The Action of Ionizing Radiations on Aqueous Solutions of Methylene Blue

The action of ionizing radiations on aqueous solutions of various dyes, and methylene blue (MeB) in particular, has been the subject of numerous investigations, and it is known that decoloration of the dye occurs on irradiation. The results obtained by different workers were seemingly not in agreement with each other. Thus Seitz (1) described the decoloration of MeB in air-free aqueous solutions which could be reversed on admission of 02 and was thus due to reduction. This reaction was further investigated by Zimmer (2). Gallico and Camerino (3) obtained similar results in aqueous but not in alcoholic solution and claimed that the addition of various organic and inorganic solutes to the aqueous solution of MeB prevented the reduction to some extent. Shekhtman et al. (4) observed the irreversible oxidative decoloration of MeB in irradiated solutions containing 02, and Collinson (5) observed a similar process as well as reduction in 02-free solutions. Day and Stein (6) showed that the reversible reduction of MeB proceeds with a very high efficiency in air-free solutions containing various added organic substances -e.g., in gels containing up to 40% gelatine and up to 0.5 M benzoate-and it was pointed out (7) that such systems are of interest from the point of view of both radiation chemistry and radiation biology, providing as they do systems in which reversible oxidation-reduction processes proceed with very high efficiency. Recently a series of results were published (8-10) using coenzyme-1, and related substrates, as well as cytochrome c in the presence of added solutes (ethanol, benzoate, succinate, etc.). These results were related to those obtained in the case of MeB. It appeared that the very high reversible reduction yields obtained are due to the intermediate formation of organic free radicals from the added solutes, these organic free radicals acting as reducing agents, as shown by Swallow (8, 11). In the meantime the efficient action of organic free radicals as reducing agents has also been investigated by Waters and co-workers (12).