The formation of unpaired electrons on sulphur atoms in irradiated dry proteins as studied by electron spin resonance

From a study of proteins containing cystine residues, it is concluded that during high-energy irradiation electrons are trapped on the cystine residues. The resultant cystine anion reacts to give the -CH₂S^• radical previously observed by other workers. These reactions are affected by freeze-drying the proteins from solutions of different pH's and can be prevented by additives which scavenge electrons. If the protein contains cysteine residues, -CH₂S^• radicals are formed also by the migration of radicals on to the sulphydryl group of the cysteine. These results have been supported by a study of the radical reactions in irradiated oxidised and reduced glutathione.