5. pH and micro-environmental H-bond network in modulating the biological ET reactions. It was found experimentally that the oxidized Trp has a pKa of 4.5. This implies that at pH=3.06 the oxidized Trp would exist in its protonated form, while at pH=7.39 the oxidized Trp would exist in its neutral radical form. Such a finding agrees with our simulations. Our simulations indicate that in the absence of tartaric acid, which corresponds to the high pH condition, Glu250 can act as an efficient base to receive the proton from the oxidized Trp, resulting in a neutral radical species Trp • (see Scheme 2a). In comparison, at low pH condition, when tartaric acid is available to strongly H-bonded to Glu250, the basicity of Glu250 can be significantly undermined, which prevents it from accepting a proton from the oxidized Trp. In such a case, the cation radical species Trp-H •+ is stabilized (see Scheme 2b). Notably, Trp-H •+ has a higher electron affinity than Trp • , thus rendering a higher driving force for the one-electron oxidation of the lignin substrate. Although the above explantion is reasonable, here we note that pH is a statistical property which does not provide molecular details. It was generally found that pH can change the protonation/deprotonation of the electron donor/acceptor directly, through which one could modulate the redox potential of the electron donor/acceptor, thereby modulating the ET reaction rate. In this work, we show that pH can modulate the redox property of an electron donor/acceptor via the H-bonding network in LIP. Such a finding provides fundamental insights into pH-regulated long-range ET processes in biological systems and has significant implications for the designs of effective ET systems.