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 ...
Radiation damage to proteins--an e.s.r. study of a single crystal of glycyl-glycine at low temperatures.
SummaryThe electron paramagnetic spectra from a single crystal of glycyl-glycine have been analysed in the temperature range between 77 and 300 k, after the crystal had been irradiated at 77 k and then gradually heated. At 77 k a decarboxylation product remains stabilized within the crystal lattice, as well as, most probably, the product of a deamination process. At about 140 k, the finally stable radical appears, although the decarboxylation product dissappears, simultaneously. The finally stable radical is due to a hydrogen abstraction at the carbon-6 position. Two other radical species also arise, one of which is reported to originate from a hydrogen abstraction from the carbon-1 position, whereas the other is thought to stem from a fragmentation process at the peptide nitrogen. Both these radicals convert, upon further heating, into the finally stable radical. The entire spectrum also contains some other resonances, which are interpreted to originate from odd σ-electron systems.
Electron spin resonance spectra of carboxylic acid radical anions at 77°K
Sodium atoms have been deposited on a series of monocarboxylic acids, R—COOH, at 77° K in the rotating cryostat and the deposits examined by electron spin resonance (e.s.r.) spectroscopy. The spectra show that the primary paramagnetic species in each case is the corresponding radical O– anion, [graphic omitted]. The spectra are similar to those observed for the primary radicals formed by X-, β- or γ- irradiation of mono-or di-carboxylic acids at 77° K and thus confirm that these radicals are the radical anions, and not cations, of the corresponding acids. The carbon-13 hyperfine coupling constants have been determined for the radical anion, [graphic omitted], of fully deuterated acetic acid. The results show that the free valence centre has a pyramidal configuration, in contrast to the planar structure found in many alkyl radicals. 
 The radical anions are able to abstract hydrogen from adjacent neutral molecules at 77° K to give secondary radicals. The extent of reaction depends on the strength of the CH bonds in the acid. Negligible reaction occurs with acetic acid which possesses only primary hydrogens, whereas the radical anion reacts completely with the tertiary hydrogen in isobutyric acid to give the radical (CH3)2CCOOH.