Substituted glucans

From the organochemical point of view, carbohydrates/polysaccharides are more or less substituted polyhydroxy aldehydes (e.g., glucose—>glucans) or polyhydroxy ketons (e.g., fructose-n fructans). From the physicochemical point of view, an enormous heterogeneity also exists in... 

Consistent with the above observations, the immunoactive glucans such as curdlan (9), lentinan (10), scleroglucan (11), schizophyllan (12) and yeast glucan (13) share a common p-D(l-3)-linked glucopyronosyl backbone. Some of these polymers also contain p-D(l-6)-linked glucopyranosyl branches through the 3,6-di-O-substituted C-6 atom of the backbone residues. 

Our original approach to polysaccharide C-13 n.m.r. spectral analysis consisted of making a minimum number of hypotheses about expected structure-to-spectra relationships (8). By then comparing spectra to known structure for a series of D-glucans, we attempted to establish the validity of these hypotheses and to establish how diverse a structural difference could be accommodated The hypotheses were as follows. Firstly, that each polymer could be considered as an assembly of independent saccharide monomers. Secondly, that these hypothetical saccharide monomers would be 0 alkylated (0 -methylated) in the same positions as the actual saccharide linked residues (it had previously been established that 0-methylation of any a-D-glucopyranosyl carbon atom position resulted in a down-field displacement of vlO p.p.m. for the associated resonance). Thirdly, that each differently substituted residue would have a completely different set of chemical shift values for each carbon atom position (different from the unsubstituted saccharide) but that only the carbon atom positions involved in inter-saccharide linkages would have A6 greater that 1 p.p.m. And, fourthly, that the hypothetical 0-alkylated residues would contribute resonances to the total spectrum proportional to their mole ratio in the polymers. 

The interaction with polysaccharides closely related structurally to galactomannans has been studied less for Xanthomonas polysaccharide than for agar. No interaction is observed between Xanthomonas polysaccharide and sodium O-(carboxymethyl)-cellulose (degree of substitution 0.7), probably because of electrostatic repulsion between the two negatively charged polysaccharides.178 Xanthomonas polysaccharide can, however, interact with polysaccharides based on /3-l>(l — 4)-linked D-glucan backbones, as... 

The enzymes involved are nucleoside diphosphohexosetransferases, probably GDP-D-mannosyltransferase (EC 2.4.1.32) and UDP-D-galactosyl-transferase. For a structurally related polymer, (1 - 6)-a-D-xylosyl-(l - 4)-/3-D-glucan, synthesis was shown to involve concurrent incorporation of D-xylose and D-glucose, and not substitution by D-xylose of a preformed (1 - 4)-/3-D-glucan chain.34 The insolubility of (1 - 4)-/3-D-glucan and man-nan would make the latter mechanism unlikely. 

A linear cell-wall glucan(s) from Sporothrix schenckii contains 3-0-, 6-0-, and 4-O-substituted /8-D-glucopyranosyI units, the /S-D-glycosi-dic configuration being determined by, 3C-n.m.r. spectroscopy,9 by virtue of the C-l signals, at relatively low field, of 8c 103.8 and 104.8. 

Cell walls of Saccharomyces cerevisiae were found to contain a (1— 6)-linked j8-D-glucopyranan this was isolated, and identified by i.r. spectroscopy52 and chemical-analysis techniques.53 The alkali-in-soluble glucan from S. cerevisiae contains this and a (1— 3)-linked /3-d-glucopyranan in the ratio54 of 1 5.7. The former, of mol. wt. 2 x 105, has 6-0- and 3-O-substituted units in the ratio of 4.4 1, and contains 14% of 3,6-di-O-substituted units.55 (A similar heterogeneity occurred... 

Degree of Side-chain Substitution in Glucans Having the General Structure 3... 

Active on substituted cellulose, Slightly lower activity P-glucan and avicel than EG I... 

EC 3.2.1.6 endo-1,3(4)-/3-glucanase Endohydrolysis of 1,3- or 1,4-linkages in /3-D-glucans when the glucose residue whose reducing group is involved in the linkage to be hydrolyzed is itself substituted at C-3... 

---