MC/MO SOC-SCF calculation of parity-violating energy of l-alanine zwitterion in aqueous solution

Abstract

A combination of Monte Carlo (MC) simulation and ab initio spin-orbit coupling SCF (SOC-SCF) method was used to evaluate parity-violating energy, E pv, of l-alanine zwitterion ( l-ALAZ) in aqueous solution. In the MC simulation, the effective pair potential functions, which include the effect of the polarization of l-ALAZ caused by salvation were determined and employed. The 100 solution structures were picked up at every 1 M steps of the MC simulation of the l-ALAZ aqueous solution, and the E pv energy was calculated for the l-ALAZ-(H 2O) n ( n=1–10) clusters in each solution structure by the SOC-SCF/6-31G method. The mean deviation of E pv for the l-ALAZ-(H 2O) 10 clusters was large, 2×10 −20 hartree, and this comes from the fact the coordination of H 2O at the CO 2 site brings about large effect on E pv. Although, the small change in the orientation of H 2O to the CO 2 group may change the sign of E pv, the expectation value of E pv of l-ALAZ is positive for thermally stable conformations in aqueous solution at 298 K. The present MC/MO calculations have shown that the E pv value of l-ALAZ is positive when the solvent effect is introduced by explicit H 2O molecules. This is same as that observed in previous calculations with the continuum model of solvation. The present calculations provide support for the destabilization of l-ALAZ compared to d-ALAZ at thermally accessible conformations in aqueous solution.