Electron-spin Resonance of an X-irradiated Single Crystal of N-acetyl-DL-valine

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Abstract
Experiments on radiation-induced radicals in single crystals of N-acetyl-DL-valine are reported. After X-irradiation at room temperature, free radicals with the structures (CH3)2ĊCH (COOH)NHCOCH3 and (CH3)2ĊCH2COOH were determined by means of e.s.r.-spectroscopy. Earlier experiments on polycrystalline samples had pointed to the formation of the radical (CH3)2ĊCH2COOH, but it was impossible to explain the structure of the second radical. In both types of radical, the paramagnetic centre is localized at the C-atom to which the two methyl groups are bonded, and not at the Cα-atom lying in the peptide chain. These two radicals, produced at room temperature, are secondary products in the chain of radiation-induced reactions. This result was obtained by e.s.r.-spectroscopy of polycrystalline samples after irradiation and measurement at 77°K. Possible reaction sequences are outlined.