The role of water in radiation-induced fragmentation of cellulosic backbone polysaccharides

Xyloglucan (XG) is a cellulosic backbone polysaccharide commerciallyused for food applications, but also widely investigated in biomedicalapplications, for its gelling properties and specific biologicalactivity. In this study, the possibility of using gamma radiation tocleave XG and generate lower molecular weight variants was explored. Theimpact of absorbed dose and irradiation conditions on the XG molecularweight distribution was investigated. Two other cellulosicpolysaccharides, hydroxypropyl cellulose (HPC) and an oxidized variantof XG (CXG), were also studied for comparison. Before irradiation, thepolymers were characterized with thermal gravimetric analysis and, afterirradiation, with gel permeation chromatography. The results showed thatfor XG irradiated in dilute aqueous solution, a dose of 10 Gy issufficient to significantly reduce the polymer molecular weight, whileHPC is less affected by irradiation under identical conditions. When thepolymers were irradiated in the solid form, either dry or humid, thereduction in average molecular weight is much less pronounced.Interestingly, for HPC the cleavage of the chains is more pronounced forthe dry than for the humid powder. A similar behavior, but lesspronounced, was observed for XG and CXG. Arguably, when water waspresent in the system as bound water it had a protective effect. This isprobably due to energy transfer from the polymer to the bound waterpreventing chain scission. Indeed, humid HPC has more bound water thanXG and CXG. Conversely, when water was present as solvent, waterradiolysis products were able to efficiently induce depolymerization.