The action of ionizing radiation on protein: radical formation in L-histidine crystals

Radicals produced at temperatures between 77 K and 300 K by X-irradiation of L-histidine.2HC1 crystals have been analysed by electron spin resonance spectroscopy. The results are compared with previous findings with L-histidine and L-histidine.HC1.H2O crystals. In these three crystals we have identified three main species, which, depending on the molecular environment, are stabilized at different temperatures and produced in different concentrations. These findings are discussed in connection with the observations made on irradiated aqueous solutions of amino acids. It is concluded that our results corroborate the mechanism for the action of ionizing radiation on dry protein proposed by Platzman and Franck (1958),