<i>Ab initio</i> calculations of proton affinities of glycine, proline, cysteine and phenylalanine: comparison with the experimental values obtained using an electrospray ionisation ion trap mass spectrometer

Abstract

The proton affinities of four characteristic amino acids, glycine, proline, cysteine and phenylalanine, have been calculated using three different types of wave function, HF, DFT and MP2. These wave functions were combined with several basis sets using the valence double- or triple-xi with polarisation functions, and included or not a diffuse d function on heavy atoms. Calculations were carried out using Gaussian 98 on a LINUX system (2 GHz, 2 GB of RAM). The calculated results have been compared with the experimental values obtained using Cooks' kinetic method, in particular on an electrospray ionisation ion trap mass spectrometer. An excellent agreement was found between the experimental values and the theoretical results obtained using the B3P86/6-31+G*//B3LYP/6-31G* level.