A 1H- and 13C-n.m.r. study of bromine-oxidised potato starch

Oxidation of lactose with bromine

Oxidation of lactose by bromine in an aqueous buffered solution was conducted as a model experiment to examine the glycosidic linkage cleavage occurring during the oxidation of oligosaccharides and polysaccharides. The resulting oxidation products, after reduction with sodium borodeuteride, were characterized by GLC-MS analyses of the per-O-methyl or per-O-Me3Si derivatives. Most of the products were carboxylic acids, of which lactobionic acid was major. Minor products, identified after partial fractionation on a BioGel P-2 column, comprised oxalic acid; glyceric acid; threonic and erythronic acids; tartaric acid; lyxonic, arabinonic, and xylonic acids; galactonic and gluconic acids; galactosylerythronic acid; galactosylarabinonic acid; galactosylarabinaric acid; galacturonosylarabinonic acid; and galactosylglucaric acid. No keto acids were identified. Galactose was detected as 1-deuteriogalactitol, the presence of which, together with the C6 aldonic acids, supported a galactosidic bond cleavage. Galactosylarabinonic acid was the major constituent (7.5%) among minors, and others constituted 0.2-3.7% of the principal lactobionic acid. These products together comprised 29% of the lactobionic acid, more than half (17%) of which were accounted for by the galactosidic linkage cleavage, supporting the significant decrease in molecular weight seen earlier in the bromine-oxidized polysaccharides by glycosidic cleavage.

Hydrodynamic properties of oxidized extracellular polysaccharides from Erwinia chrysanthemi spp

The molecular weights of the native polysaccharides of Erwinia chrysanthemi strains range from 1.8 to 7.1 x 10(6) and their hydrodynamic properties are those of polydisperse, polyanionic biopolymers with pseudoplastic, non-thixotropic flow characteristics in aqueous solutions. The effect on the hydrodynamic properties of the polysaccharides by adding carboxyl groups to increase the charge density is studied, with particular reference to their molecular weight (MW), viscosity and conformation. In general, it is found that periodate oxidation of the extracellular polysaccharides of E. chrysanthemi strains, Ech9Sm6 and Ech6S+, introduces little change in the hydrodynamic properties of the resulting polyaldehydes. However, bromine oxidation at neutral pH of the polyaldehydes results in polycarboxylate biopolymers that show significant reduction in MW and viscosity, but they are still characteristic polyanions.

Structure of dicarboxyl malto-oligomers isolated from hypochlorite-oxidised potato starch studied by 1H and 13C NMR spectroscopy

The main oxidised component in hypochlorite-oxidised potato starch was isolated by anion-exchange chromatography after enzymatic hydrolysis. The primary structure of the isolated oligosaccharides was determined by 1H and 13C NMR spectroscopy, using homonuclear and heteronuclear two-dimensional techniques. The isolated pentamer and hexamer contained one glucose unit oxidised to a dicarboxyl residue. As the hypochlorite oxidation has occurred at positions C-2 and C-3 of a glucose unit, the introduced carboxyl groups caused ring cleavage between the carbons C-2 and C-3. The ring-cleaved dicarboxyl residue had glycosidic linkages on both sides, implying that this oxidation pathway does not result in depolymerisation. The vicinal coupling constant between H-4 and H-5 in the ring-cleaved dicarboxyl residue was 3.2 Hz, showing that the gauche orientations are preferred. As a result, a different bending of the starch chain is observed and is probably, therefore, one of the reasons why hypochlorite oxidation reduces the tendency to retrogradation. The pKa values (3.0) were determined from the pH-dependent chemical shifts of H-1, H-4 and H-5 of the dicarboxylic residue.