Theoretical study of hydrated Ca2+-amino acids (glycine, threonine and phenylalanine) clusters

The hydrated clusters of complexes formed by calcium ion and three different zwitterionic amino acids, glycine (Gly), threonine (Thr) and phenylalanine (Phe) have been studied in the gas phase at MP2/6-311++G(d,p), considering the coordination numbers 6–8 for Ca2+. The clusters with the first hydration shell around calcium ion exhibiting the 8-coordination configuration were found to be most preferred for all the three hydrated systems. The relative stabilities of Gly-, Thr- and Phe-Ca2+ decrease in the order Thr-Ca2+(H2O)n > Phe-Ca2+(H2O)n > Gly-Ca2+(H2O)n upon hydration in contrast to the order without water Phe-Ca2+ > Thr-Ca2+ > Gly-Ca2+. It may demonstrate that the nature of side chain has remarkable influence on the stability of hydrated calcium–amino acid clusters. The NBO charges were analyzed to understand the bonding characteristics between the calcium ion and the amino acids. Spectroscopic characters of the most stable hydrated clusters can be assigned based on researching the calculated spectra.