The effect of low-energy electron exposure on polysaccharides and monomeric sugars

Abstract

The effect of low-energy electron irradiation on thin layers of two different polysaccharides, namely starch and chitosan as well as on their building blocks alpha-D-glucose and N-acetyl-D-glucosamine has been investigated. Exposure experiments were carried out at electron energies (E) from 5 eV to 15 eV. With this choice of energy range the reactions at E below the ionization threshold where dissociative electron attachment (DEA) plays the decisive role and at E above the ionization threshold can be compared. Low-energy electron exposure onto thin starch films leads to a change of the wetting behavior of the surface. The results obtained by means of reflection absorption infrared spectroscopy (RAIRS) and X-ray photoelectron spectroscopy (XPS) point to formation of a hydrocarbon material with low oxygen content upon exposure. At energies as low as 5 eV, this can be explained by loss of oxygen from the hydroxyl groups and consequent formation of new hydrocarbon groups. At energies above the ionization threshold, starch becomes hydrophobic by loss of hydroxyl groups in the process of ionization. In the case of chitosan, the loss of hydroxyl groups has also been observed but the amide groups are much more affected by irradiation. Thus, the side groups react faster under irradiation than the polysaccharide backbone. High resolution electron energy loss spectroscopy (HREELS) results of the exposure experiments on alpha-D-glucose and N-acetyl-D-glucosamine have also shown that side groups react first under the electron exposure. Elimination of water above the ionization threshold is effective for alpha-D-glucose as well as for N-acetyl-D-glucosamine. At energies below the ionization threshold, a loss of hydroxyl groups was observed for alpha-D-glucose while the abstraction of the amide group was observed for N-acetyl-D-glucosamine. The mechanisms of the reactions leading to elimination of water are strongly dependent on the electron energy. At energies above the ionization threshold, the abstraction of hydroxyl groups leads to the formation of CC double bonds while this process below ionization threshold is accompanied by the formation of aldehyde groups. Finally, the exposure experiments in the presence of O2 have shown that rapid oxidation of alpha-D-glucose to the products CO2 and H2O takes place, both below and above the ionization threshold.