A computational model of the glycine tautomerization reaction in aqueous solution

A theoretical study of the internal proton transfer reaction of glycine (Gly) in aqueous solution was performed by means of steered molecular dynamics (SMD) simulation with solute-solvent interaction potentials derived from AMBER van der Waals parameters and QM/MM electrostatic charges in solution. Thermodynamic calculations and the analysis of the solvation structure, dynamic properties, and vibrational spectra associated with the species involved in the tautomerization process were performed. The results obtained for the Gibbs free energy activation and reaction (ΔG(≠) =5.28 kcal mol⁻¹ and ΔG(R)=-6.65 kcal mol⁻¹), the solute-solvent interaction energy of the different glycine structures, and the hydrogen-bond lifetimes are in agreement with previous studies. These hydrations drive an increase in the diffusion coefficient and a decrease in the time of reorientation when the process takes place in the direction Z-Gly → TS-Gly → N-Gly. The vibrational spectrum associated with the normal modes of the bridge hydrogen atoms shows the N-H stretching at ν(s)=3,470 cm⁻¹ and ν(as)=3,470 cm⁻¹, the O-H stretching at 3,205 cm⁻¹, and the NHO bending at about 1,400 cm⁻¹, in agreement with previously reported data.