Nature of Glycine and Its alpha-Carbon Radical in Aqueous Solution: A Theoretical Investigation

Quantum chemistry calculations and classical molecular dynamics simulations have been used to examine the equilibria in solution between the neutral and zwitterionic forms of glycine and also of the glycyl radical. The established preference (by 30 kJ mol(-1)) for the zwitterion of glycine was confirmed by both the quantum chemical calculations and the classical molecular dynamics simulations. The best agreement with experiment was derived from thermodynamic integration calculations of explicitly solvated systems, which gives a free energy difference of 36.6 +/- 0.6 kJ mol(-1). In contrast, for the glycyl radical in solution, the neutral form is preferred, with a calculated free energy difference of 54.8 +/- 0.6 kJ mol(-1). A detailed analysis of the microsolvation environments of each species was carried out by evaluating radial distribution functions and hydrogen bonding patterns. This analysis provides evidence that the change in preference between glycine and glycyl radical is due to the inherent gas-phase stability of the neutral a-carbon radical rather than to any significant difference in the solvation behavior of the constituent species.