Towards the absolute proton affinities of 20 α-amino acids

The absolute proton affinities (APA) of 20 α-amino acids, as obtained by the MP2(fc)/6-311+G∗∗//HF/6-31G∗ + ZPVE(HF/6-31G∗) and the scaled Hartree–Fock (HFsc) models, are presented. It is shown that the α-NH2 group is protonated in all but four cases: lysine (K), proline (P), histidine (H), and arginine (R). There is a good overall agreement with experimental data measured by the kinetic method. However, there are some notable exceptions such as glutamine (Q) and lysine (K), where strong hydrogen bonds in the protonated forms occur. It is suggested that the present results and theoretical models employed could be useful for resolving such experimental ambiguities. Furthermore, it appears that the HFsc model provides an efficient tool for elucidating APAs of artificial α-AAs, derivatives of natural α-AAs and their oligomers.