. Tvaroska and Perez14 pointed out the problems of deriving an exo-anomeric energy term from DMM for use in HSEA and NBEA calculations, and concluded that this approach was only valid for anomeric torsion angles of 0-180".Consequently, these workers derived em-anomeric terms from the pyranose analogues, 2-methoxytetrahydropyran (MTHP, fig. 2) and 2-ethyltetrahydropyran (ETHP).12 The internal rotation functions were calculated from the difference in PCILO energy profiles of MTHP and ETHP for rotation about the C1-01 bond. The data available from time-consuming ab initio studies on such large systems is relatively sparse. Although we have particular concern regarding the PCILO method'' (or at least its parameters), which in relation to peptides yields energy surfaces22 which differ from those generally accepted, the above result seems to be the only well studied starting point for the derivation of parameters for use in these calculations, providing consistency with ab initio results based on fragments is retained.However, whereas separate internal rotation potentials had to be derived forA . Marsden, B. Robson and J . S. Thompson Table 3. Summary of the parametrisation of axial and equa- torial 2-methoxytetrahydropyran (A-MTHP and E-MTHP, respectively) conformer PCILO calculated # energy energy' AE 60 120 180 - 120 - 60 0 60 120 180 - 120 - 60 0 A-MTHP 0.0 0.0 1.66 1.64 2.37 2.36 7.58 > 9.58 9.10 > 11.10 7.20 > 9.20 E-MTHP 0.74 0.69 2.77 2.82 2.68 1.88 4.30 > 6.30 2.44 3.80 2.39 > 4.39 - 0.02 -0.01 -0.05 0.05 - 0.80 1.36 - - 2525 The energies are given in kcal mol-' relative to the PCILO minimum-energy conformer of A-MTHP. The PCILO energy profiles and conformer notation were taken from Tvaroska and K o ~ a r ~ ~ and Tvaroska.12 The root-mean-square fit to the PCILO energy surface is 0.32 kcal mol-'."Obtained in the present work using the program SPECIAL^. the axial and equatorial conformers, we derived a single parameter set suitable for both conformers. The geometry of ETHP was assumed to be similar to that described for MTHP.24 Published PCILO energy profiles for both optimised geometry12 and rigid show that unrealistically high energy barriers can result from the use of the latter. Parametrisation against rigid-geometry quantum-mechanical results is, however, comparing like with like, rather than parametrising against a flexible molecule in solution. We may further justify the use of rigid geometry here, not only because of the increased speed of the energy calculations but also because we parametrise against the optimum geometry profiles.Our energy calculations of present type halve the van der Waals repulsion term for vicinal (i.e. 1 + 4) contacts in systems other than C-X-X-C.18 This has the effect of softening the vicinal repulsion energy and thus reducing the barrier to rotation. This is generally believed to substitute for flexible valence angles,26 although other authors have carried out a similar correction on the basis of special electronic effects or anisotropicity of atoms. Whatever the physical justification, it provides the starting point for calibration with the understanding that the deficiencies may be overcome in the calibration of non-bonding interactions. When properties capable of at least partial experimental verification are to be calculated, this approach appears, in our general experience, to be even more justifiable for sugars than for peptide systems.Parameters for the tetrahydropyran ring atoms were derived from ETHP. The absence of the em-ring oxygen allowed the anomeric and em-anomeric effects to be neglected. MODL ether parameters were initially placed on the ring -C-0-C- atoms, while parameters for the remaining atoms were taken from an in-house data A mean experimental value of 1.72 D (range 1.55-1.89 D at 298 K23) was taken as the reference2526 Conformational Calculations on Carbohydrates Table 4. 9-6- 1 empirical potential functions used in calculations on methoxytetrahydropyran, methyl-D-glucopyranoside and D-glucopyranose name of centre type description of centre type A B ,*/A E*/kcal mol-' C/e.u.CM CG cs CR HM HG HS HR OG 0s LG LS LG*LS CM CG cs CR c 2 HM HG HS HR H HO OG 0s 0 LG LS LG*LS CH cs CR c2 HH HS HR H HO HT OT 0s 0 LT LS LG*LS methyl H anomeric H ring H ring H exo-anomeric 0 ring 0 lone-pair orbital of OG lone-pair orbital of 0s methoxytetrahydropyran methyl C 38 809 1248 anomeric C 50 968 1928 ring C 48 554 1853 ring C 39002 1488 15 44 15 22 462 110 0 0 327 145 1 450 406 27 390 35 687 0 0 specific interaction methyl C anomeric C ring C ring C aliphatic C methyl H anomeric H ring H ring H aliphatic H hydroxyl H exo-anomeric 0 ring 0 hydroxyl 0 6826 124 38 809 1248 50 968 1928 48 554 1853 39002 1488 38900 1230 327 15 145 1 44 450 15 406 22 445 15 445 15 27 390 1462 35687 1110 45770 1410 met hyl-D-glucopyranoside lone-pair orbital of OG 0 0 lone-pair orbital of 0s 0 0 specific interaction 6826 1124 D-glUCOpyranOSe anomeric C 50 968 1928 ring C 48 554 1853 ring C 39002 1488 aliphatic C 38 900 1230 anomeric H 145 1 44 ring H 450 15 ring H 406 22 aliphatic H 445 15 hydroxyl H 445 15 em-anomeric hydroxyl H 573 18 em-anomeric 0 65021 1925 ring 0 35 687 1110 hydroxyl 0 45770 1410 lone-pair orbital of OT 0 0 lone-pair obital of 0s 0 0 specific interaction 0 0 3.6 3.41 3.4 3.4 3.21 3.66 3.54 3.04 3.04 3.64 - - 2.088 3.6 3.41 3.4 3.4 3.62 3.21 3.66 3.54 3.04 3.55 3.55 3.04 3.64 3.65 - - 2.088 3.41 3.4 3.4 3.62 3.66 3.54 3.04 3.55 3.55 3.65 3.70 3.64 3.65 - - - -0.191 - 0.409 - 0.400 -0.321 - 0.005 - 0.006 - 0.003 - 0.009 -0.618 -0.159 - - -4.519 -0.191 - 0.409 - 0.400 -0.321 -0.182 - 0.005 -0.006 - 0.003 - 0.009 - 0.003 - 0.003 -0.618 -0.159 -0.199 - - -4.519 - 0.409 - 0.400 -0.321 -0.182 - 0.006 - 0.003 -0.009 - 0.003 - 0.003 - 0.002 - 0.250 -0.159 -0.199 - - - -0.264 0.146 -0.182 -0.134 0.135 0.06 1 0.124 0.067 - 0.044 0.462 -0.119 - 0.297 - - 0.264 0.146 - 0.058 - 0.067 - 0.220 0.135 0.061 0.124 0.067 0.110 0.300 - 0.044 0.462 - 0.300 -0.1 19 - 0.297 - 0.062 - 0.058 - 0.067 - 0.220 0.061 0.124 0.067 0.1 10 0.300 0.300 0.223 0.462 -0.300 - 0.290 -0.297 - a Obtained using the program SPECIAL^.A .Marsden, B. Robson and J . S. Thompson 2527 Table 5. Summary of the parametrisation results for methoxymethanol conformation ab initio calculated dipole no. # e energy energy" AE moment 60 180 180 180 180 60 60 60 60 60 - 180 - 120 - 60 0 - 180 - 120 - 60 120 0.0 9.4 7.6 3.0 2.7 4.3 4.4 6.9 2.9 0.0 8.68 7.90 3.57 2.26 3.81 5.07 7.34 2.7 1 - -0.72 0.30 0.56 - 0.44 - 0.49 0.67 0.44 -0.19 0.96 3.42 3.07 2.2 1 1.61 2.79 3.38 3.13 1.59 'The energies are given in kcal mol-' relative to the ab initio minimum-energy conformer.The tzb initio energy surface was obtained from Jeffrey et aL20 The root-mean-square fit to the ab initio energy surface is 0.50 kcal mol-'. The calculated dipole moments are given in D with the statistical average equal to 0.99 D. The fit represents more conformers than are shown below because of molecular symmetry. [1 Obtained in the present work using the program SPECIAL2. dipole moment against which the partial charges were optimised (final calculated dipole moment = 1.72 D).Optimisation of the van der Waals potentials gave a root-mean- square deviation of 1.18 kcal mol-1 for the empirical energies of eight conformations of ETHP (C1 -ethyl torsion angles = axial : 60, 120 and 180" ; equatorial : 60, 120, 180, - 60 and 0"). All parameters for the various atom types present in the ring required adjustment from their initial values, while parameters for the ethyl atom types remained constant. Parameters for the -0-CH, atom types of DMM were merged with those for ETHP to give the starting parameter set for the MTHP parametrisation. The calculated mean dipole moment (1.23 D) for the five rotameric forms of MTHP in vacuo2' was used as a reference and the optimised partial charges gave a statistically averaged dipole moment of 1.23 D.The root-mean-square fit to seven of the conformational energy points was 0.32 kcal mol-1 (see table 3), while the remaining energies were > 2 kcal mol-' above the KILO values. Since the latter were in the high-energy region of conformational space (and in peptides at least, PCILO tends to underestimate barrier heights) they can be regarded as making an insignificant contribution to the equilibrium population of conformers. As with DMM, lone-pair orbitals on the oxygen atoms were parametrised to account for most of the difference between MTHP and ETHP. Their specific van der Waals potentials are concerned with interactions between the lone-pair orbitals and not interactions involving other atoms. In contrast, they participate in electrostatic interactions with other atom types in a typical OFF manner.The parameter set obtained in this way is shown in table 4. Methoxymethanol The ab initio conformational energy surface for methoxymethanol (MOM) was calculated by Jeffrey et al. using a 4-31G basis set.20 In the present study, the atom types in MOM were parametrised against this energy surface to provide a model for the reducing terminus of mono- and oligo-saccharides. Since the energy surface of MOM was similar to that of DMM, parameters from the latter were used as starting values for the CH3-O-CH2- segment and the -OH values used were from the standard2528 Conformational Calculations on Carbohydrates parameter Standard geometry2' was used with the ether oxygen lone-pair orbitals positioned as in DMM.The direction of the lone-pair orbitals on the hydroxyl oxygen were taken as lying in the same plane as the group dipole. Thus the C-0 dipole vector angle was calculated from the resultant dipole moment of methanol (1.71 D) and the group moment of C-0-H by the method given in ref. (28). The calculated angle was 86" and gave a C-0- lone-pair valence angle of 90". Assuming that the +sc, +sc conformation is predominant and the dipole moments of the ether and hydroxyl groups are 1.30 D28,29 and 1.71 D,23 respectively, the resultant dipole moment for MOM was calculated to be 0.99 D. The agreement between the energy surfaces calculated using the parameter set and the ab initio method is shown in table 5. Testing of Parameters The all-atom energy minimisation program, SPECIAL& is an extended version (by A.M.) of SPECIAL and part of the LUCIFER suite of program^.^ It uses the SIMPLEX method to find deep energy wells and a second-derivative gradient method to locate exact minima.3 Our flexible-geometry and molecular-dynamics procedures were not employed, since the parameters presently described are for use in the rigid-geometry approximation. 2-Methox ytetrahydropyran The standard geometry of MTHP was used to determine energy minima, while torsion angles, including those in the ring, were allowed to vary. Cyclic structures are not allowed in the tree representation and so ring connectivities are discontinuous at one of the bonds. The two ring atoms normally joined by this bond are, however, maintained at the correct bond distance by a specific van der Waals interaction with a deep energy well corresponding to - 1 x lo4 kcal mol-l.This is termed a ring-closing potential. This approach is fairly standard, although in our view, some procedures in the literature are needlessly complex and may in part reflect the need to overcome minimisation deficiencies. A similar technique for applying a closing potential to a disulphide bridge has been described el~ewhere.~' It was checked that the valence geometry between groups so connected were physically satisfactory. The ring thus modelled is free to exist in any of its stable or metastable conformations. Tree structures representing the geometries of the molecules used in the present study are given in the Appendix. Energy minimisation of both forms of MTHP shown in fig.2 yielded different conformations from the idealised minima used for parametrisation. After minimisation, values of @ were 71.5" for the axial form and 65.3" for the equatorial form. This is not entirely surprising since in calibration against quantum-mechanical energy surfaces, conformations between the points of the relatively coarse grid have been interpolated, not explicitly parametrised. Even though the geometries of the two conformations are substantially maintained, the energy difference was greater (1.73 kcal mol-1 compared with a PCILO energy difference of 0.74 kcal mol-l). Strictly, the calibration of parameters so far is merely a first pass, before adjustment against experimental data.The equilibrium composition of the above conformers can be calculated from the ratio of the individual Boltzmann statistical weightings. Here, we use the GG and TG, terminology2' to represent the above axial and equatorial conformations, so to distinguish the GG : TG, ratio (which represents the two most stable conformation^)^' from experimentally derived axial : equatorial ratios. Hence swt, sw,,+sw,,' TG, = The GG : TG, ratio at 298 K for the energy-minimised MTHP was 99.95 : 0.05 compared with 78 : 22 calculated from ref. (27) by the PCILO method. An improved ratio of 8 1 : 18A . Marsden, B. Robson and J. S. Thompson 2529 Table 6. Calculated and experimental ratios of both forms of methoxytetrahydropyran in U ~ C U O and in various solvents solvent GG : TG, AEa P A & (A - E) SPECIAL2' PCILO' exptl - - in U ~ C U O -0.88 82: 18 78:22 carbon tetrachloride 2.24 - 0.05 52 : 48 77:23 77-8 3 chloroform 4.8 1 - 0.43 67:33 72 : 28 7 1-78 ethanol 20.70 - 0.55 72 : 28 72 : 28 6668 water 78.30 -0.10 54 : 46 51 :49 52 dimethyl sulphoxide 46.68 - 0.28 62:38 71 :29 74 t; is the dielectric constant of the medium.The GF:TG, ratio is explained in the text.27 PA is the experimentally derived percentage of the axial conformations in solution. The energy difference, AE, is given in kcal mol-'. L1 Energy of axial conformer minus energy of equatorial conformer. ' Obtained in the present work using the program SPECIAL^. Ref. (27). was obtained by increasing the internal dielectric constant from the usual in uacuo value of 1.0 to 2.2.The only justification for using this value here is that over a range of dielectric constants examined (both in vacuo, as above, and with solvent effects included, as below) it gave the most consistent agreement with the GG:TG, ratios. More formally, we may say that the parameters so far seem adequate within the range of uncertainty of solvent electrostatic effects and thus there is no justification for further refinement of the in vacuo energies. An important test of the parameters is to calculate equilibrium ratios (as above) of the anomeric forms present in solution. SPECIAL^ employs the Onsager reaction field,31 which estimates the contribution of the surrounding solvent to the total empirical potential energy. For aqueous solutions, the bulk dielectric constant of 78.3 is used in the reaction-field term.The relative energy difference for the two conformers in aqueous solution calculated by SPECIAL^ was 0.1 kcal mo1-I. The GG:TGl ratio was 54:46 compared with the PCILO-calculated ratio of 5 1 : 49 and an experimental anomeric ratio of 52:48. The ratios for a range of other solvents are given in table 6. Glucopyranosides and Glucopyranoses The parameters derived above for MTHP and MOM and the ring geometry established for MTHP25 have been applied to a- and p-D-glucopyranose and their methyl glycosides. The former compounds required substitution of the 0-methyl group parameters of MTHP with hydroxyl-containing MOM parameter types. This is the ubiquitous problem of parameter transferability. To maintain overall neutrality, it was necessary to adjust the partial charges on the ring atoms where a hydrogen was replaced by a hydroxyl or hydroxymethyl group.Energy minimisation was carried out using SPECIAL^ with an internal dielectric constant of 2.2 for consistency with the above studies. The preferred conformations of methyl-D-glucose were GG-like ($ = 71.0') for the a- anomer and TG,-like ($ = -63.8") for the p-anomer, showing a similar qualitative conformational preference to MTHP (see table 7 for Cremer-Pople ring-puckering parameter^^^). When compared with the corresponding crystal structures, the main difference is the conformational orientation of the hydroxyls on C3 in the a - a n ~ r n e r , ~ ~ and C3 and C4 on the p - a n ~ m e r . ~ ~ Empirical energy calculations on D-glucopyranose predict the proportion of a-anomer in aqueous solution to be 66 O h .On the other hand, experimental data suggest that the2530 Conformational Calculations on Carbohydrates Table 7. Cremer-Pople ring-puckering parameter^^^ for methoxytetrahydropyran, methyl-D- glucopyranoside and D-glucopyranose conformer medium Q/A 43/A A-MTHP E-MTHP A-MTHP E-MTHP Me-a-D-Glc Me-B-D-Glc Me-a-D-Glc Me-B-D-Glc in uacuo in uacuo water water in uacuo in uacuo water water water water methoxytetrahydropyran 0.566 0.061 0.563 0.620 0.095 0.6 13 0.545 0.110 -0.534 0.555 0.075 -0.550 meth yl-D-glucop yranoside 0.551 0.1 11 0.539 0.563 0.031 0.562 0.558 0.083 0.552 0.563 0.030 0.562 D-glucop yranose 0.622 0.100 0.614 0.605 0.054 0.602 6.2 168.3 8.8 172.2 11.7 3.1 8.5 3.1 9.2 5.1 159.0 38.9 85.0 28.0 140.4 133.6 144.4 134.8 84.3 180.0 a-anomer is less abundant (36 YO ;35 37 The reason for this discrepancy may lie in the orientation of the glucose hydroxyl groups in solution. There is evidence that hydrogen bonding to the surrounding hydration shell is more complete in the p- an~mer.~' In our calculations the reaction-field term estimates the total contribution of the surrounding medium and the net dipole on the solute to the energy, but does not necessarily influence the orientation of individual hydrogen bonds.In this instance, the hydroxy groups orientate to give the most stable intramolecular conformation, as in the in vacuo calculations. It would therefore seem necessary to represent water molecules explicitly in the empirical energy calculations in order to reproduce quantitatively the equilibrium distribution of the anomers and their correct conformations. In principle, this is accomplished by Monte-Carlo or molecular-dynamics simulations. Such studies are now being carried out ;38* 39 these techniques are currently restricted by computation speed to disaccharides or, at most, trisaccharides. The Metropolis method4' may be more time-effective.It is hoped that these techniques will eventually yield information on the pattern of conformational preferences of carbohydrates in solution which can eventually be taken into account in empirical simulations. The use of a simple, first-shell water model with the LUCIFER~ program has been studied in our laboratory and would appear likely to be able to resolve the problem, although it remains to be proven in this case that this is not purely fortuitous.Applications Detailed knowledge of the solution conformations of biological molecules is essential for a thorough appreciation of biochemical structures and functions. Approaches to the understanding of the physical chemistry of small carbohydrates have recently been The influence of solvation on saccharide shapes was discussed, and the need for caution in extrapolating from crystal structures to explain solution properties was emphasised. Generally speaking, one has available much less experimental and ab initio data than for peptides, for example, and also a certain degree of ambiguity. Current calibrations of parameters can therefore only be interim.However, there is reason to believe that even the present state of the art can provide a useful and realistic guide to oligosaccharide conformations, particularly if complementary experimental data areA . Marsden, B. Robson and J . S. Thompson 253 1 available, even though estimates of energy differences between coexisting conformers may be lacking in accuracy. N.m.r. information can be very useful here. It can be used both to restrict the range of possible conformations to be considered in a computer simulation or afterwards to narrow down the range of conformations suggested by a simulation. For example, measurements of proton NOE and coupling constants seemed to suggest that (in contrast with the situation with most peptides) single, well defined conformational minima are present in blood group A active oligosaccharides.These minima coincided with local minima predicted by three different methods of conformational potential-energy calculation^.^^ Such calculations are very sensitive to the parametrisation of non-bonded interactions. An alternative view states that quantitative NOE studies indicate that isolated N- linked oligosaccharides sample a wide variety of three-dimensional ~tructures.~~ The report contrasts this with the situation when the same oligosaccharides are bound, whether covalently or as ligands, to proteins. They then appear to be restricted to a small subset of the conformations available to the unbound material. It was suggested that the stabilisation of individual conformational subsets at different glycosylation sites of glycoproteins may explain the differential si te-specific activities of glycosyl transferases at the various glycosylation sites. It is often desirable to complement such experiments by building models of likely molecular structures.As is common in drug design work, such studies might include hypothetical structures which have neither been isolated nor synthesised. One aim of the present work is to provide methods by which identification of the minimum-energy conformations required by model- builders is reliable and cost-effective. Eventually, such studies may allow the development of rapid, empirical methods for predicting secondary structures, such as are making rapid headway for peptides and proteins. Already, simple rules exist which allow carbohydrate polymers to be assigned to conformational classes.43 A start has also been made towards establishing the relationship between the primary structures of N-linked oligosaccharides and their conformations.44 Here, the presence of three rotatable bonds between pairs of 1 -6-linked sugars confers considerable flexibility. A combination of sophisticated n.m.r. techniques and semiempirical quantum-mechanical energy calculations suggested that at least some of these structures exist as mixtures of rotamers. These techniques have been used by our research group for the conformational an- alysis of a wide variety of carbohydrates including muramyl dipeptide,2 peptidoglycan,' glycosphingolipids"9 45 and the ' high-mannose ' asparagine-linked oligosaccharides of glyc~proteins.~~ For the glycosphingolipids, solvent effects were deliberately excluded in view of the complex nature of the environment at the membrane surface where such molecules are located.Despite this, a good indication of the differences in conformation between related molecules was obtained and was sufficient to explain experimentally determined specificities of binding to activator Nevertheless, with other more hydrophilic molecules simulation of solvent was important2 and was represented by means of the reaction field.2,5 We thank Dr Eric Platt for his interest and helpful advice, and Unilever plc and the S..E.R.C. for a CASE award to A. M.2532 Conformational Calculations on Carbohydrates Appendix Rigid-geometry specifications and structures of 2-methoxytetrahydropyran, methyl-D- glucopyranoside and D-glucopyranose are listed.A description of the tree representation and column headings are given by Robson and Platt.3 Column 7(a) is used to exclude unwanted 1-3 interactions at the ring-closure point. Column 11 is optional and gives the atom labels. 1 2 3 4 5 6 ? 7 ( a ) 8 9 HM CM HM HM OG CG HG CR HR HR C* HR HR 0s cs HS HS C* HR HR LG LG LS LS HM CM HM HM OG CG HG CR HR HR C* HR HR 0s CS HS HS C* HR HR 0 1 2 2 2 5 6 6 8 8 8 11 11 6 14 15 15 15 18 18 5 5 14 14 0 1 2 2 2 5 6 6 8 8 8 1 1 11 6 14 15 15 15 18 It) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 2 5 6 14 1 1 13 15 18 20 2 5 6 14 11 13 15 18 20 axial-methoxytetrahydropyran 3 21 8 10 12 23 17 19 4 22 1 7 2 9 3 4 24 5 16 6 7 0.0 1.10 1.10 1.10 1.40 1.40 1.10 1.50 1.10 1.10 1 S O 1.10 1.10 1.40 1.40 1.10 1.10 1 S O 1.10 1.10 0.3 1 0.3 1 0.3 1 0.3 I equatorial-methoxytetrahydropyran 3 21 8 10 12 23 17 19 0.0 4 1.10 1.10 1.10 22 1 1.40 7 2 I .40 I .10 9 3 4 1.50 1.10 1.10 4 3 1.50 4 1.10 4 1.10 24 5 1.40 16 6 3 1.40 1.10 1.10 7 4 1.50 3 1.10 0.0 0.0 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 110.0 109.5 109.5 109.5 112.0 109.5 109.5 110.0 109.5 109.5 105.54 105.54 104.93 104.93 0.0 0.0 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 110.0 109.5 109.5 109.5 112.0 109.5 109.5 110.0 109.5 3 1.10 109.5 10 ~- 0.0 0.0 120.0 120.0 0.0 0.0 120.0 120.0 120.0 120.0 0.0 240.0 0.0 0.0 0.0 120.0 120.0 0.0 240.0 0.0 117.4 125.2 117.4 125.2 0.0 0.0 120.0 120.0 0.0 0.0 120.0 120.0 120.0 120.0 0.0 240.0 0.0 0.0 0.0 120.0 120.0 0.0 240.0 0.0A .Marsden, B. Robson and J . S. Thompson 2533 1 2 3 4 5 6 7 7 ( a ) 8 9 10 11 LG LG LS LS HM CM HM HM OG CG HG CR HR 0 HO C* HR 0 HO 0s cs HS c 2 H H 0 HO C* HR 0 HO LG LG LS LS HM CM HM HM OG CG HG CR HR 0 HO C* HR 0 HO 0s 5 5 14 14 0 1 2 2 2 5 6 6 8 8 10 8 12 12 14 6 16 17 17 19 19 19 22 17 24 24 26 5 5 16 16 0 1 2 2 2 5 6 6 8 8 10 8 12 12 14 21 22 23 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 6 16 2 5 6 16 12 11 14 15 17 24 22 23 26 27 2 5 6 16 12 11 14 15 17 0.3 1 0.3 1 0.3 1 0.3 1 methyl-a-D-glucopyranoside 3 28 8 10 13 30 19 20 25 3 28 7 10 13 30 4 29 1 7 2 9 3 4 5 6 31 7 18 8 21 9 10 11 12 0.0 1.10 1.10 1.10 1.40 1.40 1.10 1 S O 1.10 1.40 1 .oo 1 S O 1.10 1.40 1 .oo 1.40 1.40 1.10 1 S O 1.10 1.10 1.40 1.10 1 S O 1.10 1.40 1 .oo 0.3 1 0.3 1 0.3 1 0.3 1 met hy 1-P-D-glucopyranoside 4 29 8 9 31 1 2 3 4 4 5 3 4 6 4 7 0.0 1.10 1.10 1.10 1.40 1.40 1.10 1 S O 1.10 1.40 1 .oo 1 S O 1.10 1.40 1 .oo 1.40 105.59 105.59 104.84 104.84 0.0 0.0 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 110.0 110.0 109.5 109.5 110.0 109.5 112.0 109.5 109.5 109.5 109.5 109.5 110.0 110.0 109.5 109.5 110.0 105.54 105.54 105.54 105.54 0.0 0.0 109.5 109.5 109.5 109.5 109.5 109.5 109.5 109.5 110.0 110.0 109.5 109.5 110.0 109.5 117.4 125.2 117.4 125.2 0.0 0.0 120.0 120.0 0.0 0.0 120.0 120.0 120.0 120.0 0.0 0.0 240.0 0.0 0.0 0.0 0.0 120.0 120.0 120.0 120.0 0.0 0.0 0 .o 240.0 0.0 0.0 117.4 125.2 117.4 125.2 0.0 0.0 20.0 20.0 0.0 0.0 20.0 20.0 20.0 120.0 0.0 0.0 240.0 0.0 0.0 0.02534 Conformational Calculations on Carbohydrates 1 2 3 4 cs HS c 2 H H 0 HO C* HR 0 HO LG LG LS LS HT OT CH HH CR HR 0 HO C* HR 0 HO 0s cs HS c 2 H H 0 HO C* HR 0 HO LT LT LS LS HT OT CH HH CR HR 0 HO C* 16 15 17 19 19 19 22 17 24 24 26 5 5 16 16 0 1 2 3 3 5 5 7 5 9 9 11 3 13 14 14 16 16 16 19 14 21 21 23 2 2 13 13 0 1 2 3 3 5 5 7 5 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 1 2 3 4 5 6 7 8 9 24 22 23 26 27 2 3 13 9 8 11 12 14 21 19 20 23 24 2 3 13 9 8 11 5 19 20 - 25 25 5 7 10 27 16 17 22 25 4 7 10 6 7 7(a) 8 9 10 11 18 8 3 1.40 1.10 21 9 1 S O 1.10 1.10 10 1.40 1.10 11 4 1.50 3 1.10 12 3 1.40 1 .oo 0.3 1 0.3 1 0.31 0.31 a-D-glucopyranose 26 4 1 6 2 3 4 5 28 6 15 7 18 8 9 10 11 0.0 1 .oo 1.40 1.10 1 S O 1.10 1.40 1 .oo 1 S O 1.10 1.40 1 .oo 1.40 1.40 1.10 1 S O 1.10 1.10 1.40 1 .oo 1 S O 1.10 1.40 1 .oo 0.3 1 0.3 1 0.3 1 0.3 1 /3-D-glucop yranose 26 1 .oo 5 1 1.40 1.10 6 2 4 1.50 1.10 3 1.40 1 .oo 4 3 1.50 0.0 112.0 109.5 109.5 109.5 109.5 109.5 110.0 110.0 109.5 109.5 110.0 105.54 105.54 105.54 105.54 0.0 0.0 10.0 09.5 09.5 09.5 09.5 10.0 10.0 109.5 109.5 110.0 109.5 112.0 109.5 109.5 109.5 109.5 109.5 110.0 110.0 109.5 109.5 110.0 105.3 105.3 106.0 106.0 0.0 0.0 110.0 109.5 109.5 109.5 109.5 110.0 110.0 0.0 120.0 120.0 120.0 120.0 0.0 0.0 0.0 240.0 0.0 0.0 117.4 125.2 117.4 125.2 0.0 0.0 0.0 120.0 120.0 120.0 120.0 0.0 0.0 240.0 0.0 0.0 0.0 0.0 120.0 120.0 120.0 120.0 0.0 0.0 0.0 240.0 0.0 0.0 117.4 125.2 117.4 125.2 0.0 0.0 0.0 120.0 120.0 120.0 120.0 0.0 0.0A .Marsden, B. Robson and J. S. Thompson 2535 ~~~ 1 2 3 4 5 6 7 7 ( a ) 8 9 10 11 HR 0 HO 0s cs HS c 2 H H 0 HO C* HR 0 HO LT LT LS LS END 9 9 11 3 13 14 14 16 16 16 19 14 21 21 23 2 2 13 13 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 12 14 21 19 20 23 24 4 1.10 5 4 1.40 1 .oo 27 28 6 1.40 16 15 7 3 1.40 1.10 17 18 8 1 S O 1.10 1.10 9 1.40 1 .oo 22 10 4 1.50 3 1.10 11 3 1.40 1 .oo 0.3 1 0.3 1 0.3 1 0.3 1 109.5 109.5 110.0 109.5 112.0 109.5 109.5 109.5 109.5 109.5 110.0 110.0 109.5 109.5 110.0 105.3 105.3 106.0 106.0 240.0 0.0 0.0 0.0 0.0 120.0 120.0 120.0 120.0 0.0 0.0 0.0 240.0 0.0 0.0 117.4 125.2 117.4 125.2 C* has the same general properties as atom-type CR except that parameters A and B are adjusted to give a potential-energy well (ring-closing potential) - 1 x lo4 kcal mol-1 at the desired bond length.References 1 A. Marsden, B. Robson and J. S. Thompson, Biochem. SOC. Trans., 1986, 14, 629. 2 A. Marsden, B. Robson and J. S. Thompson, Biochem. SOC. Trans., 1986, 14, 630. 3 B. Robson and E. Platt, J. Mol. Biol., 1986, 188, 259. 4 B. Robson, R. V. Fishleigh and C. A. Morrison, Nature (London), 1987, 325, 395. 5 D. J. Ward, P. W. Finn, E. C. Griffiths and B. Robson, Int. J. Pept. Protein Res., 1987, 30, 263. 6 W. Kabsch and C. Sander, Biopolymers, 1983, 22, 2577. 7 D. A. Rees, Polysaccharide Shapes (Chapman and Hall, London, 1977). 8 E. Platt and B.Robson, in Computing in Biological Science, ed. M. J. Geisow and A. N. Barrett 9 J. S. Yadav, Int. J. Quantum Chem., 1983, 23, 1441. (Elsevier Biomedical Press, Amsterdam, 1963), p. 91. 10 C. H. Wynn, A. Marsden and B. Robson, J. Theor. Biol., 1986, 119, 81. 11 G. A. Jeffrey and R. Taylor, J. Comput. Chem., 1980, 1, 99. 12 I. Tvaroska, Carbohydr. Res., 1984, 125, 155. 13 R. U. Lemieux, K. Bock, L. T. J. Delbaere, S. Koto and V. S. Rao, Can. J. Chem., 1980, 58, 631. 14 I. Tvaroska and S. Perez, Carbohydr. Res., 1986, 149, 389. 15 E. Platt and B. Robson, J. Theor. Biol., 1982, 96, 381. 16 E. Platt, B. Robson and I. H. Hillier, J. Theor. Biol., 1981, 88, 333. 17 D. A. Rees and P. J. C. Smith, J. Chem. SOC., Perkin Trans. 2, 1975, 830. 18 I. H. Hillier and B. Robson, J. Theor. Biol., 1979, 76, 83. 19 G. A. Jeffrey, J. A. Pople, J. S. Binkley and S. Vishveshwara, J. Am. Chem. SOC., 1978, 100, 373. 20 G. A. Jeffrey, J. A. Pople and L. Radom, Carbohydr. Res., 1974, 38, 81. 21 I. Tvaroska and T. Bleha, Can. J. Chem., 1979, 57, 424. 22 B. Pullman and B. Maigret, in Conformation of Biological Molecules and Polymers, ed. E. D. Bergmann and B. Pullman (The Israel Academy of Sciences and Humanities, Jerusalem, 1973), p. 13. 23 A. L. McClellan, Tables of Experimental Dipole Moments (W. H. Freeman and Co., San Francisco, 1963). 24 T. Kozar and I. Tvaroska, Theor. Chim. Acta, 1979, 53, 9. 25 I. Tvaroska and T. Kozar, Carbohydr. Res., 1981, 90, 173.2536 Conformational Calculations on Carbohydrates 26 B. Robson and J. Garnier, Introduction to Proteins and Protein Engineering (Elsevier, Amsterdam, 27 I. Tvaroska and T. Kozar, J. Am. Chem. Soc., 1980, 102, 6929. 28 M. Davies, Some Electrical and Optical Aspects of Molecular Behaviour (Pergamon Press, Oxford, 29 N. L. Allinger and D. Y. Chung, J. Am. Chem. SOC., 1976, 98, 6798. 30 B. Robson and D. J. Osguthorpe, J. Mol. Biol., 1979 , 132, 19. 31 L. Onsager, J. Am. Chem. Soc., 1936, 58, 1486. 32 D. Cremer and J. A. Pople, J. Am. Chem. SOC., 1975, 97, 1354. 33 G. A. Jeffrey, R. K. McMullan and S. Takagi, Acta Crystallogr., Sect. B, 1977, 33, 728. 34 G. A. Jeffrey and S. Takagi, Acta Crystallogr., Sect. B, 1977, 33, 738. 35 S. J. Angyal, Aust. J. Chem., 1968, 21, 2737. 36 F. Franks, Pure Appl. Chem., 1987, 59, 1189. 37 A. Suggett, J. Solution Chem., 1976, 5, 33. 38 J. W. Brady, 8th Int. Symp. on Solute-Solute-Solvent Interactions, Regensburg, Federal Republic of 39 J. R. Grigera, J. Chem. Soc.. Faraday Trans. 1 , 1988, 84, 2603. 40 N. Metropolis, A. W. Rosenbluth, M. N. Rosenbluth, A. H. Teller and E. Teller, J.Chem. Phys., 1953, 41 C. A. Bush, Z-Y. Yan and B. N. Narasinga Rao, J. Am. Chem. Soc., 1986, 108, 6168. 42 3. P. Carver, 9th Int. Symp. on Glycoconjugates, Lille, July, 1987. 43 D. A. Rees and W. E. Scott, J. Chem. Soc. B, 1971, 469. 44 S. W. Homans, R. A. Dwek, J. Boyd, M. Mahmoudian, W. G. Richards and T. W. Rademacher, 45 C. H. Wynn, Biochem. J., 1986, 240, 921. 46 C. H. Wynn, in Gangliosides and Modulation of Neuronal Functions, NATO AS1 Series Vol. H7, ed. 1986). 1965). Germany, August, 1987. 21, 1087. Biochemistry, 1986, 25, 6342. H. Rahmann (Springer-Verlag, Berlin, 1987), p. 87. Paper 712080; Received 23rd November, 1987

ISSN:0300-9599    

DOI:10.1039/F19888402519

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Furaduy Trans. I, 1988, 84(8), 2537-2544 Correlation between the Hydrophobic Nature of Monosaccharides and Cholates, and their Hydrophobic Indices Koichiro Miyajima," Katsunosuke Machida, Toru Taga, Hiroaki Komatsu and Masayuki Nakagaki Faculty oj- Pharmaceutical Sciences, Kyoto University, Yoshida-shimoadachi-cho, Sakyo-ku, Kyoto-shi, 606 Japan Hydrophobic and hydrophilic surface areas, which are the surface areas occupied by CH and CH, groups and by OH, COO- and ether oxygen groups, respectively, are calculated for monosaccharides and cholates using the computer program developed by Hermann. Hydrophobic indices, defined by the surface-area -ratio of the hydrophobic to hydrophilic groups, correlate well with the partition coefficients of the polystyrene-water system for monosaccharides and with values of the critical micelle concentration for cholates.These facts indicate that the concept of hydrophobic index is important in considering hydrophobic interactions of flat molecules in aqueous solutions. IJp to now sugars have been regarded as typical non-electrolytes resembling urea. Generally, sugar molecules have similar numbers of CH and OH groups, and the hydration behaviour of monosaccharide has been discussed in terms of the orientation of the OH groups in the hexopyranose ring.' Recently much information has been revealed on the hydrophobic character of saccharides. Various saccharides can be partitioned in a polystyrene gel from the aqueous phase; the partition coefficients differ from each other, even though the configuration remains the same, i.e.D-glucose, D-galactose and D-mannose. On the other hand, condensed-ring compounds formed from D-glucose, such as c:yclodextrins, can include hydrophobic compounds in their cavitities, and the cavity may thus be regarded as hydrophobic. These experimental results indicate that the saccharide molecule has hydrophobic character, even if this is considerably weak. In order to confirm that the interactions mentioned above are hydrophobic, we investigated the correlation between the partition coefficients and the surface area ratio of hydrophobic to hydrophilic groups for various monosaccharide molecules, since the partitioning of the saccharide in a polystyrene gel phase is mainly based on adsorption of the saccharide molecule onto the phenyl groups of polystyrene through hydrophobic interactions, and the hydrophobic character of the saccharide molecule may originate from the hydrophobic surface formed by the CH and CH, groups of the saccharide molecule.On the other hand, cholates form micelles in aqueous solutions. The critical micelle concentration (c.m.c.) values depend on the number of OH groups and their orientation of OH groups. At low concentrations cholates are regarded as small micelles formed through hydrogen bonding3 or hydrophobic interactions.* In order to elucidate the nature of both monosaccharide-polystyrene gel and cholate4holate interactions, we calculated the hydrophobic and hydrophilic areas of seven monosaccharides and 16 cholates and determined their hydrophobic indices.The correlation between the surface area ratio of hydrophobic to hydrophilic groups, termed the hydrophobic index, the partition coefficients and the c.m.c. have been investigated for monosaccharides and cholate, respectively. 25372538 Hydrophobic Indices of Monosaccharides and Cholates /3 - plane Fig. 1. CPK model for a,a,a-trihydroxycholate: large open circles, carbon atoms; small open circles, hydrogen atoms ; dashed circles, hydroxy groups. Experimental Calculation of Surface Area The surface areas of monosaccharides and cholates were calculated using the computer program QCPE 225 MOLAREA developed by He~mann.~ The atomic coodinates used were mainly obtained from X-ray data. For furanose-type saccharides the furanose ring was assumed to be planar.Th$ van der Waals radii were used for atomic radii. The surface area at a distance of 1.4 A (the radius of a water molecule) above the spherical atop surface was calculated. The OH group was approximated as a sphere of radius 1.7 A, since the OH groups rotate around the C-0 axis. Definition of Hydrophobic and Hydrophilic Areas The total hydrophobic surface area was defined as the area occupied by the CH and CH, groups, and the hydrophilic surface area is the area occupied by the OH, -0- and C0,H groups. For the monosaccharides we assumed that the molecule is a disc which has two planes. The CH-rich plane, termed the specified hydrophobic surface area, is calculated under the following conditions. (1) For the Q-axial CH and CH, groups the specified hydrophobic surface area is equal to the calculated area.(2) For the a-axial and a-equatorial CH and CH, groups the specified hydrophobic surface area is zero. (3) For the a-equatorial CH and CH, groups the specified hydrophobic surface is half the calculated area. In calculating the surface area of ribofuranose we assumed that the five- membered ring of furanose is planar. Therefore, every CH group attached to the furanose ring is of the a- or a-type. In this case, the specified hydrophobic surface area of a-CH and CH, was assumed to be the calculated area, and that of a-CH or CH, group was taken as zero. In calculating the surface area of cholates, we determined the surface areas of the a- and a-planes separately. The two planes are shown in fig. 1. We assumed that the atoms in the a- and Q-planes contribute 100 O/O to the surface areas of their respective sides, and that atoms on the side of the molecule contribute 50% to the surface area of each side.Definition of the Hydrophobic Index As an expression of the hydrophobicity of a compound, the hydrophile-lipophile balance is frequently used in the field of surfactant physical chemistry. Its value is theTable 1. Hydrophobic indices of various monosaccharides compound position confor- of OHh anomer hydroph9bic hydropkilic hydrophobic mationa 12345 (Yo) area/A2 area/A2 index ~ a-D-glUCOSe P-D-glucose a-D-galactose P-D-galactose a-D-mannose P-D-mannose a-D-arabinose P-D-arabinose a-D-x ylose a-D-ribopyranose a-D-ribopyranose 8-D-ribopyranose 8-D-ribopyranose a-D-ribofuranose P-D-ribofuranose 2-deoxy-a-~-ri bopyranose 2-deoxy-@-~-ri bopyranose 2-deoxy-a-~-ri bofuranose 2-deoxy-@-~-ribofuranose P-D-x ylose C1 (G-G) C1 (G-G) C1 (G-T) Cl (G-T) C1 (G-G) Cl (G-G) 1c 1c Cl c1 1 c c1 1c c1 planar planar 1c 1c planar planar aeeee eeeee aeeae eeeae aaeee eaeee eeaa aeaa aeee eeee aeae eeae aaag PaaP a ea P aP .36 64 36 64 68 32 63 34 33 67 20 56 6 18 70 30 97.1 95.1 100.6 98.1 97.1 101.1 100.7 93.3 94.8 92.8 101.9 105.2 94.5 109.2 105.8 95.2 130.7 119.8 123.9 122.6 - 38.7 40.6 42.0 48.1 44. I 250.5 247.0 240.8 206.9 213.5 51.9 207.2 189.3 203.0 215.3 152.0 170.2 176.3 176.3 1 75.7 a G, gauche; T, trans. a, axial; e, equatorial.2540 Hydrophobic Indices of Monosaccharides and Cholates Table 2. Specific hydrophobic indices (SHI) compound specified hydro- phobic area specified hydrocarbon atoms" /A2 SHI a-D-glucose p-D-glucose a-D-galactose P-D-galactose a-D-mannose P-D-mannose a-D-arabinose p-D-arabinose a-D-xylose P-D-xylose a-D-ribopyranose (1 C) a-D-ribopyranose (C 1) p-D-ribopyranose (1 C) p-D-ribopyranose (C1) a-D-ribofuranose P-D-ri bofuranose 2-deoxy-a-~-ri bopyranose 2-deoxy-p-~-ribopyranose 2-deoxy-a-~-ribofuranose 2-deoxy-p-~-ribofuranose 46.3 57.3 1 56.6 67.5 39.4 45.7 69.4 "') 68.8 52.7 53.43 67.1 45.5 20.9 20.2 21.8 21.7 22.3 26.7 38.3 a Integers indicating the position of the carbon atom.mass ratio between the hydrophilic and lipophilic groups. In a similar manner, we define the hydrophobic index (HI) as the surface area ratio rather than mass ratio, because aldohexopyranoses have the same mass ratio of hydrophilic and hydrophobic groups. The hydrophobicities of a- and /?-anomers of monosaccharide, HI, and HI,, are defined by eqn (1) and (2) respectively : (1) hydrophobic surface area (a) hydrophilic surface area ( a ) HI, = x anorner YO (2) hydrophobic surface area (p) hydrophilic surface area (f?) HI, = x anomer YO.The hydrophobic index of a monosaccharide is the sum of the indices of each anomer : HI = HI,+HI,. (3) Since D-ribose and 2-~-deoxyribose have six and four isomers, respectively, the hydrophobic indices of these saccharides were calculated by an extention of the above treatment. The specific hydrophobic indices (SHI) of the a- and j?-anomers were calculated using similar equations to those for HI, and HI, using the specified area instead of the hydrophobic surface area.In the case of cholates, we calculated the hydrophobic indices of the a- and /3-planes separately, on the assumption that the atoms in the a-plane contribute lOOoh to the hydrophobic surface area of that plane and that the atoms on the side of the molecule contribute 50% to both planes.K. Miyajima et al. 254 1 Fig. 2. Relationship between the hydrophobic index and Ka,: (1) glucose, (2) galactose, (3) mannose, (4) arabinose, (5) xylose, (6) ribose and (7) deoxyribose. Results and Discussion The results of our calculations are shown in tables 1 and 2. First we investigate the correlation between the average hydrophobic surface area (= C hydrophobic surface area x anomer fraction) and the partition coefficients of the monosaccharides (Kav), obtained from polystyrene-gel-water chromatography by Janado., The magnitude of Kav increases in the order galactose > glucose > mannose > arabinose > xylose > ribose > deoxyribose.However, the magnitudes of the average hydrophobic surface areas calculated from the data of table 1 lie in the order xylose > arabinose > glucose :> mannose > galactose > ribose > deoxyribose, which is quite different from the order for Kav. On the other hand, the magnitudes of the average specified hydrophobic surface areas ( = C specified hydrophobic surface area x anomer fraction) lie in the order arabinose > xylose > mannose > galactose > glucose > ribose > deoxyribose. This order is also different from that of Kav.These results are reasonable, since the hydrophobicity of the molecule depends on the size of the hydrophobic and hydrophilic groups, as seen for values of the hydrophile-lipophile balance for surfactants. It is reasonable to assume as a first approximation that the strengths of the hydrophobicity and hydrophilicity of a saccharide molecule depend on the dimensions of the surface areas of the hydrophobic and hydrophilic groups, respectively, since the hydrophilic groups are mainly OH groups (except for the ether group) and hydrophobic groups are aliphatic CH and CH, groups. Based on this idea, we calculated the hydrophobic index as shown in table 1 . From our definition, a larger index indicates a stronger hydrophobicity. Therefore, the hydrophobicities lie in the order glucose > galactose > mannose > xylose > arabinose > ribose > deoxyribose.A reversal of the order is observed for glucose-galactose and arabinose-xylose. As can be seen in fig. 1, an almost linear relation is observed between HI and Kav. Based on the idea that the CH-rich surface interacts with a hydrophobic solute, we calculated the specific hydrophobic index using the specified surface area instead of the hydrophobic surface area. Therefore, the specific hydrophobic index refers to the preferential interaction of the CH-rich surface with a hydrophobic solute. As seen in table 2, it lies in the order galactose > glucose > arabinose > mannose > xylose > ribose > deoxyribose. Compared with the order for Kav, the position of mannose is replaced by arabinose.As shown in fig. 3, an almost linear relation is obtained between the SHI and Kav. However, we cannot judge from the above results as to which index is more closely correlated with the thermodynamic data. An important problem withTable 3. Hydrophobic indices of various cholates position, 3 7 12 - hydrophobic area/A2 a-plane /?-plane total h ydrop hilic area/ A2 hydrophobic index a-plane P-plane total a-plane P-plane total trivial name 109 112 117 121 135 127 149 139 89 105 102 115 118 130 127 143 325 324 312 315 304 3 14 288 303 320 303 303 30 1 287 286 283 270 434 436 429 436 439 441 437 442 409 408 405 416 406 416 41 1 41 3 135 137 124 120 81 64 68 64 165 150 153 109 137 95 97 82 di hydroxy 50 185 50 187 69 193 64 184 95 176 109 173 113 181 109 173 trihydroxy 50 215 64 214 68 22 1 95 204 82 219 108 203 113 210 126 208 0.80 0.8 1 0.96 1.01 1.66 2.00 2.10 1.74 0.54 0.69 0.67 1.05 0.86 1.37 1.30 1.75 6.47 6.44 4.52 4.92 3.22 2.90 2.55 2.77 6.37 4.76 4.44 3.18 3.50 2.64 2.5 1 2.14 7.27 7.27 5.48 5.93 4.88 4.90 4.60 4.5 1 6.91 5.45 5.1 1 4.23 4.36 4.0 1 3.8 1 3.89 chenodeox ycholic deoxycholic ursodeoxycholic isochenodeoxycholic isodeoxycholic isoursodeox ycholic cholic ursocholic isocholic isoursocholicK .Miyajima et al. 2543 I 1 I r J SHI 20 30 40 Fig. 3. Relationship between the specific hydrophobic index and Kav. (Symbols are as fig. 2.) c.rn.c./ l 0-3 rnol drn-3 Fig. 4. Relationship between the hydrophobic index and the c.m.c. for dihydroxycholates : (a) a-plane, (b) /3-plane and (c) total.regard to this treatment is the evaluation of the hydrophilicity of a monosaccharide molecule, i.e. the validity of the relationship between hydrophilicity and hydrophilic surface area. The hydrophilic character of a saccharide molecule arises from the hydrogen bonding of its OH groups. Water molecules form a tridymite-like structure in the liquid state. Based on the special hydration model proposed by Kabayama and Patterson,' an equatorial OH group of a pyranose ring is more favourable than an axial group for hydrogen bonding with a water molecule in aqueous solution. We cannot comment as to whether a difference in the orientation of the OH group affects the surface area of the hydrophilic group of a monosaccharide at the present stage.We did not take into account the presence of straight-chain monosaccharides through the discussion, because their concentration is low in aqueous solutions at the equilibrium state. In conclusion, the fraction of hydrophobic surface area is of primary importance in the interaction of monosaccharides with hydrophobic solutes in water. The surface areas and hydrophobic indices for various cholates are shown in table 3.6 54 2 2544 Hydrophobic Indices of Monosaccharides and Cholates n 0 0 1 1 I I I I I I I I I I 1 40 80 120 160 200 ~.rn.c./lO-~ mol dm-3 Fig. 5. Relationship between the hydrophobic index and the c.m.c. for trihydroxycholates : (a) a-plane, (6) P-plane and (c) total. The absolute values of the hydrophobic surface area are not correlated with the c.m.c. values.' The relationship between the hydrophobic index and the c.m.c. for di- and tri- hydroxycholates is shown in fig. 4 and 5, respectively. The total hydrophobic index (summed for the a- and P-planes) is correlated with the c.m.c. in both figures, except in the case of P,P-dihydroxycholate. The larger the hydrophobic index, the smaller is the c.m.c. The hydrophobic indices of the P-plane dominate over those of the a-plane. These facts suggest that the small micelles formed at low concentrations are based on a hydrophobic interaction between the p-planes in cholate molecules, which is consistent with the model of Carrey and Small. In conclusion, the concept of hydrophoic index is important for the consideration of hydrophobic interactions of flat molecules in aqueous solution. References 1 M. A. Kabayama and D. Patterson, Can. J . Chem., 1958, 36, 563. 2 M. Janado, Aqueous Size-exclusion Chromatography, ed. P. L. Dubin (Elsevier, Amsterdam, 1985), ~~ chap. 2. 3 D. G. Oakenful and L. R. Fischer, J . Phys. Chem., 1977, 81, 1838. 4 D. M. Small, Adv. Chem. Ser., 1968, 84, 31. 5 R. B. Hermann, J. Phys. Chem., 1972, 76, 2754. 6 A. Roda, A. F. Hofmann and K. J. Mysels, J . Biol. Chew., 1983, 258, Paper 712075; 6362. Receiced 23rd November, 1987

ISSN:0300-9599    

DOI:10.1039/F19888402537

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1988, 84(8), 2545-2552 Homotactic and Heterotactic Interactions in Aqueous Solutions containing some Saccharides Experimental Results and an Empirical Relationship between Saccharide Solvation and Solute-Solute Interactions Simon H. Gaffney, Edwin Haslam,* Terence H. Lilley" and Tracy R. Ward Chemistry Department, The University, Shefield S3 7HF The enthalpies of dilution of binary and ternary aqueous solutions at 25 "C containing the isomeric disaccharides cellobiose, maltose and trehalose have been investigated. The considerable variation in the enthalpic second virial coefficients appears to be related to the tendency the molecules have to hydrogen bond to the solvent, and using the results obtained, in association with literature information for other saccharide-containing systems, it is shown that a reasonable correlation can be obtained between the virial coefficients and hydrogen- bond numbers deduced from model-building studies.The properties of saccharides and polysaccharides in water have important consequences both biologically and industrially, but in spite of the large number of which have been directed towards elucidating many properties there are still considerable uncertainties regarding the role the solvent plays in influencing the conformations and reactivities of saccharidic molecules in this solvent. In this paper the results of a calorimetric study on the enthalpies of dilution of binary and ternary solutions containing the isomeric disaccharides maltose, cellobiose and trehalose (fig.1) will be presented. At the outset of the work our aim was to see if there were any obvious links between the information obtained and the structures of the interacting molecules but, as will become apparent, the links which can be made are not with the molecular structures themselves but rather with an expression of molecular solvation. Experimental The calorimeter used and its associated equipment have been described ear lie^,^ as has the general methodology of the experimental procedure. The results were obtained with the calorimeter operating at 298.15 & 0.05 K. The maltose, D-( +)-maltose, (4-0-a-~- glucopyranosyl-D-glucopyranose) was obtained from Sigma as the grade 1 monohydrate. The cello biose, D-( + )-cello biose, (4-O-/?-~-glucopyranosy1-~-glucopyranose) was an- hydrous material of puriss grade (Fluka).The trehalose (a-D-glucopyranosyl-a+- glucopyranoside) was obtained as puriss grade dihydrate from Fluka. All of the saccharides were used without further purification. Results The excess enthalpy (Hex) of a solution containing 1 kg of a solvent may be represented as a polynomial in the molalities of the solutes present. As has been shown elsewhere4 for binary and ternary systems containing only non-electrolytes the excess enthalpy may be written as (1) He" = m2(h2 + h, m + . . .) 25452546 Homotactic and Heterotactic Interactions "*A OH 0 HO OH OH maltose H O & O H V o H HO OH OH cellobiose Ho*o+/JoH HO t rehalose \OH Fig. 1. The structures of the disaccharides investigated. in which m (the osmolality) is the sum of the molalities (mA + mB) of the solutes A and B and the coefficients in this equation are of the form where yA and ys are the molality fractions of the solutes A and B, respectively (e.g.yA = m,/m), and the hijk terms are virial coefficients which represent, in a formal way, interactions between the subscripted species. In the present experiments measurements were made on systems containing only one solute or two solutes, both being equimolal. Consequently, for the single-solute and two-solute systems we have from eqn (1) and (2) The enthalpy change ( A H ) obtained on diluting a solution of initial osmolality m and (4) and table 1 lists the experimental results which were obtained. These results were fitted to eqn (4) using a least-squares analysis routine and the resulting information for the virial coefficients of the five systems investigated is summarised in table 2.It is apparent from eqn (3b) that the evaluation of the heterotactic virial coefficients necessitates having ihformation on the component homotactic coefficients. For the present systems all of the required information is available either from the present investigation or from an earlier s t ~ d y , ~ and in table 3 we have collected together the enthalpic second virial coefficients, both homotactic and heterotactic. At the present time there is not enough information available to deconvolute the h, terms for the ternary systems into their components. It should be mentioned that there are containing n moles of solute(s) to a final osmolality m' is AH = n[h,(m' - m) + h,(rnf2 - m2> + .. .]S. H . Gafney et al. 2547 Table 1. Experimental enthalpy of dilution results for aqueous disaccharide-containing solutions at 25 "C m/mol n m/mol kg-l mol kg-' AH /10-3 J m/mol n m/mol AH kg-' / mol kg-I / 1 0v3 J 0.1977 0.1977 0.4 122 0.4122 0.4122 0.6045 0.6045 0.6045 0.8063 0.8063 0.8063 0.9694 0.9694 0.9694 0.1993 0.1993 0.3893 0.3893 0.3893 0.79 I9 0.7919 0.7919 0.9555 0.9555 0.9555 0.9601 0.2002 0.2002 0.2002 0.3989 0.3989 0.5999 0.5999 (3.5999 0.7979 0.7979 0.7979 I .oooo I .oooo maltose 0.38 13 0.0969 0.3686 0.0625 0.7813 0.1971 1.5002 0.2625 0.7632 0.1290 1.0235 0.2724 2.1239 0.3815 1.1303 0.1917 2,7877 0.5 127 1.3596 0.3637 1.41 58 0.2438 1.6338 0.4338 3.2687 0.6101 1.6354 0.2847 trehalose 0.41 19 0.0993 0.3909 0.0652 0.7243 0.1883 1.4643 0.2506 0.6801 0.1 175 1.3649 0.3669 2.7783 0.5 109 1.3364 0.2349 1.6584 0.4385 3.2744 0.5894 1 S965 0.2798 3.2041 0.6022 maltosesellobiose" 0.3884 0.0999 0.7761 0.1317 0.391 8 0.0652 1.49 16 0.2585 0.7429 0.1267 1.1003 0.2842 2.1565 0.3838 1.1514 0.1972 1.4019 0.3658 2.7859 0.5045 1.3766 0.241 1 3.3464 0.6432 1.6563 0.2942 20.9 28.6 88.6 116.5 116.5 171.8 242.9 243.9 416.4 301.6 418.6 422.9 562.6 554.8 29.9 39.2 106.5 149.5 137.9 406.4 544.8 534.7 583.I 826.5 772.9 792.0 25.5 36.9 36.8 130.7 136.7 205.7 285.6 294.4 352.4 472.4 474.0 679.7 702.5 0.1993 0.1993 0.3985 0.3985 0.5996 0.5996 0.5996 0.7997 0.7997 0.7997 0.9999 0.9999 0.1992 0.1992 0.3989 0.3989 0.3989 0.5979 0.5979 0.7944 0.7944 1 .oooo 1 .oooo ma1 tose- 0.3885 0.7884 1.4894 0.7367 1.0519 2.1675 1.0604 1.4357 2.7594 1.4076 1.6893 1.6888 -trehalose" 0.0972 0.1307 0.2591 0.1256 0.2759 0.3902 0.1839 0.3783 0.5034 0.2434 0.4357 0.2955 trehalosesellobiose" 0.381 1 0.0960 0.7880 0.1304 0.7412 0.1893 1.4829 0.2558 0.7499 0.1264 2.1596 0.3809 1.0847 0.1853 2.7589 0.5018 1.3809 0.2416 3.3704 0.6207 1.7049 0.2961 25.3 32.7 112.6 114.9 177.9 229.8 244.3 305.3 432.6 425.8 440.8 633.9 27.7 36.9 103.7 143.6 143.9 31 1.0 304.4 517.3 508.3 806.9 785.8 "In the binary solute mixtures the two solutes were equimolal to within 0.1 YO.FAR I2548 Homotactic and Heterotactic Interactions Table 2. The coefficients of eqn (1) for the systems studied solute(s) h,/J kg mo1FZa h,/J kg2 m01-~~ maltose 571.1 (27.4) - 57.0 (2 1 .O) ma1 toseecello biose 682.4 (35.I ) - 69.4 (26.5) maltose-tre halose 615.3 (82.7) -77.1 (66.8) cellobiose-trehalose 722.5 (18.3) -56.1 (13.6) trehalose 794.9 (46.5) " The parenthetical terms are the 95 % confidence limits of the coefficients. Table 3. Homotactic and heterotactic enthalpic second virial coefficients for the three disaccharides studied maltose cellobiose trehalose maltose 57 1 488" 439O cellobiose 734 689' 749" trehalose 547 703 795 595" dTaken from ref. (6). bTaken from ref. (7). 'Taken from ref. (5). I I I 8 0 0 0.1 0.2 0.3 0.4 (rn -rn')/mol kg-' Fig. 2. The enthalpies of dilution of cellobiose solutions plotted using the form of eqn ( 3 ) ; 0 , ref. ( 5 ) ; 0 ref. (6).S. H. Gaflney et al. 2549 experimentally significant discrepancies between the results obtained for the homotactic virial coefficients obtained from the present investigation and those from two other recent studies6*7 (see table 3).The reason or reasons for these differences are not known at the present time, but in part almost certainly arise from different methods of data analysis. This is illustrated in fig. 2, where the available results on the enthalpy of dilution of cellobiose are displayed, and it is clear from this that there is general concordance of the primary experimental data. It is, however, apparent that there are quite marked discrepancies between these and other6V7 data for maltose and for cellobiose. Discussion The enthalpic second virial coefficients for the disaccharides studied here are, like almost all of those given in the literature1 for saccharides, positive, and this and the fact that the corresponding free energetic virial coefficients for mono- and oligo-saccharides are also positive indicates that generally bimolecular interactions between solvated saccharides are in a net sense repulsive in aqueous systems.There are several possible contributions at the molecular level which could lead to these positive coefficients and one of these will be considered in some detail below, but it should be stressed that the net interactions which are reported by the virial coefficients are small compared to thermal energies with the consequence that no one contribution can be completely dominant. The interactive behaviour of structurally simple non-electrolytes in water can often be represented tolerably well using the group-additivity approach introduced by Savage and Wood,* and there is now a very extensive literatureg where their idea is addressed to a wide range of molecular interactions. In this approach it is assumed that because no single component to the total interaction is dominant every functional group on a solute molecule A can interact with every functional group on a solute molecule B.The consequence of this assumption is that the second virial coefficient can be written as a quadratic function of the numbers of groups on the solute species. The expression which results for enthalpic coefficients has the form h,, = C n4 nj" Hii (5) where nf refers to the number of defined groups of type i present on the molecule A, n: is the corresponding term for j groups on solute B and Hii is an intensive term representing i-j interactions.Notwithstanding the relative success of the group-additivity idea when applied to many systems it is quite clear that as it stands it is strictly inapplicable to aqueous systems containing the saccharides studied here since, given that they are isomers according to the group-additivity approach, they should all exhibit the same virial coefficients. Our view is that, given that this approach is usually only semiquantitative in its predictive ability, even for saccharides it is nothing like as bad as is sometimes implied as a method of estimating solute-solute interactive energetics. However, by its very nature and conceptual basis the additivity approach is unable to cope with, for example, stereoisomeric effects and relatively subtle changes in molecular embroidery.Thus, rather than pursuing the group-additivity approach for the present systems, we thought it might be useful, given that the available free energetic information' suggests that solvation effects might well contribute significantly to the observed measures of solute-solute interaction, to explore the consequences of a suggestion made some years ag0.l' Kabayama and Patterson" suggested that the hydration of monosaccharides is dependent not only on the number of hydroxy groups in the molecules but also on their positions and orientations. They proposed that equatorial hydroxy groups have a greater facility for hydrogen bonding to an expanded tridymite water lattice.Later incorporated and extended this idea in a specific hydration model and the suggestion was made that the compatibility of a monosaccharide with a structured water 84-22550 Homotactic and Heterotactic Interactions Table 4. Estimated hydrogen-bond numbers (see text) and enthalpic second virial coefficients of some saccharides hydrogen- bond numbers h A, -~ acceptor donor /J kg rnoP raffinose trehalose maltose cello biose lactose sucrose P-D-glucop yranose /3-D-galactopyranose a-D-mannopyranose a-D-1 yxopyranose P-D-xylopyranose a-D-arabinopyranose Q-D-ribopyranose /3-D-sorbopyranose 4 3 4 4 4 4 2 1 2 1 2 4 2 2 11 11 8 9 7 8 6 4 6 4 6 2 5 5 81 1 795 57 1 689 506 577 343 133 207 243 339 177 202 395 ~~ The virial coefficients were taken from ref.(l), (2) and the present work. lattice has a major influence on the extent of hydration. We have extended this concept and for a range of saccharides of known structures have, through model-building, incorporated them into an ice lattice in such a way that hydrogen bonding to the ‘ solvent ’ was optimised for the anomer which is most favoured, while at the same time maintaining a sterically and electronically acceptable conformation. Once the models were complete the numbers of hydrogen bonds (both donor and acceptor) formed between the saccharides and the lattice were counted. The results obtained for these ‘ hydrogen-bond numbers’ are summarised in table 4. It should be stressed that there are some conflicts between the static model we have adopted and how one usually envisages the way in which molecules behave in solution where solute and solvent components are in constant motion, exploring different conformations and configurations and with hydrogen bonds constantly breaking and reforming.Consequently implicit in our method of analysis is the approximation that the particular arrangements of solvent and solute derived from the model-building procedure correspond to those which are the lowest in energy, and consequently are the most favoured. For monosaccharides the idea that one arrangement is dominant does have some upp port,'^ but for oligosaccharides (because of the possibility of rotation and flexibility about the glycosidic linkage) it seems probable that there might well be more than one conformation possible.The enthalpic virial coefficients we have obtained for the disaccharide-containing systems appear to vary in such a way that they become more positive as the number of hydrogen bonds formed with the lattice increases. Qualitatively this suggests that greater solvation leads to greater saccharide-saccharide repulsion. This does make some intuitive sense since it suggests that the better the solutes interact with the solvent the less likely are they to interact with each other. (The compromise reached between solute solvation and solute-solute interaction has been discussed from a completely different viewpoint re~ent1y.l~’ 16) Given the general trend in our disaccharide results, we decided to extend our model-building to other unsubstituted saccharides for which enthalpic virial coefficients were available.The results obtained for the ‘ hydrogen-bond numbers ’S. H . Gaflney et al. 255 1 0 100 200 n i n! Fig. 3. Plot of the enthalpic second virial coefficient (homotactic or heterotactic: units J kg rnolp2) against the hydration number product (ni nk) for saccharides. are included in table 4, and it is clear as a whole that there is a marked trend towards more positive virial coefficients with increased numbers of hydrogen bonds. There is a reasonable correlation obtained if it is assumed that the molecular interactions, which are reflected in the virial coefficients for the saccharides in water, depend upon the total number of hydrogen bonds the various solutes make with the ice-like lattice, and that the interactions obey a quadratic relationship similar to that of the group-additivity approach' viz.(6) h,, = n: nk Hhh . In this expression, which corresponds to what might be termed ' solvational group additivity', n: and nk are the hydrogen-bond numbers of solutes A and B, respectively, and Hhh is a term representing the enthalpy change which occurs when two of these hydrogen bonds are brought together. Fig. 3 shows the data from table 4 plotted according to eqn (6) and it is apparent from this that a tolerably good correlation exists. The value obtained for the Hhh coefficient is 4.0 1.6 J kg mo1-2, where the parenthetical term is the 95% confidence limit. The positive value of the coefficient represents what is in effect ' thermochemical repulsion' between the solvated species when present in water.It is worth pointing out that the relationship given in eqn (6) rationalises the empirical rule discovered some years ago by Franks and CO-workers" when it found that the heterotactic enthalpic coefficients of some hydroxy-containing systems could be described in terms of the component homotactic coefficients. If these latter can be represented by [see eqn (6)] h,, = ni2Hhh and h,, = nk2Hhh2552 Homotactic and Heterotactic Interactions then the heterotactic coefficient can be written in terms of these as which is the rule which was found ea~1ier.l~ It should be stressed that we are not suggesting that the empirical relationship we have found for a rather limited group of sugars has any universal validity, and indeed it need not apply to any other measure of solute-solute interaction than that addressed here.However, the relationship found for these sugars is certainly a better way of representing the enthalpic virial coefficients than is the Savage-Wood group-additivity approach8 since, for example, the latter predicts that because glucose and galactose are isomeric, they should have the same virial coefficients, whereas the suggested relationship recognises that although these two molecular species are structurally isomeric, they are not solvationally equivalent. Our view is that the present relationship which stresses (almost certainly overmuch) hydration effects on molecular interactions, is nothing more than a small step towards finding the rules which determine such interactions in condensed media.We acknowledge financial support from the A.F.R.C. References 1 A. Cesaro. in Thermodynamic Data for Biochemistry and Biotechnology, ed. H-J. Hinz (Springer-Verlag, 2 F. Franks, Pure Appl. Chem., 1987, 59, 1189. 3 A. Suggett, in Water: A Comprehensive Treatise, ed. F. Franks (Plenum, New York, 1975), vol. 1, 4 G. M. Blackburn, T. H. Lilley and E. Walmsley, J . Chem. Soc., Faraday Trans. I , 1982, 78, 1641. 5 S. H. Gaffney, E. Hasiam and T. H. Lilley, Thermochim. Acta, 1985, 86, 175. 6 M. Abbate, G. Barone, P. Cacace, G. Castronuovo, P. Del Vecchio and V. Elia, J. Chem. Soc., 7 M. Mathlouthi and A. M. Seuvre, J. Chem. Soc., Faraday Trans. I , 1988, 84, 2641. 8 J. J. Savage and R. H. Wood, J. Solution Chem., 1976, 5, 733. 9 See for example T. H. Lilley, in Biochemical Thermodynamics, ed. M. N. Jones (Elsevier, Amsterdam, Heidelberg, 1986), p. 177. p. 519. Faraday Trans. I , 1988, 84, in press. 2nd edn, 1988), chap. 1. 10 M. A. Kabayama and D. Patterson, Can. J . Chem., 1958, 36, 563. 11 M. J. Tait, A. Suggett, F. Franks, S. Ablett and P. A. Quckenden, J . Solution Chem., 1972, 1, 131. 12 A. Suggett and A. H. Clark, J . Solution Chem., 1976, 5, 1. 13 F. Franks, D. S. Reid and A. Suggett, J. Solution Chem., 1973, 2, 99. 14 D. A. Rees, Polysaccharide Shapes (Chapman and Hall, London, 1977). 15 T. H. Lilley, in Biochemical Thermodynamics, ed. M. N. Jones (Elsevier, Amsterdam, 2nd edn, 1988), 16 P. J. Cheek and T. H. Lilley, J. Chem. Soc., Faraday Trans. I , 1988, 84, 1927. 17 S. Ablett, M. D. Barrett, F. Franks, M. D. Pedley and D. S. Reid, in L'eau et les Systemes Biologiques, chap. 1. ed. A. Alfsen and A. J. Bertaud (C.N.R.S., Paris, 1976), p. 105. Paper 7/2 133 ; Received 3rd December, 1987

ISSN:0300-9599    

DOI:10.1039/F19888402545

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1988, 84(8), 2553-2566 Excess Enthalpy and Excess Volume in Ternary Aqueous Solutions with Sucrose-Glucose, Sucrose-Glycerol and Glucose-Glycerol at 298.1 K Norbert Daldrup and Hansjiirgen Schonert" Institut fur Physikalische Chemie, R WTH Aachen, 0-5100 Aachen, Federal Republic of Germany The excess enthalpy and excess volume in the binary and ternary aqueous solutions of sucrose, glucose and glycerol have been measured in the concentration range from dilute solutions to ca. m = 2 mol kg-l. The data were fitted to a Taylor series in the molalities and were successfully compared to a model theory in which the deviations from ideal behaviour are attributed to hydration equilibria. - Scattering experiments, spectroscopic data and simulation experiments reveal more and more details of the structure of liquids, yet a rigorous treatment of moderately concentrated aqueous solutions remains a goal to be achieved.Therefore, model theories which single out major effects are still of value. One of these effects is solvation which can be described in terms of solvation equilibria. Solvation equilibria are probably important in aqueous solutions of sugars, as pointed out by Robinson and Stokes,' who have shown that the Gibbs free energy in ternary aqueous solutions with sucrose, glucose, manni to1 and glycerol can be approximately predicted from the respective quantities of the constituting binary solutions if one applies the concept of a semi-ideal mixture of solvated species. This model theory has been extended to the enthalpy and volume of binary and ternary non-electrolyte solutions2. and will be compared with measurements in the title systems.The excess enthalpy per unit mass described phenomenologically by m3 of the two solutes:* He" = where, hex is the excess enthalpy Theory of solvent in a ternary non-electrolyte solution can be a series expansion in terms of the molalities m2 and := RT C h,mimi n, Ml i, j = O i + j 2 2 and Hex its value per unit mass of solvent; n, and M, are the amount of solvent and its molar mass, respectively. The coefficients h, are functions of temperature and pressure and can be determined calorimetrically by dilution experiments. To a solution with initial molalities m2(i) and rn3(i) and amount of solvent n, an increment An, of solvent was added. The resulting enthalpy change AH is A H = (n, + An,) M , Hex(i) - n, M , Hex(f) (2) where (f) denotes the final state.The combination of these two equation yields 25532554 Ternary Aqueous Solutions This equation allows the fitting of the experimental data with a For the excess volume vex there exists an equation analogous v""=-= Vex RT vijmimi. n1 Ml i. i-0 i+j 2 o set of coefficients hij. to eqn (1): v"" can be determined from measurements of the density p of the density p1 of the solvent by (1+m,M,+m3M3) 1 m2 Vi-m3 V;l v"" = --- P P1 where, Mi are the molal masses and Vp are the standard values of volumes of the solutes. With V i = RTv,, and V i = RTv,, we find (4) solution and the ( 5 ) the partial molal i + j > 1 Turning to the model theory, Robinson and Stokes' have shown that the solvation depends only on the activity of the solvent a,, given the molalities of the solutes.The same is true of the enthalpy changes and volume changes accompanying these eq~ilibria.~9~ Therefore, the molalities m, and m3 are linked to their values m; and m; in the constituent binary solutions of equal activity of the solvent: m %+L = 1; mi m; a, = constant. (7) This relationship is independent of the model parameters; i.e. the number of binding sites and the equilibrium constants for the solvation equilibria. Eqn (7) can be extended to the excess enthalpy and the excess volume: and (9) These relations will be used for a test of the model theory. Experimental The calorimeter was of the Tian-Calvet type (Setaram, Lyon).The measuring cell was filled with the initial solution and the amount of water Anl was delivered via a burette. The signals from the thermopile are digitized and stored for further treatment. Details are described in ref. ( 5 ) . The temperature is T = 298.16f0.02 K and the error of the measurements is ca. k 1 YO or The density of the solution was determined in a vibrating-tube densimeter (DMA 02/C, A. Parr, Graz) to ca. k (4 x lo-') g cmP3 at T = 298.16+0.008 K. The standard deviation for v"", which is due to errors in the density and the molalities, amounts to ca. k0.08 cm3 kg-l. Sucrose and glucose were of the quality ' Reinst' (Merck, Darmstadt). They were dried several days at 1 mbar. Glycerol (Merck, Darmstadt, quality for fluorescence studies) was used as delivered.0.02 J, whichever is greater.N . Daldrup and H. Schonert 2555 Table 1. Enthalpy of dilution ; glucose (2)-glycerol (3) m,(O %(i) n1 An, AH(expt1) AH(ca1d) - 1.237 1.113 1.01 1 1.958 1.613 1.372 1.055 0.946 1.134 0.854 1.404 1.223 1.084 0.973 0.882 2.273 1.865 1.581 1.372 1.212 1.085 0.982 0.897 0.802 0.588 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2.077 1.659 1.414 1.232 1.091 0.979 0.888 0.000 0 .ooo 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2.020 1.455 1.276 1.137 1.025 0.933 0.856 1.290 1.090 0.943 0.831 0.743 1.699 1.427 1.23 1 1.082 0.965 0.871 0.793 0.702 0.207 0.171 0.146 0.127 0.1 12 0.101 0.091 2.476 2.753 3.030 1.294 1.571 1.847 2.401 2.677 0.845 1.121 1.871 2.148 2.425 2.701 2.977 1.263 1.539 1.816 2.093 2.370 2.646 2.923 3.200 0.761 1.038 1.425 1.978 2.255 2.532 2.808 3.085 3.362 1 SO6 1.783 2.059 2.336 2.613 1.454 1.731 2.007 2.284 2.561 2.838 3.1 14 1.571 1.319 1.596 1.872 2.149 2.426 2.702 2.979 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 -2.35 - 1.92 - 1.71 - 5.33 - 3.80 - 2.89 - 1.72 - 1.37 - 1.77 - 1.01 - 2.79 -2.19 - 1.70 - 1.42 -1.11 - 6.37 -4.73 -3.51 -2.71 -2.18 - 1.76 - 1.59 - 1.25 - 0.84 - 0.47 - 5.73 - 3.35 - 2.63 -2.11 - 1.75 - 1.45 - 1.23 -2.58 - 1.95 - 1.47 - 1.17 - 0.94 -4.15 -3.15 -2.37 - 1.96 - 1.57 - 1.27 - 1.08 - 0.90 -6.18 - 4.66 - 3.56 - 2.76 - 2.20 - 1.80 - 1.47 - 2.32 - 1.92 - 1.61 - 5.01 - 3.62 - 2.73 - 1.72 - 1.41 - 1.66 - 1.03 - 2.86 - 2.24 - 1.81 - 1.48 - 1.24 - 6.52 -4.71 -3.55 - 2.77 - 2.23 - 1.82 - 1.52 - 1.29 -0.83 - 0.49 - 5.68 - 3.28 -2.61 -2.12 - 1.76 - 1.49 - 1.27 -2.53 - 1.89 - 1.46 - 1.16 - 0.95 -4.18 -3.11 -2.41 - 1.91 - 1.56 - 1.29 - 1.09 - 0.80 - 6.28 -4.59 - 3.49 - 2.74 -2.21 - 1.82 - 1.522556 Ternary Aqueous Solutions Table 1.(Cont.) 1.546 1.305 1.129 0.995 0.889 0.804 0.733 1.612 1.321 0.971 0.857 0.767 0.695 0.634 1.408 1.157 0.982 0.853 0.754 0.675 0.612 1.202 0.991 0.842 0.733 0.648 0.58 1 0.527 1.018 0.838 0.712 0.619 0.547 0.490 0.444 0.805 0.664 0.565 0.49 1 0.435 0.390 0.353 0.602 0.499 0.426 0.372 0.330 0.296 0.269 0.402 0.333 0.284 0.248 0.220 0.339 0.286 0.247 0.2 18 0.195 0.176 0.161 0.605 0.496 0.364 0.322 0.288 0.261 0.238 0.796 0.654 0.555 0.482 0.426 0.382 0.346 0.998 0.822 0.699 0.608 0.538 0.483 0.437 1.214 0.999 0.849 0.738 0.653 0.585 0.530 1.404 1.158 0.985 0.857 0.759 0.680 0.617 1.604 1.330 1.136 0.99 1 0.879 0.790 0.7 17 1.808 1.498 1.279 1.116 0.990 1.499 1.776 2.052 2.329 2.606 2.883 3.159 1.258 1.535 2.088 2.365 2.641 2.918 3.195 1.276 1.553 1.830 2.107 2.383 2.660 2.937 1.297 1.574 1.851 2.127 2.404 2.68 I 2.958 1.287 1.563 1.840 2.1 17 2.393 2.670 2.947 1.301 1.578 1.855 2.131 2.408 2.685 2.961 1.345 1.621 1.898 2.175 2.45 1 2.728 3.005 1.340 1.616 1.893 2.170 2.446 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 AH(expt1) AH(ca1d) -4.65 -3.41 - 2.65 -2.13 - 1.74 - 1.44 - 1.24 - 6.32 -4.75 - 2.77 - 2.23 - 1.76 - 1.51 - 1.13 - 6.41 - 4.65 - 3.5 1 - 2.75 - 2.26 - 1.82 - 1.47 - 6.36 -4.88 - 3.55 - 2.79 - 2.25 - 1.83 - 1.52 - 6.65 -4.85 - 3.64 - 2.84 - 2.29 - 1.84 - 1.54 - 6.53 -4.91 - 3.60 - 2.78 - 2.26 - 1.84 - 1.51 - 6.62 - 4.94 - 3.75 - 2.93 - 2.29 - 1.91 - 1.62 - 6.74 -4.84 - 3.73 - 2.93 - 2.39 -4.83 - 3.62 - 2.8 1 - 2.25 - 1.84 - 1.53 - 1.29 - 6.35 -4.57 - 2.68 -2.15 - 1.76 - 1.47 - 1.24 - 6.37 - 4.60 - 3.47 -2.71 -2.17 - 1.78 - 1.49 - 6.43 -4.66 - 3.53 - 2.76 - 2.22 - 1.82 - 1.52 - 6.67 -4.82 - 3.64 - 2.85 -2.28 - 1.87 - 1.56 -6.61 - 4.79 - 3.63 - 2.84 - 2.28 - 1.87 - 1.57 - 6.66 -4.87 - 3.72 - 2.92 - 2.36 - 1.95 - 1.63 - 6.69 - 4.90 - 3.74 -2.94 -2.38N .Daldrup and H. Schonert Table 1. (Cont.) 2557 m,(i) m3(9 n1 An, AnH(expt1) AH(ca1d) 0.198 0.179 0.202 0.168 0.144 0.125 0.111 0.100 0.091 0.889 0.807 2.014 1.674 1.432 1.25 1 1.111 0.999 0.908 2.723 3.000 1.362 1.639 1.915 2.192 2.469 2.745 3.022 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 - 1.94 - 1.60 - 6.66 - 4.97 - 3.76 - 2.96 -2.38 - 1.95 - 1.63 - 1.96 - 1.64 - 6.72 - 4.95 - 3.80 - 3.00 - 2.43 -2.01 - 1.69 The units are: (mi} = mol kg-l, {nl, An,} = mol, (AH) = J.Table 2. Enthalpy of dilution ; glucose (2)-sucrose (3) 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.193 0.156 0.130 0.112 0.098 0.087 0.079 0.4 10 0.330 0.277 0.208 0.190 0.174 0.161 0.604 0.489 0.410 0.31 1 0.283 0.260 0.240 0.790 0.642 0.541 0.467 0.41 1 0.367 0.316 0.512 0.733 1.491 1.491 1.924 1.924 2.443 2.443 1.933 1.558 1.304 1.122 0.984 0.877 0.790 1.813 1.460 1.222 0.922 0.839 0.770 0.712 1.603 1.297 1.089 0.825 0.752 0.691 0.639 1.406 1.142 0.962 0.831 0.73 1 0.653 1.726 1.556 1.543 1.422 1.405 1.097 1.280 1.118 1.178 1.148 1.424 1.701 1.978 2.255 2.531 2.808 1.145 1.421 1.698 2.252 2.473 2.694 2.9 16 1.173 1.450 1.726 2.280 2.501 2.723 2.944 1.200 1.477 1.754 2.030 2.307 2.584 1.733 1.678 1.679 1.679 1.681 1.680 1.676 1.678 1.678 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2214 0.2214 0.2214 0.2214 0.2767 0.2767 0.2767 0.2214 0.2214 0.2214 0.2214 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 -0.85 - 2.05 -4.29 - 15.56 - 15.75 -21.48 - 24.03 - 33.73 - 35.24 - 8.32 - 6.04 -4.35 - 3.44 - 2.73 -2.21 - 1.84 -8.21 - 6.26 -4.66 -2.36 - 1.98 - 1.70 - 1.46 - 8.29 - 6.06 - 4.49 - 2.26 - 1.95 - 1.67 - 1.45 - 8.01 - 5.90 - 4.36 - 3.43 - 2.74 - 2.25 - 0.87 -2.10 -4.22 - 15.69 - 15.61 -21.85 - 23.78 - 33.98 - 34.98 -8.16 - 5.78 -4.30 - 3.32 - 2.64 -2.15 - 1.78 -8.58 -6.12 -4.57 - 2.30 - 1.95 - 1.67 - 1.45 - 8.29 - 5.97 - 4.49 - 2.28 - 1.93 - 1.66 - 1.44 - 8.01 - 5.82 - 4.40 - 3.44 -2.76 -2.272558 Ternary Aqueous Solutions Table 2.(Cont.) An, AH(expt1) AH(ca1cd) m,(i> n1 0.331 1.019 0.805 0.665 0.567 0.494 0.438 0.393 1.219 0.995 0.841 0.728 0.642 0.574 0.519 1.398 1.150 0.977 0.849 0.751 0.673 0.609 1.591 1.319 1.126 0.983 0.872 0.783 0.71 1 I .820 1 SO4 1.281 1.1 16 1.011 0.925 0.836 2.006 1.680 1.445 1.268 1.129 1.018 0.927 0.590 1.209 0.955 0.789 0.673 0.586 0.519 0.466 1.007 0.822 0.695 0.601 0.530 0.474 0.428 0.801 0.659 0.559 0.486 0.430 0.385 0.349 0.609 0.505 0.43 1 0.376 0.333 0.300 0.272 0.403 0.333 0.283 0.247 0.224 0.205 0.185 0.200 0.168 0.144 0.126 0.113 0.101 0.092 2.860 1.042 1.319 1.596 1.872 2.149 2.426 2.703 1.230 1 SO7 1.784 2.061 2.337 2.614 2.891 1.283 1.560 1.837 2.1 14 2.390 2.667 2.944 1.342 1.618 1.895 2.172 2.449 2.725 3.002 1.316 1.593 1.869 2.146 2.368 2.589 2.866 1.425 1.702 1.978 2.255 2.532 2.809 3.085 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.22 14 0.2214 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 - 1.84 - 7.80 - 5.39 -3.91 - 2.98 - 2.36 - 1.89 - 1.57 - 7.76 - 5.71 - 4.32 - 3.37 - 2.70 -2.18 - 1.84 -7.14 - 5.41 -4.10 - 3.23 - 2.76 - 2.05 - 1.76 - 6.86 - 5.32 -4.07 - 3.27 - 2.53 -2.13 - 1.79 - 7.02 -5.17 - 3.95 -2.57 -2.12 - 2.22 - 1.84 - 6.84 - 5.06 - 3.89 - 3.09 - 2.56 - 2.02 - 1.71 - 1.89 - 7.73 - 5.40 - 3.97 - 3.04 - 2.40 - 1.94 - 1.60 - 7.67 - 5.62 -4.28 - 3.37 -2.71 - 2.23 - 1.87 - 7.30 - 5.40 -4.14 - 3.27 - 2.65 -2.19 - 1.83 - 7.09 - 5.29 -4.08 - 3.25 - 2.64 -2.19 - 1.84 -6.91 -5.10 - 3.91 -2.53 -2.13 - 2.23 - 1.86 - 6.66 -4.98 - 3.86 - 3.07 -2.51 - 2.08 - 1.76 AH(expt1) for m3 = 0, see table 1.N .Daldrup and H . Schonert Table 3. Enthalpy of dilution; glycerol (2)-sucrose (3) 2559 0.201 0.158 0.130 0.1 10 0.096 0.085 0.076 0.398 0.3 15 0.261 0.222 0.194 0.172 0.154 0.599 0.455 0.392 0.334 0.292 0.258 0.232 0.810 0.643 0.533 0.455 0.397 0.352 0.316 1.028 0.821 0.683 0.585 0.512 0.455 0.409 1.204 0.984 0.833 0.722 0.637 0.569 0.470 1.406 1.146 0.967 0.837 0.737 0.659 0.544 1.585 1.297 1.097 0.95 1 0.839 2.010 1.580 1.301 1.106 0.962 0.851 0.763 1.806 1.429 1.182 1.008 0.879 0.779 0.699 1.595 1.213 1.045 0.892 0.777 0.689 0.6 19 1.41 1 1.119 0.927 0.792 0.691 0.612 0.550 1.258 1.005 0.837 0.716 0.627 0.557 0.501 1.012 0.828 0.700 0.607 0.535 0.479 0.433 0.772 0.629 0.531 0.459 0.405 0.362 0.298 0.600 0.49 I 0.415 0.360 0.318 1.016 1.292 1.569 1.846 2.123 2.399 2.676 1.049 1.326 1.603 1.880 2.156 2.433 2.710 1.052 1.384 1.605 1.882 2.159 2.435 2.712 1.062 1.339 1.615 1.892 2.169 2.445 2.722 1.098 1.375 1.652 1.928 2.205 2.482 2.758 1.243 1.520 1.797 2.073 2.350 2.627 2.9 13 1.222 1.499 1.775 2.052 2.329 2.605 3.159 1.245 1.522 1.798 2.075 2.352 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.1660 0.2214 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 - 8.67 - 6.00 - 4.38 - 3.35 - 2.62 - 2.08 - 1.70 - 8.22 -6.11 - 4.43 - 3.43 -2.72 - 2.24 - 1.82 - 5.52 -4.63 - 4.54 - 3.33 - 2.65 -2.1 1 - 1.83 - 8.41 - 5.97 - 4.45 - 3.42 - 2.67 -2.15 - 1.76 - 8.61 - 6.26 -4.68 - 3.59 - 2.88 -2.39 -2.01 -8.16 - 6.09 -4.63 -3.61 - 2.93 - 2.38 - 1.91 - 7.62 - 5.62 - 4.23 - 3.27 -2.72 -2.17 - 1.59 - 7.67 - 5.50 - 4.09 - 3.26 - 2.68 - 8.65 - 5.95 -4.33 - 3.28 -2.58 - 2.07 - 1.70 - 8.56 - 5.99 - 4.40 - 3.37 -2.66 -2.15 - 1.77 - 5.40 -4.55 - 4.35 -3.33 - 2.64 -2.13 - 1.76 - 8.40 - 5.95 - 4.4 1 - 3.40 - 2.69 -2.18 - 1.81 - 8.65 - 6.22 -4.66 - 3.61 - 2.88 - 2.35 - 1.95 -8.17 - 6.06 -4.65 - 3.67 - 2.97 - 2.45 - 1.89 - 7.66 -5.63 - 4.29 - 3.37 - 2.72 - 2.24 - 1.59 - 7.49 - 5.51 -4.21 - 3.32 - 2.682560 Ternary Aqueous Solutions Table 3.(Cont.) m,(i> 0.751 0.679 1.778 1.463 1.243 1.109 0.978 0.874 0.790 2.017 1.668 1.421 1.238 1.097 0.985 0.893 0.284 0.257 0.393 0.323 0.274 0.245 0.216 0.193 0. I74 0.202 0.167 0.142 0.124 0.1 10 0.098 0.089 n1 2.628 2.905 1.562 1.839 2.060 2.337 2.613 2.890 1.3 19 1.596 1.872 2.149 2.426 2.704 -__ 1.285 2.98 1 An1 ___. 0.2767 0.2767 0.2767 0.2767 0.2214 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 0.2767 AH(expt1) AH(ca1cd) -2.13 - 1.81 - 7.06 - 5.30 - 3.32 - 3.30 - 2.59 -2.14 - 1.81 - 7.22 - 5.31 -4.14 -3.17 - 2.60 -2.15 - 1.75 - 2.20 - 1.85 -7.16 - 5.27 - 3.30 - 3.33 - 2.68 - 2.20 - 1.84 - 7.16 - 5.26 - 4.02 -3.17 - 2.56 -2.12 - 1.77 Data for the binary solutions, see tables 1 and 2.Results The calorimetric results are listed in tables 1-3, where, AH(expt1) is the measured enthalpy of dilution. A least-squares procedure was used to determine the minimum number of coefficients hi? in eqn (3) and their values. For each system the data of the two binary constituent solutions and of the ternary solution were simultaneously fitted to eqn (3). In this way it was also possible to evaluate the covariance matrix for the complete system. This will be used in testing the model theory. For the sake of comparison we have also included in tables 1-3 the value of AH(ca1cd) calculated from eqn (3) and the coefficients hij.The coefficients hij with units (kg mol-l)i+j-l are listed below, together with the relative standard deviation orel. of the fit. The covariance matrix can be found el~ewhere.~ (1) glucose (2)-glycerol (3) h,, = 1.5298 x lo-' h30 = -6.8840 x lo-, h,, = 1.6594 x 10-1 h,, = 3.1230~ lo-' h,, = -2.2179 x lo-' h,, = - 1.5373 x lo-, h,, = - 8.4321 x 1 OP3 orel. = 0.032 (2) glucose (2)-sucrose (3) h,, = 1.5012~ lo-' h,, =4.7297 x lo-' h,, = -5.5503 x lop3 h,, = 2,3218 x lo-' h,, = - 1.2341 x lo-' h,, = -4.7477 x lop2 h,, = -4.8546 x lop2 orel, = 0.028 (3) glycerol (2)-sucrose (3) h,, = 1.6019 x 10-1 h30 = - 5.9927 x lo-, h,, = 2.3249 x lo-' h,, = - 1.2463 x 1 O-, h,, = 5.5666 x 10-1 h,, = - 5.871 1 x lo-' h,, = -6.2919 x lo-, orel.= 0.023N . Daldrup and H. Schonert Table 4. Experimental and calculated density p(expt1) and p(ca1cd) ; glucose (2)-glycerol (3) 256 1 m2 m3 p(exptI) p(ca1cd) m3 p(expt1) p(ca1cd) 0.205 0.205 0.4 19 0.419 0.645 0.645 0.88 1 0.88 1 1.130 1.130 1.393 1.391 1.669 1.667 1.961 1.958 2.269 2.267 2.594 2.592 0.506 0.776 0.986 1.993 3.955 6.156 8.403 10.440 12.360 14.449 16.960 18.250 20.999 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.203 0.203 0.412 0.412 0.628 0.628 0.850 0.850 1.079 1.079 1.315 1.315 1.559 1.558 1.810 1.809 2.070 2.069 2.338 2.337 0.566 1.01091 1.010 96 1.024 56 1.024 65 1.038 03 1.037 99 1.051 50 1.051 62 1.064 90 1.065 13 1.077 46 1.078 60 1.09 1 00 1.091 70 1.104 15 1.105 08 1.11737 1.11807 1.131 25 1.131 70 1.029 60" 1.045 50" 1.057 50" 1.106 40" 1.177 60" 1.234 70" 1.276 70" 1.303 70" 1.326 00" 1.347 00" 1.364 60" 1.374 90" 1.385 70" 1.001 65 1.001 60 1.005 92 1,005 87 1.010 13 1.010 09 1.01442 1 ,O 14 40 1,01865 1,018 58 1.022 85 1.022 90 1.027 15 .027 08 .03146 .03134 .035 69 .035 56 .039 86 .039 76 .010 50b 1.010 54 .010 53 .024 00 .024 00 .037 44 .037 45 .050 86 .050 85 .064 25 .064 23 1.077 67 1.077 58 1.090 99 1.090 93 1.10431 1.10422 1.1 17 59 1.11751 1.13074 1.130 68 1.029 22 1.044 98 1.056 58 1.10573 .178 12 .235 24 .277 18 .305 87 .327 29 .346 15 .36440 .372 39 1.387 11 1.001 37 1.001 37 1.005 69 1.005 69 1.01001 1.01000 1.01432 1.01431 1.018 62 1.018 61 1.02291 1.022 89 1.027 18 1.027 17 1.031 45 1.031 44 1.035 71 1.035 70 1.039 96 1.039 94 1.008 78 2.005 2.005 2.005 2.005 2.005 2.005 2.005 1.767 1.767 1.767 1.767 1.767 1.767 1.767 1.767 1.510 1.510 1.510 1.510 1.510 1.510 1.510 1.510 1.249 1.249 1.249 1.249 1.249 1.249 1.249 1.249 1.002 1.002 1.002 1.002 1.002 1.002 1.002 1.002 0.746 0.746 0.746 0.746 0.746 0.746 0.746 0.746 0.505 0.505 0.505 0.505 0.505 0.505 0.505 0.501 0.750 1 .ooo 1.250 1.500 1.751 2.000 0.251 0.500 0.750 1.001 1.25 1 1.502 1.75 1 2.002 0.257 0.513 0.769 1.025 1.282 1.539 1.795 2.050 0.250 0.501 0.750 1 .ooo 1.249 1 s o 0 1.749 2.000 0.250 0.500 0.751 1.001 1.250 1 S O 1 1.751 2.000 0.250 0.500 0.750 1.001 1.250 1 SO0 1.749 2.001 0.249 0.500 0.750 1.001 1.251 1.501 1.751 1.111 84 1.11425 1.11661 1.11888 1.121 08 1.123 25 1.12535 1.098 87 1.101 57 1.10420 1.10673 1.109 19 1.111 59 1.11396 1.116 16 1.087 I 1 1.090 09 1.092 96 1.095 83 1.098 59 1.101 10 1.10362 1.106 03 1.074 38 1.077 66 1.080 80 1.083 99 1.086 70 1.08971 1.092 46 1.095 07 1.061 68 1.065 22 1.068 66 1.072 12 1.075 16 1.078 27 1.081 36 1.084 20 1.047 66 1.051 56 1.055 34 1.058 93 1.062 45 1.065 89 1.069 29 1.072 25 1.033 72 1.038 07 1.042 15 1.046 02 1.049 87 1.053 50 1.057 18 1.111 80 1.11434 1.1 16 75 1.11903 1.121 17 1.12321 1.125 11 1.098 65 1.101 53 1 .1 04 28 1.10689 1.10936 1.111 72 1.11393 1.11605 1.086 78 1.089 98 1.093 03 1.095 93 1.098 70 1.101 33 1.103 83 1.106 19 1.074 01 1.077 41 1.080 65 1.083 75 1.08671 1.089 56 1.092 25 1.094 85 1.061 32 1.064 99 1.068 52 1.071 90 1.075 12 1.078 23 1.081 21 1.084 04 1.047 45 1.051 44 1.055 28 1.058 98 1.062 49 I .065 90 1.069 15 1.07231 1.033 68 1.038 02 1.042 18 1.046 18 1.050 03 1.053 73 1.057 302562 Ternary Aqueous Solutions Table 4.(Cont.) m2 m3 0.000 0.000 0.000 0.000 0.000 0.000 0.000 2.005 1.121 2.300 2.841 3.094 4.429 6.930 10.104 0.250 1.020 41 1.039 04b 1.046 87b 1.05 1 53’ 1.068 95b 1.094 45’ 1.1 18 56b 1.10929 p(ca1cd) 1.019 38 1.039 37 1.047 53 1.051 15 1.068 50 1.094 42 1.118 58 1.109 10 m2 m3 0.505 0.256 0.256 0.256 0.256 0.256 0.256 0.256 0.256 2.002 0.250 0.501 0.75 1 1 .ooo 1.252 1 SO2 1.751 2.000 p(expt1) 1.060 5 1 1.018 77 1.023 36 1.027 76 1.032 07 1.036 40 1.040 22 1.044 11 1.047 84 p(ca1cd) 1.060 74 1.018 67 1.023 38 1.027 89 1.032 23 1.036 45 1.040 50 1.044 40 1.048 15 The units are: mi = mol kg-’, p = kg dm-3.a Data from ref. (6). Data from ref. (1 1). Table 5. Experimental and calculated density p(expt1) and p(ca1cd) ; glucose (2)-sucrose (3) m2 m3 p(expt1) p(ca1cd) m2 m3 p(expt1) p(ca1cd) 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 1.958 1.958 1.958 1.958 1.958 1.958 1.958 1.958 1.727 1.727 1.727 1.727 1.727 1.727 1.727 1.727 1.530 1.530 1.530 1.530 1.530 0.100 0.204 0.300 0.398 0.498 0.598 0.703 0.991 1.199 1.391 1.588 1.789 1.987 2.074 0.250 0.500 0.75 1 1.001 1.252 1 SO2 1.752 2.003 0.250 0.500 0.751 1.001 1.251 1 SO1 1.752 2.002 0.25 1 0.501 0.752 1.003 1.253 1.009 87 1.022 63 1.033 95 1.045 06 1.055 89 1.066 30 1.076 74 1.103 43 1.120 96 1.13603 1.15047 1.16423 1.176 97 1.182 33 1.12530 1.144 14 1.163 15 1.17746 1.192 5 1 1.206 1 1 1.21888 1.230 58 1.11597 1.135 62 1.15407 1.17053 1.185 60 1.200 1 1 1.2 1 3 00 1.226 76 1.10743 1.127 85 1.146 66 1.16371 1.179 64 1.009 95 1.022 76 1.034 1 1 1.045 22 1.056 03 1.06643 1.076 84 .lo341 .12085 .135 86 .150 32 .164 13 .177 00 .182 42 1.125 27 1.14441 1.161 87 1.177 88 1.192 63 1.206 3 1 1.21905 1.231 01 1.11569 1.13 5 64 1.1 53 79 1.17042 1.18572 1.19988 1.21307 1.225 43 1.107 27 1.127 92 1.14676 1.16396 1.179 78 1.262 1.262 1.262 1.262 1.262 1.262 1.262 1.008 1.008 1.008 1.008 1.008 1.008 1.008 1.008 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.766 0.537 0.537 0.537 0.537 0.537 0.537 0.537 0.537 0.262 0.262 0.262 0.262 0.502 0.752 1.003 1.254 1 SO5 1.756 2.007 0.25 1 0.502 0.753 1.004 1.255 1 SO7 1.758 2.009 0.25 1 0.503 0.754 1.005 1.256 1 SO8 1.759 2.01 1 0.25 1 0.503 0.755 1.006 1.258 1 SO9 1.761 2.013 0.250 0.500 0.75 1 1.001 1.11686 1.136 66 1.15463 1.171 10 1.186 70 1.200 73 1.21397 1.083 18 1.105 87 1.126 32 1.145 49 1.16269 1.178 69 1.193 54 1.207 39 1.071 16 1.095 03 1.11675 1.1 36 42 1.1 54 57 1.171 21 1.1 86 60 1.200 83 1.059 1 1 1.084 07 1.106 65 1.12745 1,146 38 1.16360 1.180 65 1.19447 1.043 69 1.070 02 1.093 94 1.11570 1.11689 1.13663 1.15468 1.171 24 1.18653 1.200 75 1.21401 1.083 05 1.10589 1.12663 1.14553 1.1 62 83 1.17880 1.19361 1.207 43 1.071 04 1.095 00 1.11670 1.13644 1.154 51 1.171 18 1.186 58 1.200 95 1.059 03 1.084 1 1 1.106 82 1.12745 1.146 3 1 1.163 63 1.179 67 1.194 59 1.043 64 1.070 12 1.093 99 1.1 1566N .Daldrup and H. Schonert 2563 Table 5. (Cont.) __________ -~~ ~ ~ _ _ _ ~ _ m2 m3 p(expt1) p(ca1cd) m2 m3 p(expt1) p(ca1cd) 1.530 1.504 1.194 13 1.19442 0.262 1.251 1.13551 1.13543 1.530 1.755 1.207 81 1.208 04 0.262 1.502 1.15362 1.15360 1.530 2.006 1.220 55 1.220 77 0.262 1.752 1.17044 1.17037 1.262 0.251 1.09548 1.095 15 0.262 2.002 1.18537 1.18595 p(expt1) for m3 = 0, see table 4. Table 6. Experimental and calculated density p(expt1) and p(ca1cd) ; glycerol (2)-sucrose (3) m2 m3 p(expt1) p(ca1cd) m2 m3 p(expt1) p(ca1cd) - 1.99 1 1.991 1.991 1.991 1.991 1.991 1.991 1.99 1 1.748 1.748 1.748 1.748 1.748 1.748 1.748 1.748 1.487 1.487 1.487 1.487 1.487 1.487 1.487 1.478 1.225 1.255 1.255 1.255 1.255 1.255 1.255 1.255 0.250 0.500 0.750 1 .ooo 1.250 1 SO0 I .750 2.000 0.250 0.499 0.749 0.999 1.249 1.499 1.749 1.998 0.250 0.500 0.749 0.999 1.249 1.499 1.749 1.999 0.250 0.500 0.750 1 .ooo 1.250 1 SO0 1 .750 2.000 1.059 87 1.083 05 1.103 98 1.12342 1.141 61 1.15801 1.173 23 1.188 00 1.056 36 1.079 95 1.101 65 1.121 25 1.140 00 1.15632 1.171 55 1.185 91 1.052 65 1.076 87 1.099 54 1.11941 '1.138 15 1.1 54 99 1.170 58 1.18492 1.048 94 1.073 97 1.096 45 1.1 1726 1.13656 1.1 53 64 1.169 30 1.18402 1.059 89 1.083 04 I .1 04 22 1.123 67 1.141 58 1.158 12 1.17344 1.187 63 1.05642 1.080 04 1.101 63 1.121 42 1.139 63 1.15643 1.171 96 1.186 36 1.052 57 1.076 75 1.09881 1.1 1903 1.137 58 1.15468 1.17047 1.185 05 1.049 06 1.073 76 1.096 28 1.1 1686 1.135 75 1.153 13 1.169 17 1.18400 0.99 1 0.991 0.99 1 0.991 0.991 0.99 1 0.99 1 0.99 1 0.757 0.757 0.757 0.757 0.757 0.757 0.757 0.757 0.512 0.512 0.512 0.512 0.512 0.512 0.512 0.512 0.260 0.260 0.260 0.260 0.260 0.260 0.260 0.260 0.250 0.500 0.750 1 .ooo 1.250 1.500 1.751 2.001 0.249 0.500 0.750 1 .ooo 1.250 1 SO0 1.749 1.999 0.250 0.500 0.750 1 .ooo I .250 1.500 1.751 2.001 0.250 0.500 0.751 1.001 1.251 1.502 1.752 2.002 1.045 17 1.071 11 1.091 09 1.11106 1.133 80 1.151 51 1.16784 1.182 74 1.041 25 1.067 30 1.090 47 1.1 12 16 1.131 76 1.1 49 55 1.16627 1.18147 1.037 08 1.063 5 1 1.087 50 1.109 60 1.I29 23 1.147 74 1.16490 1.18043 1.033 16 1.060 00 1.084 57 1.10698 1.127 41 1.14600 1.163 18 1.17922 1.044 93 1.070 25 1.093 29 1.1 1434 1.1 33 60 1.151 32 1.167 65 1.182 75 1.041 12 1.067 00 1.090 52 1.1 11 96 1.131 58 1.149 61 1.166 19 1.181 51 1.037 06 1.063 56 1.087 60 1.10951 1.1 29 53 1.14791 1.1 64 79 1.1 80 37 1.032 72 1.059 90 1.084 54 1.10695 1.12741 1.146 15 1.163 36 1.179 21 - Data for the binary solutions, see tables 4 and 5.The density measurements can be found in tables 4-6. Again, the coefficients uii of eqn (6) and their covariance matrix have been evaluated for the complete systems including the two binary solutions. The results are [units oij are dm3 J-l (kg rn01-')~+~-~] :2564 Ternary Aqueous Solutions (1) glucose-glycerol (3) : u,, = 4.5562 x u,, = - 2.3674 x 10-l' v,, = -3.5622 x lo-' v,, = 1.0994 x v,, = 2.0791 x lo-' - 2.0279 x lo-' '30 = u,, = 2.8496 x u,, = 3.8834 x lo-' u,, = 1.0776 x ore,, = 0.0005 (2) glucose (2)-sucrose (3) : ul0 = 4.5569 x u,, = 1.0896 x u,, = 8.4931 x v,, = -2.5666 x o,, = 4.1474 x low7 u,, = - 1.1 153 x lo-' u3, = - 1.9959 x 10-9 u,, = 1.1465 x v,, = -7.8097 x lo-' ore,.= 0.0005 (3) glycerol (2)-sucrose (3) : u,, = 2.8473 x u3,, = -5.6778 x 10-l' uol = 8.5501 x uO3 = -6.2398 x lo-' u,, = -7.9721 x lo-' u,, = - 4.7689 x 1 0-' v,, = 4.4399 x lo-' v,, = 4.1262 x u,, = 3.6434 x orel, = 0.0005 Discussion In the literature there are few measurements for binary solutions with which wecan compare our results. For H,O-glucose, Taylor and Robinson' measured at rn, = 1 .O 17 mol kg-l a value of Hex = 398 J kg-l compared to our fF"(ca1cd) = 376 J kg-l.For H,O-glycerol, Franks and Pedley7 found h,, = 0.1977 kg mol-l and ho, = -0.01089 kg mol-l, which gives slightly greater Hex than our fit, whereas the experiments of Lange and Mohring8 deviate rather far from these two sets. For H,O-sucrose, the values of Hex agree with those of Gucker et al.99ro Density measurements in H,0-glucose6 and H,O-glycerol'l fit so well with our data that they were included in the least-squares procedure. In comparing the results with the model theory we have fixed the binary molalities mi and rn; of equal solvent activity with osmotic coefficient data from the literature., Next, the model values of Hex and Yex were calculated according to eqn (8) and (9). The variances and covariances of the binary h,, hi, and uOi, ui, were used to evaluate the confidence interval for this model.The confidence interval for the experimental Hex and v"" values were found with the help of eqn (I) and (4) and the complete covariance matrix of h, and uij. The results are shown in fig. 1-3. In the ternary solution with glucose-glycerol experimental and theoretical values overlap. In the solution with glucose-sucrose the experimental enthalpy is slightly higher (up to 10%) than the calculated one, whereas the volume values coincide. The same is true for the last system. Although the agreement between experiment and model theory is not perfect, the results nevertheless show that solute-solvent interactions in these solutions are more important for the thermodynamic behaviour than solute-solute interactions. Financial support from the Fonds ' Der Chemischen Industrie ' is gratefully ac- knowledged.N. Daldrup and H. Schonert 2565 0.0 0.5 1 .o mz Im i Fig. 1. Excess enthalpy and excess volume for glucose (2)-glycerol (3); rn; = 1.500 mol kg-', rn; = 1.524 mol kg-l. The bars denote the experimental values f 2 a , and the two lines enclose the 95% confidence interval of the theory. 0.0 1 .o Fig. 2. The same as for fig. 1 for glucose (2 )-sucrose (3); rn; = 1.500 mol kg-', rn; = 1.386 mol kg-'.2566 - 1100 I 21000 e) 1 & 900- Ternary Aqueous Solutions 1 1 1 1 1 + I - - 1 Fig. 3. The mzim; 1.366 mol kg-'. same as for fig. 1 for glycerol (2)-sucrose ( 3 ) ; rng = 1.500 mol kg-', rn; = References 1 R. A. Robinson and R. H. Stokes, J. Phys. Chem., 1966, 70, 2126. 2 H. Schonert, 2. Phyf. Chem. N.F., 1986, 150, 163. 3 H. Schonert, 2. Phys. Chem. N.F., 1986, 150, 181. 4 L. Rafflenbeul, W. Pang, H. Schonert and K. Haberle, Z . Naturforsch., Teil C, 1973, 28, 533. 5 N. Daldrup, Thesis (RWTH Aachen, Fakultat 1, 1986). 6 J. B. Taylor and J. S. Rowlinson, Trans. Faraday SOC., 1955, 51, 1183. 7 F. Franks and M. D. Pedley, J. Chem. SOC., Faraday Trans. I , 1983, 79, 2249. 8 E. Lange and K. Mohring, Z . Elektrochem., 1953, 57, 660. 9 F. T. Gucker, H. B. Pickard and R. W. Planck, J. Am. Chem. SOC., 1940, 62, 1464. 10 L. Stroth and H. Schonert, J. Chem. Thermodyn., 1977, 9, 851. 1 1 H. Sadek, A. M. Hafez and F. Y . Khalil, Elektrochim. A m , 1969, 14, 1089. Paper 7/2082; Received 23rd November, 1987

ISSN:0300-9599    

DOI:10.1039/F19888402553

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1988, 84(8), 2567-2571 Interactions between Cations and Sugars Part 4.-Free Energy of Interaction of the Calcium Ion with Some Aldopentoses and Aldohexoses in Water at 298.1 5 K Jean-Pierre Morel," Claude Lhermet and Nicole Morel-Desrosiers Laboratoire d'Etude des Interactions Solutks-Solvants, U. A. au CNRS 12'434, Universitk Blaise Pascal (Clermont-Ferrand 11), 631 70 Aubi&e, France The free energies of interaction of ribose, arabinose, lyxose, xylose, glucose, mannose, galactose, talose and allose with CaCl, and KC1 have been determined in water at 298.15 K. The electrochemical method used for these determinations leads to reaction properties, in particular to the association constants, in sufficiently dilute solutions for these properties to be thermodynamically meaningful, unlike n.m.r.methods, which require solutions concentrated both in sugar and in salt. It appears that the constants presented here are distinctly smaller than those determined from n.m.r. data. This work also shows that, depending on the structure of the sugar and on its hydration, the interactions between the sugar and Ca2+ can be either attractive (association) or repulsive (salting-out). In the first three of this study, we have reported the thermodynamic properties characterizing the association of ribose with some cations. It is known4 that among the pentose and hexose isomers those of the pyranose form that bear a sequence of h ydroxy groups in an axiakquatorial-axial arrangement and those of the furanose form that have three cis-cis hydroxyl groups form complexes with some divalent and trivalent cations.In aqueous solutions the interactions between these sugars and the cations are rather weak, the stability constants being of the order of unity, but they are quite specific. Calcium, owing to its biological importance, is probably the cation that has been studied most.'-' Few simple sugars are liable to form such complexes, but this type of interaction becomes important for more complicated derived species. In a more general way, it is of prime importance to determine how the small sugars interact with the electrolytes in water. This is why, in the work presented here, we mainly study the interactions between simple aldoses (ribose, arabinose, lyxose, xylose, glucose, mannose, galactose, talose and allose) and the electrolytes CaC1, and KC1.The electrochemical method we use allows us to evaluate the parameters characterizing the electrolyte-non-electrolyte pair interactions, and hence to obtain the association constant of Ca2+ with the complexing forms of the sugar. This method can be used on solutions sufficiently dilute for the properties of reaction thus determined, in particular the association constant,lP3 to be thermodynamically meaningful. This is not always the case with spectroscopic methods such as n.m.r., which require solutions concentrated both in sugar and salt. Theoretical The standard free energy of transfer of an electrolyte E from pure water to a solution of a sugar S can be calculated from the e.m.f. U of the galvanic cell (1) electrode reversible to M*+ 1 MX,(m,), S(m,) I AgX I Ag 25672568 Interactions between Cations and Sugars where mE and m, are the molalities, with respect to water, of the electrolyte and the sugar, respectively.A galvanic cell of this type was used a few years ago to study the interactions in aqueous solutions of CaCl, and some amino acids." By combining liquid membrane electrodes totally reversible to cations with the AgX/Ag electrode, it is possible to perform measurements that are sufficiently sensitive and precise for one to study variations in U of only a few mV. If U(m,) is the e.m.f. in water and U(rnE,m,) is the e.m.f. in the sugar solution, then At Gg(m,) = zF lim [ U(m,, m,) - U(mE)]. mE+O In practice, for relatively low mE we directly obtain AtGg(m,) from the determination of the variation of U with m,.The free energy of transfer can be interpreted in terms of pair, triplet etc. interaction parameters l. ' where v is the number of ions of the electrolyte, g,, is the pair interaction parameter and g,,, is the triplet interaction parameter. For CaCI, At Gg(ms) = 2VgEs ms 4- 3vgESS Wg -I-. . . (3) gES = (gCa2+S+28C1 ( 4 ) and for KCl gES = k K t S +gCl-S)/2' ( 5 ) The specific interaction between Ca2+ and the complexing sequences of a sugar can be examined by different methods. For instance, the complexing sugar (ribose, R) can be compared to one of its non-complexing epimers (arabinose, A) by assuming that the non-specific interactions of Ca2+ and C1- with both sugars are the same.' The parameter g* characterizing solely this specific interaction can thus be defined as In another approach, if it is assumed that the cation K+ does not specifically interact with the complexing hydroxyl groups of a sugar S,4 the parameter g* can be written as (7) The phenomenon involving the association of a cation with a sugar is described in the usual way with a chemical model; hence, the pair interaction parameter is related to the constant p, which characterizes the reaction of association of Ca2+ with the complexing g* = 3gCaC1,S - 4gKCIS = gCa2+S - 2gK+S.hydroxyl groups, by13 /3 = -2g*/RT. Negative values of g* characterize an association process ( P > 0), whereas positive values indicate some 'salting-out ' of the sugar by the electrolyte. In the latter case, g* is related to the Setschenow constant by an expression identical to eqn ( 8 ) ." 9 " The sugars studied here exist in solution as an equilibrium mixture of mainly six isomers: the 4C1 and 1C4 conformers of each of the a and /3 anomers of the pyranose form (aP and PP) and the a and anomers of the furanose form (aF and PF). The fact that only some of these isomers do bear a complexing sequence of hydroxyl groups must be taken into account in the calculation and interpretation of the association constant. Experimental All sugars (Fluka puriss.) were dried under vacuum, stored in a desiccator and used as such. CaCl, and CaBr, (Merck pro analysi) were titrated against EDTA. All solutions were prepared by weight from triply distilled water degassed prior to use.A liquid-membrane electrode selective to Ca2+ ions (Orion 93-20) or to K+ ions (OrionJ-P. Morel, C. Lhermet and N . Morel-Desrosiers 2569 Table 1. Pair interaction parametersa of D( -)ribose and D( -)arabinose with various electrolytes in water and in aqueous electrolytic media at 298.15 K electrolyte D( -)ribose D( -)arabinose KCI - 100 - 100 NaCl 300 100 CaC1, - 2200 - 100 CaBr, - 2400 - 200 CaCl, (in 0.1 mol kg-I KCI) - 2300 - 200 CaC1, (in 0.1 mol kg-' NaC1) -2300 - 100 a 2vg,<, in J kg rno1V2 [see relation (3)]. Table 2. Pair interaction parameters of CaCI, and KCI with various sugars in water at 298.15 K sugars 6gCaC1*SU 4gKC1Sa 2g*b D( -)ribose D( - )arabinose D( - )lyxose D( + )xylose D( + )glucose D( + )mannose D( + )galactose D( + )talose D( + )allose - 2200 - 100 0 700 600 300 0 - 2700 - 800 - 100 - 100 100 100 100 - 100 - 200 0 - 2000 100 - 200 500 400 100 400 - 2700 - 800 a 2vg,, in J kg mo1-2 [see relation (3)].Calculated from relation (7). 93-19) or a glass electrode selective to Naf ions (Tacussel) was combined with the AgC1/ Ag or AgBr/Ag electrode (Tacussel). The electronic millivoltmeter used (Tacussel ISIS 20000) had a resolution of 0.01 mV. The sugar concentration was varied by adding to a solution of electrolyte at molality m E (0.005 mol kg-l) increasing amounts of a solution of sugar at molality m, containing also the electrolyte at molality m,. The concentration of the electrolyte thus remained constant, whereas the concentration of the sugar was increased to 0.4mol kg-l. The additions were performed with a special dispenser coupled to a precision balance (Mettler P160N) and were carried out in a double-walled vessel connected to a circulating bath thermoregulated at kO.01 K.The g,, parameters were obtained by extrapolating the plots of AtGg/rn, vs. m, to m, = 0. Results The parameters characterizing the free energy of interaction of ribose and arabinose with KCl, CaCl, and CaBr, in water, and with CaC1, in water containing 0.1 mol kg-l of KCl or NaCl, are listed in table 1. From these data the importance of the anion and of the interactions with the monovalent cations Kf and Na' can be estimated. The parameters characterizing the pair interactions of CaC1, or KCl with the different aldopentoses and aldohexoses studied here are compiled in table 2 together with the specific interaction parameters, g*, calculated from relation (7).The error in the pair interaction parameters is estimated to be +_ 100 J kg rnol-,.2570 Interactions between Cations and Sugars Discussion From the data in table 1 it is possible to compare the interactions of the sugars with various electrolytes. First it is seen that the non-specific interactions of arabinose are always weak. They are of the same order of magnitude whether the cation is divalent, like Ca2+, or monovalent, like K+ or Na+. The pair interactions of ribose with KC1 or NaCl are also weak; they are of the same order of magnitude as those of arabinose with the same cations. Thus ribose does not noticeably and specifically interact, through its complexing sequences of hydroxyl groups, with the monovalent cations K+ and Na+. On the other hand, the parameter characterizing the pair interactions of ribose with CaC1, is largely negative (- 2200 J kg mol-').A comparison of CaCl, with CaBr, shows that, although the anion effect is perceptible both with ribose and arabinose, it is not very important. Finally, the data of table 1 show that the values of the pair interaction parameters are not significantly modified when determined in the presence of a relatively large quantity of NaCl or KCl(O.1 mol kg-'). All this supports the idea that, in addition to the non-specific interactions normally encountered between any of those two sugars and a cation, between ribose and Ca2+ there are specific interactions of relatively greater importance.In table 2 are summarized the pair interaction parameters for ribose, arabinose, lyxose, xylose, glucose, mannose, galactose, talose and allose with CaCl, and KCl. All these sugars interact only very weakly with KCl. This means that it is thus possible to calculate the parameter g* from relation (7), which then simply reduces to g* = 3gCaClZS. It is easier in this case to extract the specific contribution by comparing the interactions CaC1,-S and KCl-S than by trying to find for each complexing sugar the non- complexing epimer which has the same non-specific interactions with CaCl,, since in most cases the sugar and its non-complexing epimer do not exist under the same isomeric forms at equilibrium in aqueous solution. Note that our g* values are consistent with the retentions of the sugars on chromatographic columns of cation-exchange resins'*-'' and with the rates of migration in paper electrophoresis'* and thin-layer ligand-exchange chromatography.l9 Thus there is a remarkable similarity between the thermodynamic and chromatographic behaviour of the sugars in the presence of Ca2+, both for the sugars which are complexed and for those which are 'salted-out'. All the sugars studied here exist in solution as an equilibrium mixture of many isomers, of which only a few bear a complexing sequence of hydroxyl groups. Theoretically, ribose, lyxose, mannose, talose and allose have such sequences of hydroxyl groups, but a study of the composition of the aqueous mixtures at equilibrium 20,21 has shown the following distribution (percentages) of the various complexing forms : ribose : 21 ~ ( a P l C 4 + aP4C 1) + 15( pPlC4) + 6.5(aF) = 43 lyxose: 0.5(PF) mannose: pP1C4 and PF are not present talose : 42(aP1 C4 + aP4C 1) + 29( pP4C 1) + 13( PF) = 84 allose: 14(ccP4C1)+3.5(aF) = 17.5.A mean association constant for the various complexing forms of a given sugar, p, can p = - (2g*/RT) ( l o o p ) (9) be calculated from where P is the overall percentage of the complexing forms at equilibrium. With the method used here the electrolyte concentration is much smaller than the sugar concentration ; accordingly there is no equilibrium shift in favour of the complexing isomers. The values thus obtained are equal to 1.9 for ribose, 1.3 for talose and 1.8 for allose. They are of the same order of magnitude for the three sugars and noticeably smaller than those found by n.m.r.spectroscopy (3.4-5.3 for ribose5). Contrary to the n.m.r. conclusions,6 no complexation of Ca2+ by mannose is observed here: there is noJ-P. Morel, C. Lherrnet and N . Morel-Desrosiers 257 1 shift of the conformational equilibrium in the direction pP4CI + BPIC4. Considering the assumptions made, the values obtained are naturally questionable. The fact that the g* values obtained by studying the interactions of Ca2+ with some sugars having no complexing sequence of hydroxyl groups (glucose and xylose) are distinctly positive (salting-out) shows, in particular, how difficult it is to take into account non-specific interactions for the evaluation of specific interactions.Conclusion This work has shown that the association constants of Ca2+ with the complexing sequences of hydroxyl groups of the sugars studied here are distinctly smaller than those determined from n.m.r. data. In fact, although n.m.r. methods allow one to distinguish between the various anomers and to obtain their respective association constants, the experimental conditions are such that the values obtained are not thermodynamically meaningful. This work also shows that the various carbohydrates studied here interact very differently with the same cation, Ca2+ in the present case. According to the structure of the sugar, and also to the nature of the hydration peculiar to this structure, these interactions can be either attractive (association) or repulsive (salting-out).This underlines the fact that the interactions in dilute aqueous solutions are very different from those in concentrated solutions (or in the solid state). References 1 J-P. Morel and C. Lhermet, Can. J . Chem., 1985, 63, 2639. 2 J-P. Morel, C. Lhermet and N. Morel-Desrosiers, Can. J . Chem., 1986, 64, 996. 3 A. Maestre Alvarez, N. Morel-Desrosiers and J-P. Morel, Can. J . Chem., 1987, 65, 2656. 4 S. J. Angyal, Aust. J. Chem., 1972, 25, 1957. 5 R. E. Lenkinski and J. Reuben, J . Am. Chem. Soc., 1976, 98, 3089. 6 M. C. R. Symons, J. A. Benbow and H. Pelmore, J . Chem. SOC., Furuduy Trans. I , 1982, 78, 3671. 7 M. C. R. Symons, J. A. Benbow and H. Pelmore, J . Chem. Soc., Faraday Trans. I , 1984, 80, 1999. 8 L. G. Ekstrom and A. O h , Acta Chem. Scand., Part A, 1977, 31, 838. 9 A. Vesala and H. Lonnberg, Acta Chem. Scand., Part A, 1981, 35, 123. 10 C. C. Briggs, T. H. Lilley, J. Rutherford and S. Woodhead, J . Solution Chern., 1974, 3, 649. I I J. E. Desnoyers, M. Billon, S. Leger, G. Perron and J. P. Morel, J . Solution Chem., 1976, 5, 681. 12 G . Perron, D. Joly, J. E. Desnoyers, L. Avedikian and J. P. Morel, Can. J. Chem., 1978, 56, 552. 13 R. H. Wood, T. H. Lilley and P. T. Thompson, J . Chem. Soc., Furaduy Trans. 1, 1978, 74, 1301. 14 J. K . N. Jones and R. A. Wall, Can. J . Chem., 1960, 38, 2290. 15 R. W. Goulding, J . Chromatogr., 1975, 103, 229. 16 S. J. Angyal, G. S. Bethel1 and R. J. Beveridge, Curbohydr. Res., 1979, 73, 9. 17 A. S. Serianni, personal communication. 18 S. J. Angyal and J. A. Mills, Aust. J. Chem., 1979, 32, 1993. 19 J. Briggs, P. Finch, M. C. Matulewicz and H. Weigel, Carbohydr. Res., 1981, 97, 181. 20 S. J. Angyal, Angew. Chem., 1969, 8, 157. 21 S. J. Angyal, Adv. Curbohydr. Chem. Biochem., 1984, 42, 15. Paper 712084; Received 23rd November, 1987.

ISSN:0300-9599    

DOI:10.1039/F19888402567

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

t Interaction of Divalent Cations with Polyuronates Attilio Cesaro,* Franco Delben and Sergio Paoletti Dipartimento di Biochimica, Biojisica e Chimica delle Macromolecole, Universita di Trieste, 1-34127 Trieste, Italy The interaction of some divalent cations (Ca2+, Cu2+, Pb2+, Zn2+ and Cd2+) with two polyuronates (alginate and polygalacturonate) has been studied by thermodynamic and structural methods. From the ther- modynamic results, the enthalpy of interaction is claimed to reveal the changes in the conformation and/or association of polymeric chains, while the dilatometric results are related with the solvent release due to ion binding. The degree of cooperativity of the ion-binding process (with Ca and Pb) is evidenced by the calorimetric and dilatometric data and, more quantitatively, by the Scatchard plot of the single-ion activity data (with Cu).Theoretical evaluation of the electrostatic energy change accompanying the process of mixing polyuronates with ions and/or of the conformational energy of the polysaccharide is carried out to discriminate the cases in which the binding process is complicated by other effects (chain aggregation or conformational change). Further structural information is obtained by studying the perturbation on the circular dichroic bands of the chromophore arising from the binding process. Thermodynamic and spectroscopic properties are of fundamental importance for the understanding of the mode and the extent of the specific interaction of uronates and polyuronates (natural and ionic derivatives of neutral polysaccharides) with ions [e.g.see ref. (l)]. Attention has been drawn, in particular, to the role of divalent cations (especially calcium) in the gel formation of polyuronates containing either guluronate or galacturonate residues. ’-’ A detailed study dealing with the binding of divalent cations (Ca, Pb, Zn, Cd, Cu, Mg etc.) to pectins and their oligomeric fragments4y7-’ shows a dramatic difference in the activity coefficient of the cation upon the nature of the cation itself. For example, for oligo-D-galactosyluronates (D, < 7 ) the activity coefficient of the single ion y(M2+) decreases in the order Sr = Ca > Zn > Cd > Cu > Pb.’ These data have been interpreted to propose that the more weakly bound ions, i.e. Ca, Sr and Zn, are bound to oligouronates through intramolecular electrostatic interaction, whereas Cd, Cu and Pb form some sort of complex.’ Indeed the polymers reported in the present study, i.e.alginate and pectate, are claimed to interact ‘quite specifically’ with calcium ions. Alginates are non-random copolymers of p-D-mannuronate (man) and a-L-guluronate (gul) and they have been recently recognized as a class of polymers having not only different fractional content of the monomeric units, but also different sequence distribution of gul and man units. The latter quantity is even more important for the formation of chain clusters, which entrap cations in the so-called ‘egg-box’ structure.’ The same model has been proposed also for pectate, in which homopolymeric sequences of a-D-galacturonate (gal) are interrupted by a series of defects of neutral sugars, including rhamnose, galactose and methyl- galacturonate-ester.‘ Without going into structural details, it is also accepted that, in the initial stage of the ‘egg-box’ formation, two chains associate together to include the mediating cations.25732574 Polyuronate-Ion Interactions From the standpoint of the thermodynamics of the binding and of the accompanying dimerization processes, it has to be emphasized that, whatever the driving force, when the two linear charged polymers are brought together (‘ zippered ’), the formal, linear charge density is doubled. In some cases, perturbation of the system due to pH, ionic strength etc., may trigger large conformational changes which could also dramatically alter the inter-residue repeat distance.Therefore ion-polymer interactions will be strongly dominated by the occurrence of chain dimerization (or polymerization), a phenomenon very common among polysaccharides. A mathematical framework has been provided by various theories for predicting thermodynamic properties of an aqueous system containing small ions interacting with a polyelectrolyte with known structural features.1°-14 In particular, attention has been given to the derivation of the changes of enthalpy (and of volume) arising from the mixing of a polyelectrolyte solution with a simple salt s01ution.l~ In the present paper we report on the interactions of a series of divalent cations with alginate and pectate investigated especially by calorimetry and dilatometry, in addition to other techniques, where applicable.Among the cations, cadmium has been selected because it is often claimed to be ‘vicarious’ to calcium in crystallographic structures, and lead because of its U.V. band which becomes disymmetric upon interaction with polysaccharides. l5 Thermodynamics and Structure of Polysaccharide-Ion Complexes in Solution Experimental Approach for Studying PoiyuronateIon Binding Few crystal structures of monomeric carbohydrate complexes with cations are known, and most of them refer to calcium ion. Data in solution mainly concern n.m.r. spectra or binding affinities. In all cases, it has been demonstrated that the interaction of cations with monomeric sugars is weak.16 On the other hand, homo- and co-polymeric acidic glycans have been extensively studied with ions.In these systems the electrostatic interaction is enhanced (poly- electrolytic effect) and possible topological constraints to the binding on the chain are introduced (conformational effects). Indeed, when speaking of the polymer-ion interaction, it is mandatory to recognize and, possibly, to factorize the contributions of different sources of interactions, namely (i) electrostatic interactions among charged groups, (ii) non-electrostatic contributions to binding, (iii) stereochemistry and conformation of monomer unit(s), (iiii) cation coordination geometry. Experimentally, therefore, a combination of different methods has to be used to get information on the energetics of the binding process to be accounted for in terms of electrostatic interactions, stereochemical requirements and conformational pertur- bations of the chain upon binding.The properties studied and partially reported here, including the methods currently employed, are : ( a ) single-ion activity coefficients by ion- selective potentiometry ; (b) the extent and mode of binding by polarographic determination of the free species; (c) extent of binding by equilibrium dialysis; (6) enthalpy and volume changes on mixing by microcalorimetry and dilatometry, respectively ; (e) U.V. and/or visible absorption and circular dichroism. Some methods [(a), (b), (e)] can be used only with suitable cations. In all cases, owing to the nature of polymeric ligands, it is necessary to follow the entire course of the titration curve (at the equilibrium) of the various cations with acidic polysaccharides.The operational variable used is the concentration ratio of ion-to-polymeric units, A, usually expressed as (moles of ion)/(polymer equivalent).A. Cesaro, F. Delben and S. Paoletti 120 60 + -60- -120- 2575 - - 0- -120 -60 0 60 120 4 Fig. 1. Conformational energy contour diagram for the dimeric segment shown in the insert (digalacturonic). Energy contours are drawn at 2, 7, 17 and 32 kcal mol-I above the energy minimum ; dashed contour defines the negative absolute energy space. Theoretical Approaches Two theoretical approaches can be routinely followed for the understanding of the structural requirements and of the conformational properties of the complexes. The first is the classical conformational calculation of the non-bonding energies as a function of the rotational angles about the glycosidic bonds,17 which substitutes empirical visual model-building.In the random conformation, no explicit account is commonly given for hydrogen bonds which, however, may be considered in the final conformational state upon cation coordination. The most probable dimeric structure (from the assumed structural parameters) can establish the site and the coordination for cation binding. In addition to this canonical, albeit rigid, approach, statistical-mechanical calculations on properly averaged properties of individual conformations also provide the average chain trajectory and the average monomeric repeating distance. The latter quantity defines the average charge density of such a ‘realistic’ chain and determines, therefore, the overall electrostatic potential.All the results of the above calculations are used to discriminate between different models and also to produce chain parameters to be used in the following calculation. The second approach, further improved in our l a b o r a t ~ r y , l ~ ~ ~ ~ * ~ ~ makes use of the polyelectrolyte model of the counterion-condensation theory1°-13 in order to evaluate the excess thermodynamic functions due to the ‘ unspecific effects ’ arising in a polyelectrolyte solution when the ionic strength is changed. The comparison of the ‘expected trend’ with the experimental values of the activity coefficients, enthalpy and volume changes upon mixing a polyuronate with ions is of fundamental importance.In fact, the theoretical thermodynamic functions appear to depend strictly on the polymer ‘ charge per unit length’, and therefore allow one to discriminate the systems which undergo a ‘ phase transition ’. Ultimately, this corresponds to changes in the conformation of the glycosidic bonds, to chain-chain interaction or both, giving rise to formal changes in the2576 Poly ur ona te- Ion In t e rac t ions Fig. 2. Electrostatic energy surface calculated for an infinite charged polygalacturonate chain (as in fig. 1). Energy contours are marked on the figure. value of the charge per unit length, as is known to occur in some of the present cases [for a review, see ref. (14), (20), (21)]. Therefore, within the framework of polyelectrolyte theory one can derive the enthalpy change of electrostatic origin accompanying the mixing of a polyelectrolyte solution with a simple salt solution.'* The net heat effect, corrected for dilution of both polymer and low-molecular-weight electrolyte us.solvent, has been calculated for a system containing a univalent14 and a divalent salt (unpublished results). In all cases AHmix is a smooth positive function of logR, for each given value of the intrinsic charge density parameter, ( [see detailed calculations given in ref. As an example of the abovementioned approaches, in fig. 1 and 2 the results obtained for the a- 1 + 4 -galacturonic acid dimer are reported. Fig. 1 shows the surface energy map calculated from the non-bonding interactions only, according to the procedures outlined by Brant.17 Fig.2 shows the electrostatic potential well calculated with eqn (1) of ref. (14) for a polygalacturonate chain having a fixed conformation defined by the angles $ and t,v (without added salt). The case of polygalacturonate is particularly interesting since it is evidenced that major changes of the electrostatic energy may almost exclusively derive from chain association. In fact, within the conformational limitations imposed by the glycosidic angles, the charge density parameter, 5, may only vary between 1.57 and 1.66. (1411. Materials The sample of alginate, A, of algal origin was purchased from Fluka (sold as alginic acid). Oligomeric fragments of algal alginate, (man), and (gul),, were gifts of Dr G. Skjik-Brzk, University of Trondheim.A sample of bacterial alginate from Azotobacter vinelandii, obtained from Tate and Lyle and characterized 23 was also used. Pectate was purchased from Sigma (sold as polygalacturonic acid). Purification of theA . Cesaro, F. Delben and S. Paoletti 2577 Table 1. Composition and molecular weights of the polyuronates polyuronate composition (%) molecular weight /dalton (method) algal alginate man, 55; man doublets, 40; 5.2 x lo4 (viscometry) gul, 45; gul doublets, 30 bacterial man, 37; man doublets, 24; 1.3 x lo5 (viscometry) alginate gul, 63; gul doublets, 50; acetyl groups, 8 mannuronate man, 95; (4-6) x lo3 (g.p.c.") guluronate man, 5; (4-6) x lo3 (g.p.c.26) polygalacturonate gal, > 95; esterified gal 2.1 x lo4 (osmometry) gul, 5 gul, 95 and/or neutral sugars, < 5 samples and preparation of the solutions were carried out as described previo~sly.~~ Composition parameters and molecular weights estimated by various methods (osmometry, viscosity or gel-permeation chromatography) are reported in table 1.All salts were used as perchlorates because of the high U.V. transmittance of this anion, when optical measurements have been carried out. Concentration of the salt solutions was determined by complexometric titration by the alkaline EDTA method and proper indicators. All solutions were freshly prepared by using deionized doubly distilled water. Methods Calorimetric and dilatometric experiments were performed using LKB 10700-2 batch- type microcalorimeter and Carlsberg type dilatometers, respectively, as modified by Lindestrarm-Lang.Some of the calorimetric measurements were performed or duplicated with a LKB 10700-1 flow-type microcalorimeter. All the experiments were carried out according to standardized procedures already described elsewhere. 2 4 3 25 Circular dichroism (c.d.) measurements were performed with a Jasco J-500A dichrograph, equipped with a DP-500 data processor. At least four spectra were cumulated after optimization of the cell pathlength, transmittance and c.d. signal parameters. In each experiment the change of the measured property ( i e . AH, AV or [8] was determined for a controlled increase of the concentration of divalent cation. All quantities were normalized to the polyuronate concentration (expressed as equivalent of carboxylate groups per dm3 of solution) and not to the divalent cation concentration, since the actual bound fraction was depending upon all experimental conditions and therefore was not known.Results and Discussion The whole set of results obtained by microcalorimetry and dilatometry is reported in Fig. 3 and 4 show the enthalpy change measured upon mixing dilute polymer solutions with various salt solutions to reach the final salt-to-polymer ratio, R (alginate, fig. 3; polygalacturonate, fig. 4). In all cases, the ionic strength is 0.05 mol dm-3 (as NaC10,). Cations like Zn and Cd perturb the polymeric chain slightly, giving a heat effect which can be accounted for by the electrostatic contribution calculated for the addition of fig. 3-6.2578 Polyuronate-Ion Interactions -1.0 0 0.3 0.6 0.9 1.2 1.5 R Fig.3. Enthalpy of mixing of alginate with divalent cations in aqueous 0.05 mol dmP3 NaCIO,, as a function of R (ratio of moles of ion per monomoles of polymer). 0, Pb; 0, Ca; 0, Zn; 0, Cd; A, Cu; A, oligoguluronate-Cu. I 0 E I I I 1 I 0. 0 -1.5 I I I I 0 0.3 0.6 0.9 1.2 1 R Fig. 4. Enthalpy of mixing of polygalacturonate with divalent cations in aqueous 0.05 mol dm-3 NaClO, as a function of R. Symbols as in fig. 3. divalent salt to the polyelectrolyte system. l4 Similarly, a smooth and positive enthalpy of interaction is obtained upon mixing oligomannuronate with all the divalent cations considered. On the contrary, the enthalpy of mixing Ca and Pb with both alginate and pectate are anomalous with respect to the theoretical prediction (exothermic enthalpy change). Moreover, these enthalpy changes show a sigmoidal trend which cannot be traced back to a Langmuir-type binding.A .Cesaro, F. Delben and S. Paoletti 1 6 1 2579 0 0.1 0.2 0.3 0.4 0.5 0.6 R Fig. 5. Volume change upon mixing alginate with divalent cations. Conditions and symbols as in fig. 3. Enthalpic behaviour of the opposite sign is exhibited in the interaction of Cu with the two classes of polymers. In the case of alginate polymers (both algal and bacterial alginate and guluronate-rich oligomer) a positive AH is exhibited by Cu, in agreement with all other already reported data on the interaction enthalpy with synthetic polycarb~xylates.~~ This trend seems, therefore, to be traced back to the complex formation of Cu with the carboxylate groups.On the contrary, the sign of AH of mixing is exothermic (negative) in the case of the interaction of Cu” ions with polygalacturonate. It is worthwhile to recall that the only stereochemical difference between the homopolymeric sequences of galacturonate and guluronate, respectively, is limited to the orientation of the hydroxy group on carbon 3. At a first glance, the very unusual negative value of AHmix, in the case of pectate, might be ascribed to the particular orientation of OH(3) involved in the complex formation. However, it is speculative to associate this large negative contribution (ca. - 3.6 kca1.F mol-1 Cu) entirely to a specific interaction of Cu2+ with the hydroxy group on C(3) of a single sugar unit. In fact, a positive enthalpy for Cu binding has been found in the case of monomeric galacturonate.27 From all these findings, the picture emerges of a specific contribution of ‘the polymeric chain’ when it is made by an otherwise ‘normally behaving’ ligand. Support for the involvement of the stereochemistry of the dimeric sugar moieties comes out from the conformational calculation. Inspection of the conformational energy map calculated for the dimeric gal-gal showed that in the conformational space available to the dimer two different pairs of the rotational angles give hydrogen bonds between OH(3) and O(5’) and between COOH(6) and OH(2’). One must conclude that a specific polysaccharide conformation can be reached by polygalacturonate upon Cu t 1 cal = 4.184 J.85 FAR I2580 Poly uronate-Ion Interact ions 16 Y 14 12 I - g 10 5 8 m E Q 6 4 2 n "0 0.2 0.4 0.6 0.8 1.0 1.2 R Fig. 6. Volume change upon mixing polygalacturonate with divalent cations. Conditions and symbols as in fig. 3 . binding. The possibility of the hydrogen bond OH(3)-O(5') is ruled out for the near- mirror-image structure of guluronate dimer, because of the axial orientation of OH(3). A similar conclusion can be reached from the structure proposed for the oligo (gu1uronate)-Ca complex in the solid state by Mackie et al.28 Dilatometric results (fig. 5 and 6) always show a positive change of the volume of mixing polysaccharide with ions, in agreement with the general observation that ion-ion interaction occurs with (at least partial) desolvation of the interacting species.The absolute values and the shapes of the dilatometric curves (AVmix us. R) indicates that a strong desolvation seems to occur with Pb and Cu, and that the trend is more regular for polygalacturonate than for alginate. In both cases A V is in the order: Cu = Pb > Cd > Ca. It is worth noting the different behaviour of Zn with respect to pectate and alginate. In fact, while with alginate a little change is observed (ca. 4cm3 per mole of 'nominal bound species'), in the case of pectate the initial lower trend raises up to a A V value of cn. 27 cm3 per bound mole. Volume changes of mixing of polysaccharide with ions have been mainly, if not exclusively, associated with the binding to a specific site. In other words, possible extended modifications in the chain conformation or intrachain interaction are not detectable (or barely detectable) whith dilatometric measure- m e n t ~ .~ * * ~ ~ In this respect, the dilatometric behaviour of the Zn-pectate system is somehow anomalous with respect to the enthalpic behaviour. In all cases, either mere electrostatic (or even site-binding) interactions produce a positive change of entropy, related with the number of water molecules liberated because of the strong desolvation. From the thermodynamic data presented in fig. 3-6, it appears that only some cases indicate a significant cooperativity of the ion-polymer interaction. In particular, both alginate-Ca and alginate -Pb systems appear to be cooperative. while dilatometric and enthalpic data obtained with Cu apparentIy do not exhibit cooperativity at all, not even with pectate.To check this statement, it is necessary to resort to independent (e.g. electrochemical) data and in particular to the spectroscopic perturbation of chromo- phores on binding. The absence of cooperative behaviour of the AH and A V data could in fact stem from one relative insensitivity of such thermodynamic parametersA . Cesaro, F. Delben and S. Paoletti ,20[ loo- 80- A t, m 2 2 60- 40- I 0.05 0.10 0. I5 0.20 r 258 1 Fig. 7. Scatchard plot of the interaction of Cu2+ with alginate (a) and polygalacturonate (b), using the ion-selective electrode data of ref. (30). (and by the large absolute values) with respect to more subtle but complicated eflec t s . The interaction of copper with polyuronates has been studied by ion-selective electrode potentiometry, differential pulse polarography, spectrophotometry and circular dichroism meas~rernents.~~~ 31 It is clear that the uncritical application of a simplistic association model to the experimental activity coefficient data could be misleading if the entire deviation of y (with respect to that of the polymer-free solution containing copper at the same concentration) was referred to an unspecified binding phenomenon.Indeed, a significant amount of this deviation is expected to arise from the interaction of Cu2+ ions with the strong electrical field of the polyelectrolyte chains, even in the absence of specific site binding. This electrostatic effect decreases smoothly with increasing binding on specific (independent) binding sites, unless the local charge density increases abruptly by chain dimerization. Owing to the latter process, in addition to further uptake of ions in the electrostatic domain, other chelating sites with stronger affinities may be formed. Ultimately, the whole process of binding must present some degree of cooperativity with respect to the fraction of ' bound ' ions.Cooperativity of the binding process is found for Cu with most of the polyuronates so far investigated in our laboratory. This conclusion is evident from analysing the experimental e.m.f. data previously r e p ~ r t e d ~ ~ , ~ ~ in the form of a Scatchard plot, as can be seen from the well pronounced upper curvative of the binding data reported in fig. 7. While a detailed report on the experimental potentiometric and polarographic data has been already p ~ b l i s h e d , ~ ' , ~ ~ we also mention here that different binding levels are 85-22582 Poly uronate-Ion Interactions I 200 220 240 260 X/nm Fig.8(a). For description see opposite. obtained by using the free and bound ion concentrations obtained by polarography on one hand, or by ion-selective potentiometry and dialysis on the other. This reflects the fundamental difference in the determination of the ' free species ', obtained as an average of different ionic sub-populations, among which some techniques are apt to discriminate. Evidence for a complex situation during the course of the binding process also comes from the development of the charge-transfer band, as evidenced by U.V.absorption and circular-dichroic The most informative of the cations studied, Pb", revealed interesting features concerning the extrinsic Cotton effect in the U.V. region.15 In particular, the single degenerate band of Pb2+ loses its symmetry, giving rise to (up to) four disymmetric bands originating from the different strengths of interaction of Pb orbitals with the chiral centres (see fig. 8 for some c.d, spectra of the complex). This means that the distances between the Pb ion and the surrounding oxygens are not equal (as indicated by the shift in the wavelength maxima), and that several disymmetric centres are involved. The fact that most of the c.d. bands have opposite sign with near-mirror-image sugars is also relevant : oligo-guluronate and alginate have all the bands opposite in sign with respect to polygalacturonate, except for that centred at ca.240 nm. All these results are interpreted in term of a well defined binding site of (almost) opposite chirality. Moreover, the general conclusions drawn for the cooperativity of the binding process in the case of Cu2+ have an interesting indirect support in the c.d. data of Pb2+. It has beenA . Cesaro, F. Delben and S. Paoletti 2583 - 3 I 1 I 200 220 240 26 0 200 220 240 260 X/nm X/nm Fig. 8. Circular dichroic spectra of polygalacturonate in the presence of increasing amounts of Zn2+ (a), Cd2+ (6) and Pb2+ (c). shown that the intensity of some bands develops linearly upon increasing Pb2+ concentration, while other bands emerge at a higher value of Pb/polymer ratio.It is straightforward to associate this phenomenon to chain dimerization which occurs only after a partial cation binding. In the case of the other ions, it is worth mentioning that Cd2+ and Zn2+ affect the c.d. bands of the polyuronates in a similar way to that already reported for Ca2+. The data obtained with polygalacturonate are reported in fig. 8. For these cations, dilatometric and spectroscopic results suggest an effective perturbation of the solvation sphere around the carboxylic/cationic groups, but always in the range of binding which is classified as electrostatic from the enthalpic point of view. Conclusions Cation binding by polyuronate in solution has to be studied by different approaches capable of discriminating between electrostatic and specific interactions.Micro- calorimetry appears to be very sensitive to the occurrence of polymer conformational processes and is supported by a formal, easily accessible, theory. Dilatometric data on2584 Polyuronate-Ion Interactions mixing ions with polyuronate are easily correlated with the number of water molecules released upon interaction, providing a direct evaluation of the local changes in the solvation spheres. Circular dichroism is structurally more informative, although it is limited to the presence of suitable cations and by the difficulty of model calculations. The authors are indebited to Dr A. Luchetta for carrying out some of the calorimetric and dilatometric measurements. The work has been carried out with the financial support of the Italian Ministry of Public Education (MPI) and of the University of Trieste.References 1 D. A. Rees, E. R. Morris, D. Thom and J. K. Madden, in The Polysaccharides, ed. G. 0. Aspinall 2 A. Haug and 0. Smidsrerd, Acta Chem. Scand., 1970, 24, 843. 3 E. R. Morris, D. A. Rees, D. Thom and J. Boyd, Carbohydr. Rex, 1978, 66, 145. 4 R. Kohn and T. Sticzay, Collect. Czech. Chem. Commun., 1977, 42, 2372. 5 R. Kohn, I. Furda, A. Haug and 0. Smidsrerd, Acta Chem. Scand., 1968, 22, 3098. 6 E. R. Morris, D. A. Rees and D. Thom, J . Chem. SOC., Chem. Commun., 1973, 245. 7 R. Kohn, Pure Appl. Chem., 1975, 42, 371. 8 A. Malovikova and R. Kohn, Collect. Czech. Chem. Commun., 1983, 48, 3154. 9 R. Kohn, Carbohydr. Res., 1987, 160, 343. (Academic Press, New York, 1982), vol. 1, pp.196-290. 10 F. Oosawa, Polyelectrolytes (Marcel Dekker, New York, 1971). 11 G. S . Manning, J. Chem. Phys., 1969, 51, 924. 12 G. S. Manning Acc. Chem. Res., 1979, 12, 443. 13 G. S. Manning, in Ions in Macromolecular and Biological Systems, Colston Papers No. 29, ed. D. H. 14 S. Paoletti, A. Cesiro, F. Delben, V. Crescenzi and R. Rizzo, in Microdomains in Polymer Solutions, 15 A. Cesaro, F. Delben, A. Flaibani and S. Paoletti, Carbohydr. Res., submitted. 16 S. J. Angyal, Chem. Soc. Rev., 1980, 9, 415. 17 D. A. Brant, in Biochemistry of Plants, ed. J. Preiss (Academic Press, New York, 1980), vol. 3, 18 A. Cesaro, F. Delben, A. Flaibani and S. Paoletti, Carbohydr. Res., 1987, 160, 355. 19 A. Cesaro, S. Paoletti, R. Urbani and J. Benegas, to be submitted. 20 A. Cesaro, in Thermodynamic Data for Bi0chemistr.y and Biotechnology, ed. H-J. Hinz (Springer, 21 G. S. Manning and S. Paoletti, in Industrial Pol.vsaccharides, ed. S. S. Stivala, V. Crescenzi and I. C. M. 22 S. Paoletti, A. Cesiro, A. Ciana, F. Delben, G. Manzini and V. Crescenzi, in Solution Properties of 23 F. Delben, A. Cesaro, S. Paoletti and V. Crescenzi, Carbohydr. Res., 1982, 100, c46. 24 A. Cesaro, A. Ciana, F. Delben, G. Manzirj and S. Paoletti, Biopolymers. 1982, 21, 431. 25 V. Crescenzi, F. Delben, S. Paoletti and J. Skerjanc, J . Phys. Chem., 1974, 78, 607. 26 H. Grasdalen, B. Larsen and 0. Smidsrerd, Carbohydr. Res., 1981, 89, 179. 27 R. Aruga, Bull. Chem. Soc. Jpn, 1981, 54, 1233. 28 W. Mackie, S. Perez, R. Rizzo, F. Tardvel and M. Vignon, Int. J . Biol. Macromol., 1983, 5, 329. 29 J. C. Fenyo, F. Delben, S. Paoletti and V. Crescenzi, J . Phys. Chem., 1977, 81, 1900. 30 G. Manzini, A. Cesaro, F. Delben, S. Paoletti and E. Reisenhofer, Bioelectrochem. Bioenergetics, 1984, 12, 443. 31 E. Reisenhofer, A. Cesaro, F. Delben, G. Manzini and S. Paoletti, Bioelectrochem. Bioenergetics, 1984, 12, 455. Everett and B. Vincent (Scientechnica, Bristol, 1978), p. 157. ed. P. Dubin (Plenum Press, New York, 1985), pp. 159-189. chap. 1 1 . Heidelberg, 1986), pp. 177-207. Dea (Gordon and Breech, New York, 1987), pp. 305-324. Polysacchurides, ed. D. A. Brant (A. C. S. Symp. Ser., 1981), pp. 379-386. Paper 7/2134; Received 3rd December, 1987

ISSN:0300-9599    

DOI:10.1039/F19888402573

出版商:RSC    

年代:1988

数据来源: RSC