Temporal control of xyloglucan self-assembly by radiation-induced degradation

Abstract

Xyloglucan is a natural polysaccharide present as storage material in seeds and structural materials in the primary cells walls of higher plants. Xyloglucan from tamarind seed is composed of B(1,4)-D-glucan backbone, partially substituted by a-(1,6)-linked xylose units, some of which are B-D-galactosylated at O-2. In its native form the polymer is water-soluble and forms gel in the presence of moderate amounts of alcohols. When a given fraction of galactosyl residues are removed, the polymer acquires the ability to form gels in aqueous solution at physiological temperatures, a property of great interest for biomedical/pharmaceutical applications. Gelation on increasing temperature can be ascribed to hydrophobic interactions that drive the system into a region of thermodynamic instability. In the highly concentrated regions generated by demixing, the dangling segments of polymeric chains "condensate" into additional intra- and inter molecular bonds forming a more compact structure. The drammatic viscosity increase prevents phase separation to occur and leads to gel formation. Gel morphology and properties are controlled by the relative rate of condensation and gelation, and they are both affected by the molecular weight distribution of the biopolymer. Gamma irradiation of the solid polymer in the form of a dry powder has been carried out to tailor the molecular weight of the polymer without significant alteration of its chemical structure. As a result, temperature induced gelation leads to gels with vaious degrees of structural order, from disordered porous networks for the non-irradiated system to a fairly ordered stack of thin membranes for the system irradiated at 20 kGy. Gamma irradiation of the polymer as a solid materials in the presence of different gaseous atmospheres and in aqueous solution have been carried out in the attempt to elucidate the radiation chemistry of xyloglucan.