Proton affinities of methyl esters of N-acetylated amino acids

Abstract

The proton affinities of the methyl esters of N-acetylated amino acids, Ac-Xxx-OMe, where Xxx is Gly, Leu, Phe, Pro, Glu and Arg have been calculated and characterized at B3LYP/6-31++G(d,p) and compared against experimentally measured values by the kinetic method. The proton is attached to the carbonyl group in five cases out of the six derivatized amino acids. Proton affinities of 15 derivatized amino acids have been measured experimentally. The average absolute deviation between theory and experiment is 1.2 kcal mol −1. The proton affinities of all derivatized amino acids are higher than those of the corresponding underivatized parents, reflecting the ionic hydrogen bonding stabilization. A Bader analysis of the intramolecular hydrogen bonding within these protonated amides is presented.