A structural study of Acacia nilotica and Acacia modesta gums

Abstract

Superficially similar carbohydrate polymers from similar sources can have dramatically different charac-teristics. This work seeks to examine the molecular properties responsible for these differences. Protonsresponsible for cross-polarization in the anomeric region of Acacia nilotica (AN) were replaced easily bydeuterium, but not for Acacia modesta (AM). Time constants describing the mobility and cross-polarizationtransfer were both found to be lower for AM. Variable contact time experiments showed poorer fits andmore heterogeneity for AN. Solution state HSQC experiments showed a lower number of environmentsin the anomeric region for AM. The relaxation time T2of AM solutions had a lower value consistent witha higher viscosity. The Tg of solutions were −14.5◦C AN and −18.5◦C AM. These results form a largelyself-consistent picture of molecular differences between AN and AM, suggesting a more compact butheterogeneous structure for AN and more branching in the case of AM.