AB-INITIO CALCULATIONS OF METHIONINES AND THEIR PROTONATED FORMS

Ab initio calculations of molecular and electronic structures of neutr al molecules and protonated forms of methionine and its derivatives in the gaseous phase were carried out by the Hartree-Fock method using t he 6-31G basis set with full geometry optimization. Proton affinities of methionine (1), methionine sulfoxide (2), and methionine sulfone ( 3) were calculated for different modes of coordination of the proton. The results of calculations demonstrated that in protonated forms of 1 and 3, bonding between the proton and the N atom is most favorable, w hile in protonated form of 2, bonding between the proton and the O ato m of the SO group is most favorable. The proton affinities of the amin o acids are as follows: 223.2 (1), 241.2 (2), and 221.5 (3) kcal mol(- 1), i.e., methionine sulfoxide 2 exhibits the highest proton affinity in the series of the amino acids under consideration.