Protonation of glycine and alanine: proton affinities, intrinsic basicities and proton transfer path

Proton affinities and intrinsic basicities for nitrogen and oxygen protonation 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 approximations. All methods gave good agreement for proton affinities for nitrogen protonation for both amino acids. However, dramatic differences were 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 characteristics of oxygen protonation in glycine and alanine. An investigation into the source of these differences revealed that electron correlation effects are chiefly responsible for the differences in calculated oxygen proton affinities between the various methods. It has been found that proton transfer between nitrogen and oxygen protonation sites in both amino acids occurs without a transfer path barrier when correlated methods were used to calculate the path energetics.