As we mentioned in the introduction, the low pH condition (pH ~3) can speed up the one-electron oxidation of lignin by up to ~10 folds compared to that at the neural pH condition (pH ~7). In experiments, tartaric acid has been widely used as the acid buffer to control the solution pH. Tartaric acid is a diprotic acid, with two pKa values of 3.0 and 4.3, respectively. Thus, diprotic acid will have half population singly protonated and another half doubly protonated at pH ~3. As an organic acid, tartaric acid can act as a proton donor or H-bonding donor to a plausible base. Because the deprotonated Glu250 is the only available base at the surface of LiP which is exposed to the solvent, tartaric acid is likely to form an H-bond with it (see Figure ). As such, we have investigated both ET steps with or without tartaric acid (Figure ). Notably, as our simulations focus on the interaction of Glu250 with a protonated carboxylic group of diprotic acid, either the singly protonated or the doubly protonated form would be reasonable for our simulations. In the present study, we treated the diprotic acid doubly protonated in our simulations.