A THEORETICAL-ANALYSIS ON THE INTRAMOLECULAR PROTON-TRANSFER OF ALPHA-ALANINE IN AN AQUEOUS-MEDIUM

Intramolecular proton transfer from oxygen to nitrogen atoms in the al pha-alanine amino acid has been studied by ab initio methods at the HF /6-31G, HF/6-31 ++ G** and MP2/6-31 ++ G** levels of calculation incl uding the solvent effects by means of self-consistent reaction field t heory. An analysis of the results based on the natural bond orbital ch arges shows that the transition structure presents an imbalance in the sense that the charge shift lags behind the proton transfer and that the bond formation is always in advance with respect to the bond cleav age. All calculation levels show that the barrier height associated wi th the conformational change on alpha-alanine is larger than the proto n transfer process. (C) 1998 Elsevier Science B.V. All rights reserved .