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

Proton affinities and gas-phase basicities of glycine, serine and cysteine have been computed using the three-parameter B3LYP density functional appro ach, For that, the geometry and vibrational frequencies of several conforma tions of neutral and protonated glycine, serine and cysteine have been expl ored. The preferred site for protonation in all aminoacids is the amino gro up. The lowest conformation always shows an intramolecular hydrogen bond be tween NH3+ and the carbonylic oxygen. For serine and cysteine, additional h ydrogen bonds may be formed, the favored interaction being that in which th e oxygen or sulfur atoms of the side chain interact. as proton acceptor, wi th NH3+. The computed B3LYP proton affinities and gas-phase basicities are in very good agreement with the known experimental data. (C) 2001 Elsevier Science B.V. All rights reserved.