Molecular mixtures containing different proportions of ribonuclease and glycyl-glycine were irradiated with X-rays a t 77" K and the electron spin resonance spectra were observed a t this temperature and after heat-treatment for 10 minutes a t 60".
Electron-spin-resonance spectroscopy was used to observe the free radicals induced in reduced penicillamine and reduced glutathione both by x-irradiation and bombardment by thermal hydrogen atoms. X-irradiation at 77°K indicated that free radicals were initially located on a-carbon atoms in both molecules. When irradiated penicillamine was gradually warmed, E.S.R. centres were completely transferred to sulphur at about 200°K. In the case of irradiated glutathione, however, 8 days at room temperature were required for transfer of E.S.R. centres to sulphur. Hydrogen-atom bombardment resulted in the formation of the organo-sulphur radical in both compounds without evidence of hydrogen abstraction from a-carbon atoms.
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