Comprehensive theoretical study towards the accurate proton affinity values of naturally occurring amino acids

Abstract

Systematic quantum chemical studies of Hartree–Fock (HF) and second-order Møller–Plesset (MP2) methods, and B3LYP functional, with a range of basis sets were employed to evaluate proton affinity values of all naturally occurring amino acids. The B3LYP and MP2 in conjunction with 6-311+G(d,p) basis set provide the proton affinity values that are in very good agreement with the experimental results, with an average deviation of ∼1 kcal/mol. The number and the relative strength of intramolecular hydrogen bonding play a key role in the proton affinities of amino acids. The computational exploration of the conformers reveals that the global minima conformations of the neutral and protonated amino acids are different in eight cases. The present study reveals that B3LYP/6-311+G(d,p) is a very good choice of technique to evaluate the proton affinities of amino acids and the compounds derived from them reliably and economically.