Synthesis of unsaturated polysaccharide via selective desulfation of oxidized κ-Carrageenan

Abstract

Here, κ-Carrageenan was selectively C6 oxidized to convert β-d-Galp-4-sulfate units into its uronic acid derivative using 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and trichloroisocyanuric acid (TCCA), giving an oxidized κ-Carrageenan derivative, which was submitted to alkaline β-elimination reaction to produce a 4,5-unsaturated carrageenan derivative via selective C4 desulfation. The Carrageenan derivatives were characterized by ¹H and ¹³C NMR (1D and 2D), UV, HPSEC-MALLS-RI and Thermogravimetric analysis. Investigation of the formation of unsaturated Carrageenan showed that rate of production was pH dependent and increased 6-fold in the presence of Ca²⁺. Also, deuterium exchange experiment indicated a fast and irreversible H5 abstraction at β-d-GalAp-4-sulfate units during the reaction.