摘要:

The synthesis of (E)-1-methoxy-2-azabuta-1,3-diene-4-carbonitriles was performed by methylation of N-alkenylamides9and11. TheZisomers were obtained by treatment of (E)-1-methylthio-2-azabuta-1,3-diene-4,4-dicarbonitriles with sodium methoxide in methanol. We also describe the reactions of (E)-1-methylthio-2-azabuta-1,3-diene-4,4-dicarbonitriles with pyrrolidine, which afforded 1-(1-pyrrolidinyl) derivatives20,21, and23. X-ray crystallographic analyses of21and23established theEstereochemistry of the C—N double bond.Keywords: 2-azabuta-1,3-diene-4-carbonitriles: stereoselective synthesis, nucleophilic substitution and X-ray diffraction; N-alkenylamides: methylation.

ISSN:0008-4042    

DOI:10.1139/v96-032

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The influence of pH (3.0–8.0), ionic strength (0.005–0.10 M), and concentration (ca. 50–300 mg L−1) on the viscosimetric behaviour of solutions of a soil fulvic acid has been determined. The results show a complex influence of the concentration on the specific viscosity with maxima at about 80–100 mg L−1and a minimum at 140–160 mg L−1. The viscosity is minimum at pH ca. 6 and decreases when the ionic strength increases. The results are interpreted by a rod-type model from which the dimensions of fulvic acid molecules are calculated.Keywords: soil fulvic acids, viscosity, configurational changes.

ISSN:0008-4042    

DOI:10.1139/v96-033

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The values of the rate constants of the reactions of es−with the efficient scavengers nitrobenzene and acetone are ≥ 2 × 106 m3 mol−1 s−1in the whole range of 1-butylamine–water mixtures at 298 K; the reaction rates in the mixed solvents vary approximately as the solvent fluidity. In pure butylamine at 298 K,k2(es− + nitrobenzene) = 84 × 106 m3 mol−1 s−1andk2(es− + acetone) = 7.3 × 106 m3 mol−1 s−1. The values of the rate constants of the reactions of es−with the inefficient scavengers phenol and toluene are < 2 × 105 m3 mol−1 s−1in the whole range of 1-butylamine–water mixtures at 298 K and have a maximum at 50 mol% water and a minimum at 99 mol% water. In pure 1-butylamine at 298 K,k2(es− + phenol) = 1.0 × 104 m3 mol−1 s−1andk2(es− + toluene) = 0.28 × 104 m3 mol−1 s−1. The reaction rates with inefficient scavengers show strong dependence on the solvent composition and selective solvation of electron and scavenger. In the amine-rich region (0–30 mol% water), the rate constants increase with theincreaseof viscosity, indicating the chemical participation of solvent molecules in the reaction. In the water-rich region from 50 to 99 mol% water, the decrease of the rate constants indicates the nonhomogeneous solvation of the electrons by water and of the organic solutes by 1-butylamine. From 99 mol% to pure water the rate constant increases rapidly, which we attribute to insufficient 1-butylamine to coat the phenol or toluene molecules. The variation of the activation energiesE2for the efficient scavengers, 14–27 kJ mol−1, are similar to the variation ofEηin the mixed solvents. The values ofE2for the inefficient scavengers are from 15 to 38 kJ mol−1for phenol and from 6 to 21 kJ mol−1for toluene. Bothk2andE2for the inefficient scavenger reactions show a correlation with the temperature coefficient −dEAmax/dTof the optical absorption of es−in the mixed solvents, but the reason is obscure.Keywords: 1-butylamine–water solvent, solvated electron, organic solutes, reactivity, solvent effects.

ISSN:0008-4042    

DOI:10.1139/v96-034

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Nucleophilic aromatic substitution reactions of 3-nitrophthalonitrile yield 3-hydroxyphthalonitrile and 3-neopentoxyphthalonitrile, the latter of which condensed to 1,8,15,22-tetraneopentoxyphthalocyanine as a mixture of isomers. Bisphthalonitriles such as 1,3-bis(2′,3′-dicyanophenoxy)-2,2-dipentylpropane, 1,3-bis(2′,3′-dicyanophenoxy)-2,2-diethylpropane, 1,3-bis(2′,3′-dicyanophenoxy)-2,2-dioctylpropane, and 1,3-bis(2′,3′-dicyanophenoxy)-2-methyl-2-trityloxymethylpropane all gave bis-crown-like 1,11,15,25-tetrasubstituted phthalocyanines as pure compounds when treated with lithium octoxide in 1-octanol at 196 °C. A host of nine other bisphthalonitriles including 1,5-bis(2′,3′-dicyanophenoxy)-3-oxapentane, 1,1-bis(2′,3′-dicyanophenoxymethyl)cyclohexane, 1,2-bis(2′,3′-dicyanophenoxymethyl)benzene, and 2,5-bis(2′,3′-dicyanophenoxymethyl)furan did not dimerize to mononuclear phthalocynaines. The "gemdimethyl" effect was suggested as a reason for the successful macrocyclizations.Keywords: nucleophilic aromatic substitution, phthalonitriles, bisphthalonitriles, 1,11,15,25-tetrasubstituted phthalocyanines.

ISSN:0008-4042    

DOI:10.1139/v96-035

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

A recently developed technique for the probing of the combining sites of lectins and antibodies, to establish the structure of the epitope that is involved in the binding of an oligosaccharide, is used to study the binding of methyl α-isomaltoside by the enzyme glucoamylase. The procedure involved the determination of the effects on the kinetics of hydrolysis of both monodeoxygenation and mono-O-methylation at each of the seven hydroxyl groups in order to gain an estimate of the differential changes in the free energies of activation, ΔΔG≠. As expected, from previous publications, both deoxygenation andO-methylation of OH-4 (reducing unit), OH-4′, or OH-6′ strongly hindered hydrolysis, whereas the kinetics were virtually unaffected by either the substitutions at OH-2 or structural changes at C-1. The substitutions at OH-3 caused increases of 2.1 and 1.9 kcal/mol in the ΔΔG≠. In contrast, whereas deoxygenation of either OH-2′ or OH-3′ caused much smaller (0.96 and 0.52 kcal/mol) increases in ΔΔG≠, the mono-O-methylations resulted in severe steric hindrance to the formation of the activated complex. The relatively weak effects of deoxygenation suggest that the hydroxyl groups are replaced by water molecules and thereby participate in the binding by contributing effective complementarity. Methyl α-isomaltoside was docked into the combining site of the X-ray crystal structure at 2.4 Å resolution of the complex with the inhibitor acarbose. A fit free of steric interactions with the protein was found that has the methyl α-glucopyranoside unit in the normal4C1conformation and the other glucose unit approaching a half-chair conformation with the interunit fragment defined by the torsion anglesThe model provides a network of hydrogen bonds that appears to well represent the activated complex formed by the glucoamylase with both maltose and isomaltose since the structures appear to provide a sound rationale for both the specificity and catalysis provided by the enzyme.Keywords: monodeoxy and mono-O-methyl derivatives of methyl α-isomaltoside, enzyme binding domain, functioning of glucoamylase, differential changes in free energy of activation, characteristics of hydrogen bonding networks.

ISSN:0008-4042    

DOI:10.1139/v96-036

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The formation of singlet dioxygen (1O2) in the reaction between the doublet excited states of tris(bipyridine)chromium(III) and triplet dioxygen in deuterium oxide was detected using near-infrared luminescence corresponding to singlet dioxygen decay. In addition, luminescence intensity measurements were used to determine the quantum yield of singlet dioxygen produced. A value of 0.86 ± 0.08 was obtained for the yield of1O2. Based on these quantum yield measurements, it is proposed that the major quenching mode is energy transfer quenching. However, considering that the1O2yield is less than unity and electron transfer quenching is not a viable pathway, it is proposed that, in addition to the energy transfer quenching, physical quenching involving spin-catalyzed deactivation of Cr(bpy)33+may also be operative as a minor quenching pathway.Keywords: Cr(bpy)33+, singlet dioxygen, quenching, quantum yield.

ISSN:0008-4042    

DOI:10.1139/v96-037

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

An improved method for the synthesis of dithioesters of amino acids and peptides has been developed. The syntheses have been carried out from the nitriles. The addition of thiol to the nitrile derivative in the Pinner step of dithioester synthesis was activated with hydrogen fluoride. A few examples of dithioester synthesis using liquid HF are described. Some novel dithioesters, which are model compounds for resonance Raman spectroscopic studies of dithioacylpapain intermediates, are described.Keywords: dithioesters, amino acids, Pinner reaction, HF, isotopes.

ISSN:0008-4042    

DOI:10.1139/v96-038

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Complete characterization of the thickness – shear mode (TSM) acoustic wave sensor coated with films of nucleic acid and subjected to conductive solution loading is achieved through network analysis of impedance measurements. The responses of coated sensors with respect to series resonant frequency can be separated into two regions. When 0 < σ < 0.5Ω−1 m−1the frequencies for sensors with films of single strand (ss) DNA or polycytidylic (5′) (Poly C) in place rise significantly with increasing conductivity. The motional resistances for sensors coated with both species peak at the same value of conductivity where the frequency has the most rapid shift. In the second region (0.5 < σ < 12Ω−1 m−1), the frequency response for the ss DNA system exhibits a bell-shaped curve, with a minimum at σ = 4Ω−1 m−1. The motional resistances for both nucleic acid layers show reversible behaviour, whereas TSM sensors with bare PdO and Au electrodes yield little change. Models that consider only the bulk properties of solutions fail to explain the observed changes in frequency. On the other hand, an equivalent circuit model incorporating the rheological properties of nucleic acid layers on the sensor surface is successful in explaining the various responses.Keywords: DNA sensor, network analysis, thickness – shear mode sensor, DNA interfacial properties.

ISSN:0008-4042    

DOI:10.1139/v96-039

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The acid–base properties of phthalic acid (H2L) have been determined by simultaneous spectroscopic, potentiometric, and polarographic measurements. Stability constants of the complexes CdHL+, CdL, CdL22−, CdL34−, CdH−2L24−and their distribution versus pH were determined at 25 °C and ionic strength in 0.1 M NaNO3. The advantages of each method are pointed out in such a way as to describe more precisely the interactions between cadmium and phthalic acid over a wide range of ligand and metal concentrations. The crystal structure of the [CdL] complex has been established by single-crystal X-ray diffraction. Cd(C8H4O4)•H2O is orthorhombic, space groupPbca,a = 23.463(5),b = 9.293(2),c = 7.730(1) Å,Z = 8, and ρ = 2.323 g cm−3. The structure was refined to a finalRvalue of 0.054 for 3231 unique reflections withI > 3σ(I). The crystal is a three-dimensional polymeric network in which the Cd atom is seven-coordinated. The first ligand coordinates in a bidentate carboxylate (O,O′) mode (Cd—O = 2.372(5) and 2.445(5) Å); a second ligand is part of a seven-membered ring formed with its two carboxylate groups (Cd—O = 2.317(5) and 2.341(5) Å); two other phthalic ligands make one coordination bond (Cd—O = 2.255(5) and 2.449(5) Å) while the seventh bond is a water molecule (Cd—O = 2.333(6) Å).Keywords: crystal structure, cadmium complexes, phthalic acid complexes.

ISSN:0008-4042    

DOI:10.1139/v96-040

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The reaction of acridine with S(IV) species (SO2•H2O, HSO3−, and SO32−) to form the adduct acridine–S(IV) has been studied spectrophotometrically throughout the pH range 2.6–8 in aqueous solutions. The observed pseudo-first-order rate constants,kobs, were determined at 25 °C and ionic strengthI = 0.11 M, and the pH profile of the rate reached a maximum at pH ≈ 6.1. At constant pH thekobsvalues were a linear function of the total S(IV) concentration with slopes that increased significantly with pH. These data are consistent with the rate-determining attack of SO3H−and SO32−upon the C-9 of the acridinium cation. A nonlinear least-squares fitting of the experimental values to the model equation, within the overall pH region studied, yields the pH-independent rate constantsk1 = 3.7 ± 0.1 andk2 = (6.24 ± 0.04) × 104 M−1s−1for the attack of these two species, respectively. The experimental results agree very well with the kinetic model. Due to the experimental conditions used we did not detect any possible pseudobase formation in the pH range studied. The reactivity of the S(IV) species with acridine follows the order:The value obtained for the ratiok1/k2is similar to the results given for other addition reactions of S(IV) species to the double bond of carbonyl compounds such as benzaldehyde and formaldehyde.Keywords: covalent addition, acridine, acridine – S(IV) adducts, kinetics and mechanis

ISSN:0008-4042    

DOI:10.1139/v96-041

出版商:NRC Research Press    

年代:1996

数据来源: NRC