The mean-square electronic coupling dependence on the distance between the two porphyrin rings (90° angle between porphyrin rings): XH(P,W), proteins and water included in the coupling calculation at the extended-Hückel level; XH(P), only protein mediation is included in the extended-Hückel coupling calculations. The three distance ranges correspond to the direct contact regime, structured water–mediated regime, and bulk water–mediated regime, respectively. The insets show configurations of the interfacial water molecules typical for each regime. The error bars show the sampling error estimated using a renormalization group-based method (39).