Multiwater-Assisted Proton Transfer Study in Glycinamide Using Density Functional Theory

Abstract

Multiwater-assisted proton transfers (PTs) involving two and three water molecules from the amide nitrogen to carbonyl oxygen atom in model peptide compound glycinamide have been investigated employing the B3LYP/6-311++G** level of theory. The thermodynamic and kinetic parameters, such as tautomeric energies, equilibrium constants, barrier heights, and rate constants, have been predicted, respectively. The relevant quantities associated with the proton-transfer processes, such as geometrical structures, interaction energies, and intrinsic reaction coordinate (IRC) calculations, have also been studied. In addition, the factors influencing the thermodynamic and kinetic parameters, such as temperature dependences, solvent effects, and deuteration effects, have also been explored qualitatively, respectively. Computational results show that the PT barrier heights are 16.93 (4.98) and 18.95 (6.67) kcal/mol in the forward (reverse) directions with the assistances of two and three water molecules, which are reduced significantly by 28.43 (25.95) and 26.41 (24.26) kcal/mol compared with those of direct intramolecular PT, respectively. Both of the PT processes proceed with a concerted mechanism, reflecting the bifunctional roles of the water, that is, it can accept a proton from the donor site in glycinamide and transfer a different proton to the acceptor site in glycinamide. The optimal numbers of water molecules directly participating in the PT may be two compared with the barrier heights among the various water-assisted PT cases. Applications of the IPCM model within the framework of the self-consistent reaction filed (SCRF) theory indicate that the bulk solvent has a subtle influence on the thermodynamic and kinetic properties.