ENDOR study of irradiated histidine HCl.

Free‐radical formation was studied in x‐irradiated single crystals of histidine HCl using conventional electron spin resonance (ESR) as well as double‐resonance (ENDOR) spectroscopy. The spectra are attributed to a free radical formed by the addition of hydrogen to the C2 position on the imidazole ring of histidine.