Protonation of glycine, serine and cysteine. Conformations, proton affinities and intrinsic basicities

Abstract

Proton affinities and gas-phase basicities of glycine, serine and cysteine have been computed using the three-parameter B3LYP density functional approach. For that, the geometry and vibrational frequencies of several conformations of neutral and protonated glycine, serine and cysteine have been explored. The preferred site for protonation in all aminoacids is the amino group. The lowest conformation always shows an intramolecular hydrogen bond between NH 3 + and the carbonylic oxygen. For serine and cysteine, additional hydrogen bonds may be formed, the favored interaction being that in which the oxygen or sulfur atoms of the side chain interact, as proton acceptor, with NH 3 +. The computed B3LYP proton affinities and gas-phase basicities are in very good agreement with the known experimental data.