Dimer cation radicals of N-acetyl methionine: E.S.R. and ENDOR studies

Abstract

ENDOR spectra of a sulphur-centred radical (β) produced in a single crystal of N-acetyl DL-methionine by γ-irradiation at 77 K in the dark and of another sulphur-centred radical (β*), converted from β by heat-treatment at 115 K, were obtained and the hyperfine tensors of the methyl protons determined. It was found that the methyl groups in the two radicals rotate rapidly at 77 K (and even at 4·2 K at least for β), and the isotropic parts of the hyperfine tensors for the methyl protons were 0·69 and 0·82 mT for β and β*, respectively. Results of the E.S.R. spectral simulation using the experimentally determined coupling constants indicate that the β and β* radicals must be dimer cation radicals of the form

in which two out of the four methylene protons besides the six methyl protons interact preferentially with the unpaired electron localized on the sulphur atoms to give thirteen almost isotropic hyperfine lines. From the consideration of the crystal symmetry, it is concluded that the dimer cation is formed from a pair of D and L molecules of N-acetyl methionine. The ³³S-hyperfine tensor determined for the β* radical showed that the s-character of the unpaired orbital on sulphur is quite small (0·036) and that the p-character is not very large (0·51), either. The small p-character of approximately 0·5 is consistent with the formation of a disulphide cation in which the unpaired electron is shared equally by two sulphur atoms. It is concluded that the unpaired electron occupies an antibonding σ_SS* orbital constructed from 3p orbitals of two sulphur atoms and that the formation of a disulphide cation radical is an energetically favourable process. The two optical absorption bands of the disulphide cation radical with λ_max at 730 nm and at 350 nm for β (or λ_max at 630 nm and at 330 nm for β*) are now correctly assigned to the electronic transitions n ₊→σ_SS* and n _-→σ_SS*, respectively, where n ₊ and n _- represent different combinations of sulphur non-binding orbitals. The electronic structures of the β and β* radicals are discussed in some detail based on the reassigned optical absorption bands and the experimentally obtained g tensors.