Molecular Aggregation of Four Modified Xyloglucan Models in Aqueous Solution

Abstract

Tamarind seed xyloglucan (TSX) is extracted from tamarind seeds and is one of the most useful natural polymers. It has been used as a drug carrier in drug delivery because of its non-toxicity, biodegradability, and biocompatibility. A solution of TSX can become a thermally reversible TSX hydrogel when its galactose molecules are partially removed. It was found that a galactose-removed TSX hydrogel is formed when two modified TSX chains form parallel-like structure in aqueous solution. Many molecular associations via hydrogen bonds between two modified TSX chains can promote its molecular aggregation. Moreover, it was reported that the lack of galactose molecules might be able to promote the aggregation of the b-glucan main chains through the hydrophobic interactions, resulting in the gelation. In this study, four double-chain TSX models with 50% galactose removal were used for the study of molecular aggregation in aqueous solution at its gelling temperature. Each model was built from eight monomer units of nonasaccharide (N, XLLG) and eight monomer units of heptasaccharide (H, XXXG), where G, X, and L are glucose (Glu), xylose (Xyl) substituted with glucose, and galactose (Gal) substituted with xyloglucose, respectively. The numbers of N and H are equal in all models, but the arrangements of N and H monomer units in each model are different. The simulated SAXS profiles of all models were also carried out. The roles of molecular structures, intermolecular interactions, and molecular arrangements in molecular aggregation of 50% galactose removal of two TSX chains are discussed.