ESR and INDO study of radical conformations in irradiated single crystals of L-histidine free base

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Abstract
Radicals produced in x-irradiated single crystals of L-histidine free base in the temperature range 77–300 K have been investigated using ESR spectroscopy at 9.5 and 35 GHz. The main species trapped at 77 K are radical anions located at the carboxyl group. These are converted to deamination radicals upon warming. Conversion of the anion into the deamination radical is not found if the former is bleached by uv illumination at 77 K. At room temperature, only radicals resulting from deamination are observed. Two different conformations of the deamination have been found. The one obtained at room temperature is produced from a precursor with identical composition but different conformation. The latter, which is stable at reduced temperatures, is in the open conformation like both the precursor anion radical and the parent molecule. The room temperature radical however adopts a closed conformation. The identifications are supported by MO-SCF calculations at the INDO level. A detailed analysis of the anion radical and the deamination radical by INDO is presented.