E.S.R. and INDO study of radicals produced in irradiated single crystals of histidine hydrochloride monohydrate at low temperature

Radicals produced in X-irradiated single crystals of histidine hydrochloride monohydrate in the temperature range 77–300 K have been investigated using E.S.R. spectroscopy at 9·5 GHz and 35 GHz. The main species trapped at 77 K are the radical anions located at the carboxyl group. These are converted to deamination radicals upon warming to 180 K. Conversion of the anion into the deamination radical is not found if the former is bleached with ultra-violet illumination at 77 K. At room temperature radicals formed by hydrogen addition at the imidazole ring are found after irradiation both at 77 K and 300 K. These have been identified previously. The identifications are supported by MO-SCF calculations at the INDO level. They show that the trapped anion species is represented by the structure RCH(NH2)ĊOOH- rather than RCH(NH3/+)ĊOOH- and the deamination radical by R′CH2ĊHCOOH. The anion radical is characterized by principal values of hyperfine splittings of (20·1, 24·5, 23·0) G for the β proton at the neighbouring C atom and (23·2, 15·6, 11·9) G for the proton of the OH group. In the deamination radical the corresponding values are (-8·5, -18·0, -32·5) G for the α proton; 42·6 G and 7·3 G for the isotropic splittings of the two β protons.