Silver Ion Binding Energies of Amino Acids: Use of Theory to Assess the Validity of Experimental Silver Ion Basicities Obtained from the Kinetic Method

Abstract

The complexes of silver ion, Ag+, with the twenty naturally occurring amino acids have been calculated using hybrid density functional theory at the B3LYP/DZVP level. For all of these silver complexes, several possible structures were examined, but as there are remarkable similarities between all the structures at the global minima, only summarized data are reported. All of the complexes, except that with proline, are solvated ions. Amino acids containing only hydrocarbon side chains are bidentate, coordinating through the amino and carbonyl groups and the remaining amino acids (with the exception of proline) are tricoordinate with the same two interactions as in the simpler amino acids and an additional interaction through the side chain. The proline complex contains zwitterionic proline with the Ag+ ion attached to the carboxylate anion. Enthalpies (at 298 K) for dissociation of Ag+ from the complexes range from 49.3 kcal mol-1 for glycine to 80.4 kcal mol-1 for arginine. Free energies for these reactions are in the range of 40.7 kcal mol-1 for glycine to 70.3 kcal mol-1 for arginine. Comparison of the calculated free energies (relative to that of glycine) with those measured by the kinetic method shows good agreement, with the largest discrepancy being 3.9 kcal mol-1 for aspartic acid. There are some systematic trends with theory giving lower values than experiment for amino acids containing aromatic groups in the side chains (phenylalanine, tryptophan, and tyrosine) and higher values for the four amino acids with carbonyl groups in their side chains (aspartic acid, asparagine, glutamic acid, and glutamine).