PROTONATION OF GLYCINE AND ALANINE - PROTON AFFINITIES, INTRINSIC BASICITIES AND PROTON-TRANSFER PATH

Proton affinities and intrinsic basicities for nitrogen and oxygen pro tonation in the gas phase of the amino acids glycine and alanine were calculated using density functional theory (DFT) and ab initio methods at different levels of theory from Hartree-Fock (HF) to G2 approximat ions. All methods gave good agreement for proton affinities for nitrog en protonation for both amino acids. However, dramatic differences wer e found between DFT, MP4/MP2, and G2 results on one hand, and MP4//HF results on the other to the calculation of structural and energetic ch aracteristics of oxygen protonation in glycine and alanine. An investi gation into the source of these differences revealed that electron cor relation effects are chiefly responsible for the differences in calcul ated oxygen proton affinities between the various methods. It has been found that proton transfer between nitrogen and oxygen protonation si tes in both amino acids occurs without a transfer path barrier when co rrelated methods were used to calculate the path energetics. (C) 1998 Elsevier Science B.V.