Radiation damage in single crystals of cysteine

Abstract

The technique of electron spin resonance (e.s.r.) has been used to study the free radicals produced by γ radiolysis of cysteine single crystals at 77°K. By controlled warming or further irradiation with ultra-violet light we have obtained six different spectra, four of which have been shown to arise from one radical, RCH₂S·. The differences in these spectra are explained by conformational and environmental differences in the free radical. The conformational changes lead to an interaction of the unpaired electron with a neighbouring sulphydryl group giving rise to uniquely large anisotropies in the spectroscopic splitting factor g. Warming from 77°K causes appreciable increases in radical concentration suggesting that much of the primary radiation damage is non-paramagnetic. Further warming results in a near complete disappearance of all radicals which is consistent with dimerization occurring in the solid state.