TEMPO-mediated oxidation of maltodextrins and D-glucose: effect of pH on the selectivity and sequestering ability of the resulting polycarboxylates.
Maltodextrins were oxidized to polyglucuronic acids with the ternary oxidation system: NaOCl-NaBr-2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO). The chemoselective oxidation at the primary alcohol groups was shown to be strongly pH dependent. Oxidation of polysaccharides was best achieved at pH 9.5 in order to minimize depolymerization, whereas oxidation of oligosaccharides required stronger alkaline conditions (pH 11-11.5). The resulting sodium polyglucuronates present interesting sequestering properties, the best of which being obtained from maltodextrins with the highest degrees of polymerization. The same oxidation process allowed the convenient conversion of D-glucose to D-glucaric acid in high yield (> 90%), under strongly basic conditions (pH > 11.5).