A theoretical study of solvent effects on the conformational equilibria of neutral glycine in aqueous solution

Abstract

In this work conformational equilibrium of neutral glycine in solution is systematically investigated by using DFT and MP2 methods combined with solvent continuum models. A systematic exploration of the potential energy surface and full geometry optimizations for several conformers have been carried out in the gas phase and aqueous solution at the MP2/6-31+G** and B3LYP/6-31+G** levels. Zero-point and thermal contributions to the free energy have been obtained at the B3LYP level. Both theoretical levels lead to very similar results, in geometrical and energetic terms, both in the gas phase and in solution. Solvent effects play an important role on the conformational equilibria of neutral glycine; so the absolute free energy minimum in solution is not the same as in the gas phase. The changes found when passing from gas phase to aqueous solutions can have important consequences also on the tautomeric equilibrium between the neutral and zwitterionic forms of glycine.