摘要:

AbstractThe conformation of poly(α‐L‐aspartic acid) was investigated on a sample in which β‐bonds were not detected. CD and ir spectroscopy showed that poly(α‐L‐aspartic acid) passes through a conformational change induced by changes of the degree of ionization that is accompanied by precipitation; the precipitate is probably highly helical. The change was also detected by potentiometr

ISSN:0006-3525    

DOI:10.1002/bip.360210602

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1982

数据来源: WILEY

摘要:

AbstractA strict analytical theory has been developed describing the behavior of a model lattice polymer chain of arbitrary stiffness in a slitlike pore at polymer–adsorbent interaction energies –ε. The thermodynamic characteristics of the system were calculated. It was shown that the transition of the macromolecule from the solution volume inside a pore occurs by the first‐order phase transition with evolution of latent heat of adsorption. The transition point –ε = –εcis determined by the chain stiffness and is independent of the pore widthD. It is shown that in the precritical range, –ε<–εc, the free energy ΔFof the macromolecules in the pores is adequately described by the universal dependence ΔF= ΔF(D*/A), whereD* is some effective pore width depending on the value of –ε, andAis the length of the Kuhn segment. At high attraction energies, –ε ≫ –εc, the macromolecules are bonded to the pore walls by a great number of units and their free energy depends only on –ε and the chain stiffness, ΔF= ΔF(A, ε). Close to the critical energy –ε ≃ –εc(transition range), ΔFis determined by both the stiffness of the macromolecule and the pore widthD: ΔF∼A2D−1for fairly high values ofAandD. The possibilities of using porous media as protein stabilizers are discussed, and the value of the stabilizing

ISSN:0006-3525    

DOI:10.1002/bip.360210603

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1982

数据来源: WILEY

摘要:

AbstractThe diameters of the fibrin fibers in a clot are not uniform. Morphometric analysis of transmission electron micrographs show a bimodal distribution. The effect of polydispersity of fiber diameter on mass–length ratio calculated from the turbidity and permeability of a clot has been investigated with the aid of a two network model, the networks being called Major and Minor. The fibers in the Major network are many times thicker than those in the Minor network. In a model in which Major network fibers are 10 times thicker than fibers of the Minor network, the fibers in the Minor network make a negligible contribution to the turbidity of the clot. However, they may have a marked effect on its permeability. Experiments with clots made from human fibrinogen show that the Minor network is stabilized by α‐polymer and γ‐γ linkages and that without such linkages it is washed away during permeation. It remains relatively intact in crosslinked clots. In agreement with the theoretical model, when mass–length ratios calculated from the turbidity are compared with those calculated from the permeability, the latter were reduced in crosslinked clots with an intact Min

ISSN:0006-3525    

DOI:10.1002/bip.360210604

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1982

数据来源: WILEY

摘要:

AbstractA formalism is presented to represent the hydrodynamic and diffusion behavior of segmentally flexible macromolecules with a high degree of flexibility. The development specifically treats open assemblies of cylindrically symmetric segments joined at their end points by frictionless swivels. Three‐dimensional diffusion and frictional tensors that govern the rotations of each segment and the translations of some chosen swivel are established and related by a Stokes–Einstein relation. The dependence of these tensors on the choice of swivel is examined, and simplified expressions are obtained by omitting hydrodynamic interactions between segments. These simplified expressions are used to analyze the rotational behavior of a Y‐shaped body in detail. If there is complete flexibility, the rotational motions of each segment reduce, for all practical purposes, to those of a cylindrically symmetric rigid body. Estimates are provided to relate segment rotations to internal motions and restrictions on flexibility. A procedural framework for generating rotational diffusion coefficients for segments of more complicated bodies is applied to a four‐segment two‐swivel myosin model. Comparison of calculated and experimental relaxation times for myosin and its fragments indicates the head–tail junction is highly flexible, with at least a ±60° angular extent of free bending between heads allowed, while the tail is neither straight and rigid nor hi

ISSN:0006-3525    

DOI:10.1002/bip.360210605

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1982

数据来源: WILEY

摘要:

AbstractWe studied the kinetics of the heat denaturation (at 50°C) of met‐hemoglobin in the presence of various monohydric alcohols. The denaturation rate was slowed by the presence of small concentrations of methanol and ethanol; in all the other cases, i.e., at high concentrations of methanol and ethanol, and in the whole concentration range studied foriso‐ andn‐propanol, an increase in the denaturation rate was observed. Following a procedure already applied to the study of the effects of the same alcohols on the reaction of hemoglobin with oxygen, we separated the overall observed effects into contributions related to the variation of the bulk dielectric constant of the medium (bulk‐electrostatic contributions) and contributions not related to variations in this parameter (nonbulk‐electrostatic contributions). For all the alcohols, we found a unique correlation law connecting the above nonbulk‐electrostatic contributions with the analogous ones previously reported for the T → R transition of hemoglobin. This fact strongly supports the validity of the procedure used and suggests that nonbulk‐electrostatic contributions, relative to these two different processes, have a common background and are univocally determined by the extent of the “solvent perturbation” imposed by the presence of the p

ISSN:0006-3525    

DOI:10.1002/bip.360210606

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1982

数据来源: WILEY

摘要:

Abstract2,2,′,2″,2‴‐Tetrapyridineiron(III) complex ions anchored to poly(L‐glutamate) (FeL) or poly(D‐glutamate) (FeD) were used as catalysts for the H2O2oxidation ofL(+)‐ascorbic acid at pH 7 and varying complex:polymer‐residue molar ratios [C]/[P]. Evidence is produced that the reaction is a composite process reflecting contributions from parallel pathways, one of which corresponds to a catalytic route and is [H2O2]‐independent and the other to an uncatalyzed electron‐transfer process between the ascorbate anion and hydrogen peroxide. Stereospecific effects in the catalysis are observed on increasing the complex:polymer ratio, which corresponds to an increase of the amount of α‐helical fraction in the polypeptide supports (xa). Thus, at [C]/[P] = 0.01 (xa<0.05),kFeD/kFeL= 1.0; but at [C]/[P] = 0.20 (xa≈ 0.70),kFeD/kFeL= 4.0 ± 0.5,kFeDandkFeLbeing the second‐order rate constants of the electron‐transfer reaction between the FeD or FeL isomer of the asymmetric catalyst and theL‐ascorbate anion. The activation energies were found to increase markedly on going from the former to the latter complex:polymer ratio but, at the same time, to exhibit equal values with both enantiomeric catalysts. Stereoselectivity therefore appears to be an entropy‐controlled process, arising from the conformational rigidity of the precursor complex, which very likely sees the substrate molecules bound to the chiral residues of the ordered polymer surrounding the active sites. The implications of the stereochemical features of the substrate–catalyst adduct on the mechanism of electron transfer are also discussed. Evidence is presented that the asymmetric [Fe(tetpy)(OH)2]+–polyelectrolyte systems play the additional role of environmental controller of the uncataly

ISSN:0006-3525    

DOI:10.1002/bip.360210607

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1982

数据来源: WILEY

摘要:

AbstractAmino acids are known to differ in their individual preferences for each of the four positions of the β‐turn conformation formed by tetrapeptide segments. Proline and glycine show relatively high preferences for positions 2 and 3, respectively, of the β‐turn. Using tripeptides of the typeN‐acetyl‐Pro‐Gly‐X‐OH, whereX= Gly, Ala, Leu, Ile, and Phe, we have sought to study the influence of the 4th residueXon the stability of the β‐turn conformation in these tripeptides. Our nmr and CD results show that the β‐turn stability is quite significantly governed by the nature of the amino acid residue at this position in the following order:

ISSN:0006-3525    

DOI:10.1002/bip.360210608

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1982

数据来源: WILEY

摘要:

AbstractThe effects of pressure on the composition density distribution in a binary density gradient at sedimentation equilibrium in the analytical ultracentrifuge are rigorously examined. A computer algorithm is described for the necessary iterative computations. The pressure effect is found to be significant in runs where a long column length, high angular velocity, and a salt with a high pressure correction coefficient are simultaneously employed. Such conditions are sometimes encountered in current studies in which high precision is required to measure the compressibility of proteins.

ISSN:0006-3525    

DOI:10.1002/bip.360210609

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1982

数据来源: WILEY

摘要:

AbstractSix different sequential polypeptides with the repeating unitsL‐lysyl‐L‐DOPA,L‐DOPA‐L‐lysine,L‐lysyl‐L‐lysyl‐L‐DOPA,L‐DOPA‐L‐DOPA‐L‐lysine,L‐lysyl‐L‐lysyl‐L‐lysyl‐L‐DOPA, andL‐DOPA‐L‐DOPA‐L‐DOPA‐L‐lysine have been synthesized by solution polymerization of thep‐nitrophyenyl esters of the corresponding di‐, tri‐, and tetrapeptides. TheO,O′‐dimethyl andN‐ε‐2‐chlorobenzyloxycarbonyl groups were used to protect side chains ofL‐DOPA andL‐lysine. The monomers for the polytripeptides and polytetrapeptides were prepared by stepwise elongation, using the dicyclohexylcarbodiimide coupling method. Moderately high molecular weight sequential polypeptides were obtained. The addition of 1‐hydroxybenzotriazole increased their molecular weights, but not so effectively. The protected groups of the side chains were removed simultaneously by use of boron tribromide i

ISSN:0006-3525    

DOI:10.1002/bip.360210610

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1982

数据来源: WILEY

摘要:

AbstractThermodynamic studies of the binding of adamantanecarboxylate to cyclodextrins have been made as a function of temperature and added organic cosolvent (methanol) using flow microcalorimetry. The negative heat capacity change associated with the adamantanecar‐boxylate/β‐cyclodextrin interaction and the fact that the interaction is weakened by the addition of methanol implicate the binding process as being a hydrophobically driven one. The negative enthalpy change (ΔH0= −5.5 kcal/mol) and near‐zero entropy change (ΔS0= 1.5 cal/mol deg) are quite different from the values normally expected for a hydrophobic bond, indicating that other bonding forces are important in addition to the hydrophobic effect. The relative contribution of the hydrophobic effect and other bonding forces (most likely van der Waals forces) to the overall binding was judged from an analysis of the dependence of the thermodynamics of the association process on the surface tension of the water–methanol mixtures following a model for “solvophobic” bonding described by Sinanoglu [Molecular Associations in Biology(1968) Academic Press, New York, pp. 427–445]. From this analysis, adamantane–carboxylate/cyclodextrin complex formation is found to be driven to the extent of −1.9 kcal/mol by the hydrophobic effect. Furthermore, the hydrophobic driving force is found to be characterized by a positive ΔS0of 10 cal/mol deg. The remaining free energy of binding (and the ΔH0of binding of ∼−6 kcal/mol) is then due to the intrinsic (surface‐tension‐independent) van der Waals interaction between th

ISSN:0006-3525    

DOI:10.1002/bip.360210611

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1982

数据来源: WILEY