Direct simulation of proton-coupled electron transfer across multiple regimes

Abstract

The coupled transfer of electrons and protons is a central feature of biological and molecular catalysis, yet fundamental aspects of these reactions remain poorly understood. In this study, we extend the ring polymer molecular dynamics (RPMD) method to enable direct simulation of proton-coupled electron transfer (PCET) reactions across a wide range of physically relevant regimes. In a systembath model for symmetric, co-linear PCET in the condensed phase, RPMD trajectories reveal distinct kinetic pathways associated with sequential and concerted PCET reaction mechanisms, and it is demonstrated that concerted PCET proceeds by a solvent-gating mechanism in which the reorganization energy is mitigated by charge cancellation among the transferring particles. We further employ RPMD to study the kinetics and mechanistic features of concerted PCET reactions across multiple coupling regimes, including the fully non-adiabatic (both electronically and vibrationally nonadiabatic), partially adiabatic (electronically adiabatic, but vibrationally non-adiabatic), and fully adiabatic (both electronically and vibrationally adiabatic) limits. Comparison of RPMD with the results of PCET rate theories demonstrates the applicability of the direct simulation method over a broad range of conditions; it is particularly notable that RPMD accurately predicts the crossover in the thermal reaction rates between different coupling regimes while avoiding a priori assumptions about the PCET reaction mechanism. Finally, by utilizing the connections between RPMD rate theory and semiclassical instanton theory, we show that analysis of ring-polymer configurations in the RPMD transition path ensemble enables the a posteriori determination of the coupling regime for the PCET reaction. This analysis reveals an intriguing and distinct "transient-proton-bridge" mechanism for concerted PCET that emerges in the transition between the proton-mediated electron superexchange mechanism for fully non-adiabatic PCET and the hydrogen atom transfer mechanism for partially adiabatic PCET. Taken together, these results provide a unifying picture of the mechanisms and physical driving forces that govern PCET across a wide range of physical regimes, and they raise the possibility for PCET mechanisms that have not been previously reported.

On the mechanism of photoinduced dimer dissociation in the plant UVR8 photoreceptor

“…Accurate theoretical models of proton transfer in proteins have been developed (30)(31)(32)(33). For a simple and rough estimate of differences in the free energy barrier for proton transfers (34) from Arg + to Asp − we consider an expression for the reaction free energy barrier, adapted from electron transfer theory, (ΔG 0 + λ) 2 /4λ, where ΔG 0 is the standard free energy of the proton transfer reaction and λ is the equivalent "reorganization energy" (35).…”

Deep proton tunneling in the electronically adiabatic and non-adiabatic limits: Comparison of the quantum and classical treatment of donor-acceptor motion in a protein environment

“…Recent work using ring polymer MD appears to address some of these important issues. 63,64 ACKNOWLEDGMENTS This work is supported by NSF Award Nos. CHE-1243948 (PMC) and CHE-1026369 (PMC and JTS).…”

Non-equilibrium dynamics from RPMD and CMD

“…Two widely used methods based on imaginary-time path integrals are centroid molecular dynamics (CMD) [24][25][26][27][28] and ring-polymer molecular dynamics (RPMD). [29][30][31][32] Although both methods have known artifacts, such as the spurious-mode effect in RPMD [33][34][35] and the curvature problem 35,36 in CMD, they have proven effective for a vast range of chemical applications including the calculation of thermal rate constants, 30,[37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] diffusion coefficients, 31,[56][57][58][59][60][61] and vibrational spectra. [33][34][35][36][62][63][64]…”