Electron spin resonance studies of electron irradiated peptides. A single crystal of N-acetylglycine at 77 K.
Free radicals formed in single crystals of N-acetylglycine, irradiated and observed at 77 K, have been studied using ESR spectroscopy. Two different radical species were found. One radical (called ...
An electron spin resonance study of gamma-irradiated polycrystalline methionine-containing peptides
Abstract The radicals formed on γ-radiolysis of several methionine containing peptides in the polycrystalline state at 77 K and their reactions on subsequent warming over the temperature range 77–293 K are reported. For all peptides the radical species present at 77 K are peptide anion radicals. For peptides of the type x -met ( x = gly, ala, val, leu, ser), the radical anions decay to produce deamination radicals on warming to 120–230 K. On further warming to 293 K, the presence of decarboxylation radicals and H-abstraction radicals at the α-carbon of the methionine residue were detected for gly-met, ala-met, and val-met, leu-met, ser-met, respectively. For peptides of the type met- x ( x = gly, ala, leu, ser, asp, glu, gly-gly) the H-abstraction radicals at the α-carbon of the C-terminal amino acid residue are the only radical species present at room temperature. For val-met, leu-met, ser-met and met- x peptides the H-abstraction radicals in the side chain of methionine residue over the temperature range 180–230 K were also observed. It is proposed that the primary localization of the positive charge on the sulphur atom accounts for the radical reaction pattern in the cationic pathway.
Electron Spin Resonance of ?-Irradiated Dicyandiamide
The X-band e.s.r. spectra of single crystal and powder samples of γ-irradiated dicyandiamide, (H2N)2CNCN have been observed. The results show that three radical species are formed on irradiation at room temperature. One of these radicals, which gives a single broad line spectrum of width 8.5 G and g = 2.0040, has not been identified. The other two species are considered both to have the same chemical formula but one is a π-electron radical whereas the other is a σ-electron radical with a very large proton hyperfine splitting in the range of 73–81 G. The production of two types of radicals by radiation damage to the same —NH2 group of dicyandiamide is explained in terms of the ease of formation of π- and σ-electron radicals as influenced by geometrical considerations stemming from the hydrogen bonding in the dicyandiamide crystal.