Abstract
Free radicals produced by ..gamma.. irradiation at 77K in a single crystal of N-acetyl-DL-valine (CH/sub 3/)/sub 2/CHCH(NHCOCH/sub 3/)COOH were investigated by ESR spectroscopy. The aim of the experiments was to determine whether the paramagnetic centers observed at room temperature (i.e., (CH/sub 3/)/sub 2/CCH(NHCOCH/sub 3/)COOH and (CH/sub 3/)/sub 2/-CCH/sub 2/COOH) have precursor radicals of a different type than the peptide radicals -NH-CR-CO-. The anion free radical (CH/sub 3/)/sub 2/CHCH(NHCOCH/sub 3/)C(O/sup -/)OH and the free radical (CH/sub 3/)/sub 2/CHCHNHCOCH/sub 3/, probably formed by a decarboxylation process from the cation radical were demonstrated at 77 K. These are the same types of radicals that were shown to be the precursors of the peptide radicals. Suppositions about the reaction mechanism leading to the finally stable radicals in N-acetyl-DL-valine are discussed. If the ..gamma..-irradiated crystals were photobleached by uv light the radicals were destroyed and other types of paramagnetic centers were formed. The radicals CH/sub 3/ and (CH/sub 3/)/sub 2/CHCH(NHCO)COOH could not be found after uv irradiation alone; they require the ..gamma..-produced paramagnetic centers as precursors.
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