CONFORMERS OF GASEOUS PROTONATED GLYCINE

Ab initio geometry optimizations were performed on gaseous protonated glycine using the second-order Moller-Plesset perturbation theory with the 6-31G, 6-31G**, 6-31 + G**, and 6-311 + G** basis sets. Eight en ergy minima and 12 saddle points in the low-energy region of the elect ronic potential energy surface were characterized. The global minimum was an amino N-protonated conformer containing an ionic H bond between the -NH3+ and O=C < groups. The lowest energy O-protonated conformer was stabilized by a conjugative attraction between the nitrogen lone-p air electrons and the positively charged planar fragment -C(OH)(2)(+). Relative electronic energies of the nine N- and 11 O-protonated speci es fall in the ranges of 0-10 and 30-40 kcal mol(-1). At room temperat ure the equilibrium distribution contained the most stable N-protonate d conformer almost exclusively. Additional subjects for investigation include the effects of basis set and electron correlation on the predi cted structures, nonbonded interactions that influence the relative st ability of protonated conformers, conformational interconversions base d on intrinsic reaction coordinate calculations, and kinetic pathways fur protonation and associated changes in Gibbs free energy. The work provides geometric, energetic, and thermodynamic data pertinent to the study of gas-phase ion chemistry of amino acids and peptides. (C) 199 8 John Wiley & Sons, Inc.