ELECTRON-SPIN-RESONANCE EVIDENCE FOR INTRAMOLECULAR AND INTERMOLECULAR SIGMA-SIGMA-STAR BONDING IN METHIONINE RADICALS - RELATIVE STABILITIES OF S-CL, S-BR, S-N, AND S-S 3-ELECTRON BONDS

An electron spin resonance investigation of three-electron sigma-sigma* bond formation in methionine, methionine peptides, and model structures in frozen aqueous solutions is presented. Irradiation of 12 M LiCl or 6.3 M LiBr solutions at 77 K results in the Cl2.- and Br2.- radicals, respectively. These species are found to attack the methionine sulfur atom to form [GRAPHICS] and [GRAPHICS] adducts. In basic media both these species are found to react on further annealing to form the cyclic intramolecular adduct, [GRAPHICS] Further annealing results in the more stable intermolecularly bonded disulfide radical adduct, [GRAPHICS] Experiments performed to compare the stabilities of the various three-electron bonded species show the following order in increasing stability: [GRAPHICS] UV-vis spectra of the various intermediates taken suggest that the species identified in this work are those identified in previous pulse radiolysis investigations in aqueous solutions at ambient temperatures. Results found for methionine peptides and model structures are similar to those found for methionine with the exception that intramolecular bond formation in the peptides was not as facile. Each of the radical species found are analyzed for hyperfine splittings and g values. Ab initio molecular orbital calculations of optimized geometries and hyperfine couplings for model compounds for [GRAPHICS] bonding are presented. The calculations clearly show the localized sigma-sigma* nature of the bonding and further predict that the [GRAPHICS] species is stable toward deprotonation from the amine group.