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

The main oxidised component in hypochlorite-oxidised potato starch was isol ated by anion-exchange chromatography after enzymatic hydrolysis. The prima ry structure of the isolated oligosaccharides was determined by H-1 and C-1 3 NMR spectroscopy, using homonuclear and heteronuclear two-dimensional tec hniques. The isolated pentamer and hexamer contained one glucose unit oxidi sed 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 cau sed ring cleavage between the carbons C-2 and C-3. The ring-cleaved dicarbo xyl residue had glycosidic linkages on both sides, implying that this oxida tion pathway does not result in depolymerisation. The vicinal coupling cons tant between H-3 and H-5 in the ring-cleaved dicarboxyl residue was 3.2 Hz, showing that the gauche orientations are preferred. As a result, a differe nt bending of the starch chain is observed and is probably, therefore, one of the reasons why hypochlorite oxidation reduces the tendency to retrograd ation. The pK(a) values (3.0) were determined from the pi-I-dependent chemi cal shifts of H-1, H-4 and H-5 of the dicarboxylic residue. (C) 1999 Elsevi er Science Ltd, All rights reserved.