pH Accelerates the Long-Range Electron Transfer for Lignin Degradation via Second-Sphere H-Bonding Interactions

pH plays versatile roles in both biology and chemistry. For instance, pH can regulate the long-range electron transfer (ET) in many proteins. However, the mechanistic basis of many pH-dependent long-range ET processes remain unclear. In this study, we unravel the critical role of pH in accelerating the long-range ET in lignin peroxidase (LiP) using QM/MM MD simulations and the nonadiabatic ET and PCET theories. As a key enzyme involved in lignin degradation, LiP accomplishes the one-electron oxidation of lignin via two consecutive long-range ET reactions: the first one involves ET from Trp171 to the active species of Compound I, affording the Trp171 radical species, while the second one involves ET from the lignin substrate to the Trp171 radical, affording the lignin cation radical species. Our study demonstrates that pH can remarkably modulate the mechanism and kinetics of both ET reactions. Specifically, in the absence of the pH buffer of tartaric acid, our study shows that the first ET leads to a neutral Trp• radical, agreeing with the EPR experiments. Surprisingly, the addition of the tartaric acid stabilizes the Trp-H•+ cation radical via second-sphere H-bonding interactions and accelerates the ET rate for lignin oxidation by several orders of magnitude. These findings are consistent with the experimental information available, which not only expand our understanding on pH-dependent ET reactions in both biology and chemistry, but also provide valuable insights on tryptophan-mediated biological ET reactions.