摘要:

The stabilization effect of the Me group with respect to H has been determined for peptides of the type HCO-(NH-CHR-CO)n-NH2for 1 ≤ n ≤ 3. It was found that for the Me group the stabilization energy was in the vicinity of 5.4 kcal/mol. The site of substitution had no appreciable effect on the stabilization energy. The stabilization energy for monomethyl substitution (R=CH3) ranged from 5.21 to 5.60 kcal/mol. For dimethyl substitution the values were between 10.53 and 10.81 kcal/mol, and for trimethyl substitution it was 15.99 kcal/mol.Keywords: stabilization effects of substituents, oligopeptide conformations, ab initio study on HCO-(NHCHRCO)n-NH2.

ISSN:0008-4042    

DOI:10.1139/v95-194

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The properties and application of the median filter are investigated using both simulated and real data for signals evolving with time. Comparison is made with existing numerical techniques for drift compensation and noise reduction in analytical measurements such as the moving average and Savitzky–Golay digital filters. The median filter provides a means for dealing with "spiky" noise and separating peaks from a slowly changing baseline, even when the exact nature of the drift and noise distribution is not known. Median filtering is a useful and complementary addition to existing digital filtering techniques, being mathematically robust and readily implemented on any computer platform.Keywords: median filtering, signal noise, signal processing, digital filters, chemometrics, data smoothing.

ISSN:0008-4042    

DOI:10.1139/v95-195

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The mixture of hexoses and carboxylic acids produced by alkaline degradation of fructose was analyzed quantitatively by high-performance liquid chromatography. A pulsed amperometric detector with two Carbopac Dionex columns in series was employed in the assay of sugars (glucose, fructose, mannose, sorbose, and psicose), while a conductimeter with an HPICE AS1 Dionex column was employed for carboxylic acids (lactic, glyceric, glycolic, formic, and acetic). This sensitive and reproducible method obviated sample pretreatment. The results were compared with those of enzymatic assays and HPLC determinations using Biorad HPX87C and HPX87P columns in series for the sugars, and an Interaction ORH 801 column for the carboxylic acids. The behaviour of fructose, glucose, and mannose in alkaline medium was compared.Keywords: hexoses, polyhydroxylic acids, isomerization, alkaline degradation, high-performance ionic liquid chromatography.

ISSN:0008-4042    

DOI:10.1139/v95-196

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The crystal structure of bright-red crystals of the bidimensional polynuclear diamagnetic (µ-chloro)(µ-pyrazine)copper(I) complex has been analyzed from X-ray diffraction data. The structure was solved by the heavy-atom method. Full-matrix least-squares refinement based on 673 reflections withF > 4σ(F) converged to a finalR = 0.043 andRw = 0.048. Crystals of this complex are monoclinic, space groupP2/c,a = 3.814(1),b = 6.356(1) andc = 11.497(5) Å; β = 96.04(2)°,Z = 2. The structure consists of planes of copper(I) atoms linked by pyrazine and Cl−bridges, each Cu(I) being in a distorted tetrahedral environment formed by two chloride ions and two nitrogen atoms belonging to two pyrazine molecules, with bond distances of Cu—Cl = 2.349(1) and Cu—N = 2.038(4) Å. Vibrational and electronic spectroscopy of the title compound is also reported.Keywords: two-dimensional copper(I)–pyrazine complexes.

ISSN:0008-4042    

DOI:10.1139/v95-197

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The thermodynamic behavior of NiCl2in NiCl2–CoCl2–KCl ternary solutions was investigated using formation cells of the type, (−) Ni/NiCl2–CoCl2–KCl/C, Cl2(gas, 1 atm)(+). The values of the NiCl2activity coefficients indicate a non-ideal system consonant with the formation of complex species. The NiCl2activity coefficients at 1000 K were fitted to the polynomial equation,whereThe experimental data were limited to the ranges 0 ≤ t ≤ 0.80 and 0.10 ≤ y ≤ 0.5.Keywords: molten salt, thermodynamic properties, nickel chloride, non-ideal solution, complex species.

ISSN:0008-4042    

DOI:10.1139/v95-198

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The O-chain polysaccharide produced byProteusmirabilisstrain 7570 was shown by chemical analysis and 1D and 2D NMR spectroscopy to be a high molecular weight acidic linear polymer of tetrasaccharide repeating units, composed ofD-galacturonic acid, 2-acetamido-2-deoxy-D-galactose, and 2-acetamido-2-deoxy-D-glucose (2:1:1). In addition, native O-chain was randomly substituted by O-acetyl groups.Keywords:Proteus, lipopolysaccharide, O-chain, structure, NMR.

ISSN:0008-4042    

DOI:10.1139/v95-199

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

In characterizing the UV–visible absorption properties of a series of seven anilino class squaraine dyes in dimethyl sulfoxide (DMSO) – water mixtures, spectral features characteristic of aggregation were observed. These included hypsochromic and hypochromic shifts of the absorption maximum, relative to the nonassociated state of these dyes. Previously aggregation of this class of squaraine dyes had only been reported in studies of Langmuir–Blodgett (LB) films prepared with squaraine dyes. In the present study two distinctly different and novel solution dye aggregates, designated as type A and type B, were identified on the basis of their characteristic absorption properties. The type A aggregate was characterized by an absorption maximum that was hypsochromically and hypochromically shifted, in comparison with that of the nonassociated dyes, and had a secondary absorption occurring at approximately the same wavelength at which nonassociated squaraine dyes absorb. These spectral features are believed to be the result of a "face-to-face" alignment of molecules in the aggregate. The type B aggregate featured a broad absorption typically from 525 to 700 nm. Based on similarities between the spectral features of type B aggregates and LB films of surfactant squaraines, the alignment of molecules in the solution aggregate was believed to be such that the electron-deficient squarylium moieties interacted with the electron-rich dialkylamino phenyl moieties. Certain squaraine dyes that were investigated as part of this study were found to form both type A and type B aggregates, depending upon the composition of the DMSO–water medium. Typically, the type A aggregate formed preferentially in media of intermediate DMSO content (50–70% DMSO) and the type B aggregate in more water-rich media (<20% DMSO). In the intervening region it was possible to monitor spectroscopically a dynamic conversion from the type B aggregate to the type A aggregate. An examination of structure–property relationships indicated that dyes with less hydrophobic N-alkyl substituents formed only type A aggregates. Squaraine dyes with more hydrophobic N-alkyl substituents were found to form type B aggregates in water and type A aggregates in some DMSO–water mixtures. A model of squaraine aggregation was proposed in which the type A squaraine aggregate was "thermodynamically" preferred while the type B aggregate was "kinetically" preferred. The stability of the type B aggregate was proposed to be enhanced by increasing hydrophobicity of the N-alkyl substituents and decreased by increasing amounts of DMSO in the solvent system.Keywords: squaraine dyes, aggregation, crystallochromy, solvent effects.

ISSN:0008-4042    

DOI:10.1139/v95-200

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

A general synthetic route to 2-alkyl- and 2,2-dialkyl-1,3-dioxan-5-ones, using tris(hydroxymethyl)-nitromethane as the starting material, is described. Deprotonation of these compounds was studied. It was established that these dioxanones could be deprotonated with LDA; however, the reduction of the carbonyl group via a hydride transfer from LDA, giving the corresponding dioxanols, often competed with deprotonation. The reduction could be minimized by using Corey's internal quench procedure to form silyl enol ethers and was less pronounced in 2,2-dialkyldioxanones (ketals) than in 2-alkyldioxanones (acetals). Self-aldol products were observed when dioxanone lithium enolates were quenched with H2O. Addition reactions of lithium enolates of dioxanones to aldehydes werethreo-selective as predicted by the Zimmerman–Traxler model. Dioxanones having two different alkyl groups at the 2-position were deprotonated enantioselectively by chiral lithium amide bases with enantiomeric excess (ee) of up to 70%.Keywords: 1,3-dioxan-5-ones, enantioselective deprotonation, chiral lithium amides.

ISSN:0008-4042    

DOI:10.1139/v95-201

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The rearrangement of organic thionitrate to sulfenyl nitrite potentially mediates the release of nitric oxide from organic nitrates, such as nitroglycerin, in the presence of thiol. The biological activity of these nitrovasodilators is proposed to result from release of nitric oxide in vivo. The thionitrate rearrangement bears analogy to the rearrangement of peroxynitrous acid to nitric acid, which has been proposed to mediate the biological toxicity of nitric oxide and superoxide. In this paper, the two concerted rearrangement processes and competing homolytic reactions are explored using molecular orbital calculations at levels up to MP4SDQ/6-31G*//MP2/6-31G*. Examination of structure and energy for all conformers and isomers of RSONO2(R = H, Me), models for organic thionitrates and their isomers, demonstrates that structures corresponding to thionitrates and sulfenyl nitrates are of similar energy. Free energies of reaction for homolysis of these compounds are low (ΔG0 < 19 kcal/mol), whereas the barrier for concerted rearrangement is large (ΔG≠(aq.) = 56 kcal/mol). The larger barrier for concerted rearrangement of peroxynitrous acid to nitric acid (ΔG≠(aq.) = 60 kcal/mol) again compares unfavourably with homolysis (ΔG0 < 11 kcal/mol for homolysis to NO2or•NO). The transition state structures, confirmed by normal mode and intrinsic reaction coordinate analysis, indicate that considerable structural reorganization is required for concerted rearrangement of the ground state species. These results suggest that concerted rearrangement is not likely to be a viable step in either biological process. However, rearrangement via homolysis and radical recombination may provide an energetically accessible pathway for peroxynitrous acid rearrangement to nitric acid and rearrangement of thionitrate to sulfenyl nitrite. In this case, NO2will be a primary product of both reactions.Keywords: thionitrate, nitric oxide, peroxynitrite, nitrovasodilator, nitrate.

ISSN:0008-4042    

DOI:10.1139/v95-202

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

As part of continuing studies on the structural features responsible for the intense sweetness ofD-fructose, 3-deoxy-D-erythro-hexulose (3-deoxy-D-fructose,1) was prepared, its solution composition was determined, and its taste was evaluated. In aqueous solution, 3-deoxy-D-fructose exists as a complex mixture of five tautomeric forms in which the β-D-pyranose form is preponderant (52.5% at 22 °C) and the α-D-pyranose form is the least abundant (5%). Quite remarkable is the behavior of the open-chainketoform of1: its content increases from 7.5% at 22 °C, to 36% at 82 °C, and to 47% at 97 °C, making this form the preponderant one at high temperatures. 3-Deoxy-D-fructose was found to be sweet, albeit probably not as sweet asD-fructose. The hydroxyl group at C-3 is thus not an essential function of the glycophore ofD-fructose. The significance of this result is discussed in relation to the evidence already available and the divergent theories that have been proposed to explain the origin of the sweet taste ofD-fructose.Keywords: 3-deoxy-D-erythro-hexulose (3-deoxy-D-fructose), sweetness, solution composition.

ISSN:0008-4042    

DOI:10.1139/v95-203

出版商:NRC Research Press    

年代:1995

数据来源: NRC