interactions which can be done when more structural information is available, but they do provide order of magnitude estimates of interaction strengths. Evidently, considerable rearrangement of charge driven by reduction significantly raises the effective dielectric. It seems likely that centres S-2 and FR-2 are apparently low-potential clusters merely because they are the middle clusters in the sequence of electron transfer. Interactions in the equilibrium state favour the separation of electrons; at lower levels of reduction, transient electron transfer through the intermediate cluster could be feasible without energetic cost.