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 degreesC 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 H-1 -and C-13 NMR. In particular H-1 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, C-13 NMR spectra of the oxidized products revealed that the alpha-1,6-glucoside bonds present in the original starch as branched structures were mostly cleaved during the oxidation, thus giving almost pure alpha-1,4-linked polyglucuronic acid. (C) 2003 Elsevier Science Ltd. All rights reserved.