Yoksan, R., Akashi, M., Miyata, M. and Chirachanchai, S. Optimal γ-Ray Dose and Irradiation Conditions for Producing Low-Molecular-Weight Chitosan that Retains its Chemical Structure. Radiat. Res. 161, 471–480 (2004).
This study focuses on the optimal conditions for γ irradiation to reduce the molecular weight of chitosan but still retain its chemical structure. Chitosan was irradiated under various conditions, i.e. flake solid state (condition 1), flake dispersed in water (condition 2), flake dispersed in 0.05, 0.1, 1 and 2% aqueous K2S2O8 solution (conditions 3a, 3b, 3c and 3d, respectively), flake dispersed in 0.5, 1 and 2% aqueous H2O2 solution (conditions 4a, 4b and 4c, respectively), and chitosan acetic acid solution (condition 5). Comparative studies were done using three types of chitosans with molecular weights of the order of 105 Da with degrees of deacetylation of 0.80, 0.85 and 0.90%. For all conditions, after irradiation, there were two regions of molecular weight reduction. A severe degradation occurred in the first region with decreases in the molecular weight of 80% for radiation doses up to 50 kGy for conditions 1, 2 and 3 (3a–3c) and 20 kGy for condition 4. In the second region, a slow degradation occurred, which resembled a plateau stage. The results for conditions 3d and 5 were the most dramatic, since the primary structure of chitosan was changed after the irradiation. The degradation of chitosan by γ rays was found to be most effective for the amorphous structure. The retention of the structure of chitosan after γ irradiation makes it possible to produce a low-molecular-weight chitosan that retains its functionality, as demonstrated by its activity in the coupling reaction with N,N′-carbonyldiimidazole.