The geometries of biologically important ions in water clusters: ab initio molecular orbital study

摘要

The purpose of this review is to (1) determine to what extent the geometries and charges on atoms of biologically important ions in aqueous solution are different from those in the gas phase, and (2) show the causes for the differences. For this purpose, first we determined the geometries and charges on atoms of the aqueous clusters of model ions (RCOO~- (R=H, CH_3 and F), CH_3O~-, CH_3S~-, CH_3NH_3~+, CH_3PH_3~+, CH_3OH_2~+ and CH_3SH_2~+) for biologically important ions using ab initio molecular orbital method. Next, we predicted the geometries and electronic structures of those ions in aqueous solution based on those geometries and charges of the aqueous clusters. For the model ions, two types exist. (1) The geometries of RCOO~- (R=H, CH_3 and F), CH_3O~-, CH_3NH_3~+ and CH_3OH_2~+ in aqueous solution are considerably different from those in the gas phase. It is the primary cause of those differences that the contributions of resonance structures, R~- … CO_2,H~-…CH_2=O, CH_3~+…NH_3 and CH_3~+…OH_2, to those ions in aqueous solution are smaller than those in the gas phase due to the charge transfer from the ions to water molecules. (2) The geometries of CH_3S~-, CH_3PH_3~+ and CH_3SH_2~+ (except for the S-H bond lengths) in aqueous solution are similar to those in th gas phase. It is the primary cause of a little difference that the contributions of resonance structures, H~-…CH_2=S, CH_3~+…PH_3 and CH_3~+…SH_2, to these ions in aqueous solution are close to those in the gas phase due to a little charge transfer from the ions to water molecules. These findings are useful for predicting the geometries of biologically important ions such as amino acids and peptides in aqueous solution.