Oxidation process of water-soluble starch in TEMPO-mediated system

Water-soluble starch was oxidized with NaClO and catalytic amounts of 2,2,6,6-tetramethylpiperidinyloxy radical (TEMPO) and NaBr in water below 4 °C at pH 10.75, and the oxidation process was studied by NMR analysis of the oxidized products. The degree of oxidation at the C6 primary hydroxyl group was controlled stepwise from 0 to 100% by controlling the amount of NaClO added or the reaction time. It was possible to distinguish intermediate aldehyde and hemiacetal structures as well as carboxyl and alcohol groups at C6 by 1H and 13C NMR. In particular 1H NMR spectra of incompletely oxidized products indicated that significant amounts of C6 hemiacetals were formed as intermediate structures. These hemiacetals were eventually oxidized to carboxyl groups by sufficient reaction time or sufficient amounts of reagents. Based on the composition ratios of C6 structures and NaOH consumption, a new scheme of the TEMPO-mediated oxidation for polysaccharides was proposed. Furthermore, 13C NMR spectra of the oxidized products revealed that the α-1,6-glucoside bonds present in the original starch as branched structures were mostly cleaved during the oxidation, thus giving almost pure α-1,4-linked polyglucuronic acid.