Electron spin resonance study of the reactions of hydrogen atoms in aqueous sulphuric acid glasses. Part 2.—Reaction with amino acids
Reactions of hydrogen atoms with amino acids in vitreous 6 mol dm–3 sulphuric acid have been investigated. Glycine and alanine yield the species NH+3ĊR—in which the ammonium group adopts random fixed orientations. Radicals of the form CH3ĊRCR′R″— are observed from norleucine, norvaline, leucine, valine, isoleucine, isovaline and 2-amino-n-butyric acid. Glutamic acid, glutamine, aspartic acid, asparagine and 4-amino-n-butyric acid yield HO2CĊHCHR— radicals, while the species —CH2ĊHCH2— is identifiable in the spectra derived from lysine, ornithine, proline and 4-amino-n-butyric acid. Arginine and citrulline yield both—NHĊHCH2— and —CH2ĊHCH2— radicals. The spectra derived from serine and threoinine are ascribed to RĊ(OH)— radicals. Interpretation of spectra of the radicals formed from 2-aminoisobutyric acid, β-alanine, histidine, homoserine and hydroxyproline is also undertaken.
In several cases, evidence drawn from computer simulation of spectra indicates that the trapped radicals adopt a range of conformations; notable examples are the cases of norleucine, leucine, isoleucine and threonine. When a radical can adopt a range of conformations, apparently anomalous spectra arise due to the angular dependence of β-atom hyperfine interactions.