The present study provides fundamental insights for how pH buffer modulates the long-range ET at the molecular level. It was generally found that pH could modulate the redox potential of the electron donor/acceptor via its protonation/deprotonation directly. In comparison, our study show that pH buffer can effectively modulate the mechanism and kinetics of long-range ET via second-sphere Hbonding interactions with the ET donor/acceptor. These findings should have far-reaching implications for biological ET reactions. For instance, the redox-active residues, especially Tyr and Trp, are widely involved in biologically important proteins such as PSII, Ribonucleotide reductase (RNR), 100-102 LPMO, 103, 104 and peroxidases. In these proteins, the redox-active Tyr or Trp residues usually form H-bonding interactions with adjacent bases (e.g. Asp, Glu, His). In such cases, the adjacent base may mediate a PCET process or stabilize the cation radical intermediate via H-bonding interactions. As demonstrated in our study, the second-sphere H-bonding interactions, either with acid buffers or other second-sphere residues, could significantly modulate the mechanism and kinetics of the ET reactions. Specifically, for lignin peroxidase, the presence of a single H-bond with the acid buffer of tartaric acid can accelerate the ET rate by several orders of magnitude. Similarly, the second-sphere H-bonding interaction between Asp235 and His175 in cytochrome c peroxidase (CcP) may well stablize the active site Trp191-H •+ cation radical (see Figure ), which could remarkably accelerate the longrange ET from Cytochrome c to the active site of CcP. We would expect pH could effectively regulate the redox property of the redox-active residues via similar intermolecular H-bonding interactions, especially for the redox-active residues which are solvent-exposed or could form Hbonding interactions with the solvent-exposed residues. In sum, our study revealed the mechanistic basis for the pH-dependent biological ET reactions in lignin peroxidase. The obtained knowledge could not only benefit the manipulation of biological ET reactions via engineering the second-sphere residues but also provide guidance for the relevant catalyst designs in the long run.