As discussed above, in the absence of tartaric acid, our QM/MM metadynamics simulations suggested that the second ET step, which is from the lignin substrate to Trp171, follows a concerted PCET mechanism instead of a pure ET mechanism. To investigate the effect of the single H-bond on the second ET step, we also carried out the QM/MM MD simulation in the presence of the tartaric acid, just as we did for the first ET reaction. Again, we found the tartaric acid could form a persistent and strong H-bond with Glu250, with an average O--H distance of 1.52 Å (see Figure ). In addition, we observed a spontaneous ET from the lignin substrate to the Trp171 cation radical, which yielded the lignin cation radical and the intact Trp171 (Figure ). Such a finding implies that the H-bond between the tartaric acid and Glu250 could change the mechanism of the one-electron oxidation of the lignin substrate. However, how does this H-bond cause such a significant change?