Free radical formation in x-irradiated histidine HCl

ENDOR spectroscopy was used to identify three primary radicals in single crystals of histidine hydrochloride x irradiated at 4.2°K. Oxidation leads to decarboxylation producing the radical ${\mathrm{R–CH}}_2{\mathrm{–CH–NH}}_3^{+}$ , where R stands for the protonated imidazole ring. Another oxidation product is produced by removal of an electron from the imidazole ring. The reduction process yields a carboxyl anion by addition of an electron to the carbonyl oxygen. ENDOR spectroscopy was also used to characterize more completely the final hydrogen‐adduct radical that is obtained on warming the crystals to room temperature.