Gas-phase acidities of the 20 protein amino acids

Abstract

The gas-phase acidities of the 20 protein amino acids (PAAs) have been determined using an electrospray ionization–quadrupole ion trap instrument. Three different methods were used to determine both the absolute acidities and the relative acidity ordering of the PAAs. The extended kinetic method was used to determine absolute acidities for all 20 PAAs with substituted carboxylic acids and substituted phenols as reference acids. Acidities were obtained with an average uncertainty of ±10 kJ/mol, which is large compared to some of the differences between amino acids with similar acidities. To determine the relative acidity ordering, single-reference kinetic method experiments were performed using both the reference acids from the absolute acidity studies and tryptophan and threonine as reference acids. Additional ordering information was obtained from kinetic method experiments in which proton-bound dimer ions comprising pairs of amino acids were generated and dissociated in the ion trap. The recommended acidity ordering is Gly < Ala < Pro < Val < Leu < Ile < Lys < Trp < Phe < Tyr < Met < Ser < Thr < Cys < Gln < Gln < Arg < Asn < His < Glu < Asp. Isodesmic acidity values were also obtained at the B3LYP/6-311++G**//B3LYP/6-31+G* level of theory with acetic acid as the reference acid. The theoretical acidities are in excellent agreement with the absolute acidities obtained from the extended kinetic method studies. The calculations predict that the preferred isomer for protonated cysteine and tyrosine is not a carboxylate anion, but rather a thiolate anion and a phenoxide anion, respectively.