Showing papers on "Pyranose published in 1979"

Flexibility of the pyranose ring in α‐ and β‐D‐glucoses

Abstract: Conformational energies of α- and β-D-glucopyranoses were computed by varying all the ring bond angles and torsional angles using semiempirical potential functions. Solvent accessibility calculations were also performed to obtain a measure of solvent interaction. The results indicate that the 4C1 (D) chair is the most favored conformation, both by potential energy and solvent accessibility criteria. The 4C1 (D) chair conformation is also found to be somewhat flexible, being able to accommodate variations up to 10° in the ring torsional angles without appreciable change in energy. Observed solid-state conformations of these sugars and their derivatives lie in the minimum-energy region, suggesting that the substituents and crystal field forces play a minor role in influencing the pyranose ring conformation. Theory also predicts the variations in the ring torsional angles, i.e., CCCC < CCCO < CCOC, in agreement with the experimental results. The boat and twist-boat conformations are found to be at least 5 kcal mol−1 higher in energy compared to the 4C1 (D) chair, suggesting that these forms are unlikely to be present in a polysaccharide chain. The 1C4 (D) chair has energy intermediate between that of the 4C1 (D) chair and that of the twist-boat conformation. The calculated energy barrier between 4C1 (D) and 1C4 (D) conformations is high—about 11 kcal mol−1.

Conformational studies of proteoglycans: theoretical studies on the conformation of heparin.

Abstract: Various models proposed for heparin have been examined by a stereochemical approach involving contact distance criteria and potential energy calculations. The present study suggests that the model favored by Atkins and coworkers [Biochem. J. (1973) 135, 729-733 and (1974) 143, 251-252] is not stereochemically satisfactory. An alternative model has been proposed with N-acetyl-D-glucosamine and one of the uronides in the $4C_1$ conformation and the other uronide (probably sulfated) in the $1C_4$ conformation. The observed variations in the tetrasaccharide periodicities (16.5-17.3 ) in different crystalline modifications of heparin have been attributed to possible changes in the rotational angles about the interunit glycosidic bonds rather than a change in the pyranose ring conformation. The proposed model is also independent of the observed variation in the relative composition of uronic acid residues in heparin. These conclusions are in disagreement with those of earlier workers.

Furanose derivatives of 3-deoxy-D-manno-oct-2-ulosonic acid

Abstract: Acylation of methyl 3-deoxy-D-manno-oct-2-ulosonate leads essentially to furanose derivatives, whereas acetylation of the free acid or of its ammonium salt gives pyranose derivatives. Upon treatment with HBr in acetic acid, the pentabenzoate of methyl 3-deoxy-D-manno-oct-2-ulofuranosonate yielded a crystalline 2-bromo-derivative which, when treated with MeOH–Ag2CO3 was transformed into a single methyl furanoside of undetermined anomeric configuration. Reaction of methyl 3-deoxy-D-manno-oct-2-ulosonate with MeOH in the presence of an acidic catalyst gives a mixture of pyranoside(s) and furanosides separable by g.l.c.; they can be identifed by the characteristic fragments at m/e 217 for the furanosides and m/e 158 for the pyranoside(s).

Artificial carbohydrate antigens. Synthesis and conformation of a Shigella flexneri trisaccharide hapten

Abstract: The trisaccharide 8-methoxycarbonyloctyl 2-O-[2′-acetamido-2′-deoxy-3′-O-(α-L-rhamnopyranosyl)-β-D-glucopyranogyl]-α-L-rhamnopyranoside has been synthesised in good yield utilising silver trifluoromethanesulphonatepromoted Konigs–Knorr reactions. 3,4-Di-O-benzyl-β-L-rhamnopyranose 1,2-(methyl orthoacetate) provided access to the 2-O-substituted rhamnoside, 2-O-(2-acetamido-2-deoxy-β-D-glucopyranosyl)-α-L-rhamnopyranoside. Introduction of the amino-sugar was achieved with tri-O-acetyl-2-deoxy-2-phthalimido-D-glucopyranosyl bromide in conjunction with silver trifluoromethanesulphonate and 2,4,6-trimethylpyridine. Selective conversion of the 2-deoxy-2-phthalimido glucoside to a 2-acetamido-2-deoxy glucoside blocked at positions C-4 and,-6 of the pyranose ring by a benzylidene acetal gave the trisaccharide, following reaction with tri-O-acetyl-α-L-rhamnopyranosyl bromide. Proton and 13C n.m.r. evidence indicated that, despite a sterically crowded environment, the orientation of both the 2-acetamido-2-deoxy-glucose and terminal rhamnose residues is dictated by the exo-anomeric effect.

Intermolecular Transglycosylation of Cyclodextrin Glycosyltansferase

Abstract: The acceptor specificity in the intermolecular transglycosylation of cyclodextrin glycosyltransferase (CGT-ase) was investigated using 18 kinds of monosaccharides and their derivatives. It is concluded on the basis of the experimental results that the requirement for an acceptor of the intermolecular transfer reaction catalyzed by CGT-ase is the pyranose structure having the same configurations of the C2-, C3- and C4-hydroxyl group as D-glucopyranose. Furthermore, it was showed that CGT-ase transfered the glycosyl residues only to C3-hydroxyl group of L-sorbose as acceptor and C4-hydroxyl group of D-glucose, D-xylose and 2-deoxy-D-glucose as acceptor. On the other hand, in the case of D-galactose as acceptor which is different from D-glucopyranose in the configuration of the hydroxyl group at C4 position, CGT-ase transfered the glycosyl residues to C1-, C3- and C2- (C4-) hydroxyl group of D-galactose in the proportion of 26 : 10 : 1.

Flexibility of the pyranose ring in \alpha and \beta -glucoses

Abstract: Conformational energies of \alpha and \beta -D-glucopyranoses were computed by varying all the ring bond angles and torsional angles using semiempirical potential functions. Solvent accessibility calculations were also performed to obtain a measure of solvent interaction.The results indicate that the $4C_1$ (D) chair is the most favored conformation, both by potential energy and solvent accessibility criteria. The $4C_1$ (D) chair conformation is also found to be somewhat flexible, being able to accommodate variations up to 10 Deg in the ring torsional angles without appreciable change in energy. Observed solid-state conformations of these sugars and their derivatives lie in the minimum-energy region, suggesting that the substituents and crystal field forces play a minor role in influencing the pyranose ring conformation. Theory also predicts the variations in the ring torsional angles, i.e., CCCC < CCCO < CCOC, in agreement with the experimental results. The boat and twist-boat conformations are found to be at least $5 kcal mol^{-1}$ higher in energy compared to the $4C_1$ (D) chair, suggesting that these forms are unlikely to be present in a polysaccharide chain. The $1C_4$ (D) chair has energy intermediate between that of the $4C_1$ (D) chair and that of the twist-boat conformation. The calculated energy barrier between $4C_1$ (D) and $1C_4$ (D) conformations is high-about $11 kcal mol^{-1}$.