Tamarind seed polysaccharide: preparation, characterisation and solution properties of carboxylated, sulphated and alkylaminated derivatives

Abstract

A range of derivatives of tamarind seed polysaccharide has been prepared, characterised and selected solution properties examined. Following oxidation of terminal sidechain galactose residues with galactose oxidase, subsequent oxidation and reductive amination have been used to prepare a range of carboxylated and alkylaminated derivatives respectively. Sulphated derivatives have been prepared by reaction with a sulphur trioxide-pyridine complex in dimethylformamide. The nature and extent of substitution have been characterised by potentiometric titration, infrared and 1H and 13C NMR spectroscopy. From the dependence of intrinsic viscosity on ionic strength (Smidsrød & Haug, 1971), carboxylated and sulphated derivatives are found to have characteristically stiffened backbones as found previously for the native polysaccharide (Gidley et al., 1991). Binding of divalent cations to carboxylated derivatives is shown to be relatively weak, although polymer precipitation was noted in the presence of Pb 2+. Alkylaminated polysaccharides show only a modest decrease in surface and interfacial tension compared with the native polysaccharide, although significant foam formation and stabilisation was found for a nonylaminated sample. Following enzymic depolymerisation, this material showed a marked decrease in surface and interfacial tension suggesting that interfacial activity in alkylaminated tamarind polysaccharide is only apparent under disruptive solution conditions. The results of 1H NMR line width and T 1 measurements before and after depolymerisation suggests that this is due to solution viscosity rather than specific interaction effects.