Modulation of Electric Field and Interface on Competitive Reaction Mechanisms

Recently, much evidence has accumulated, showing that electric fields and water interfaces influence the characteristics and alignment of biomolecules and greatly boost reaction rates. The prototropic tautomerism is a fundamental process in biological systems; however, a comprehensive understanding of the electric field effects and interfacial effects on it is still lacking. In this work, we performed a theoretical study of the modulation of the electric field and the interface on the tautomerism dynamics of solvated glycine by using deep potential molecular dynamics technology with enhanced sampling. The deep learning potentials used were trained to integrate long-range electrostatic interactions in order to better describe the electric field effect. We observed that an external electric field of 10 mV/Å barely changed the key structures involved in tautomerism reactions but significantly influenced their relative free energies and consequently made the transformation from zwitterionic ([Z]) to neutral ([N]) glycine more achievable both thermodynamically and dynamically and altered the optimal reaction mechanism from intramolecular proton transfer (Intra-PT) to intermolecular proton transfer (Inter-PT) involving a separate cationic-glycine-hydroxide ion pair. The detailed analysis revealed that the electric field increased the thermodynamical stability of [N] relative to [Z] by 7 kJ/mol due to the entropy effect and promoted the Inter-PT pathway by electrostatically facilitating the separation of ion pairs, causing the free-energy-barrier decrease of the rate-determined step by approximately 10 kJ/mol. Interestingly, in the air-water interface, due to the interfacial propensity of the glycine and water self-ions, the separation of ion pairs is restricted, slowing the Inter-PT pathways. Nevertheless, the interfacial interconversion between the [Z] and [N] forms of glycine is dynamically accelerated via the Intra-PT pathway due to partial solvation. These findings provide new insights into how the electric field and interfaces modulate thermodynamics, kinetics, and the mechanism of chemical reactions.