Fig. 5. (A) Electrostatic potentials depicted on electron density iso-surfaces for each cluster system in the ααββ spin pairing combination in the oxidized state. Red regions show volumes of high negative charge while green regions show volumes of neutral charge. [4Fe–4S]HisH is observed to be much less negatively charged compared to [4Fe–4S]Cys. (B) Depictions of lowest unoccupied molecular orbitals (α-LUMOs) for the three clusters in the oxidized state, indicates regions with high probability for hosting the electron upon reduction. The red and blue delineate the wave-function as being either negative or positive, respectively. (C) The presence of HisH elongates the molecular orbitals of the [4Fe–4S], shown by superposition of the[4Fe–4S]HisH LUMO map (white shade) onto the CaFd/CaI electron transfer complex. Orientation of the electronic structure towards the coordinated water molecule imparts directionality to electron transport between CaFd and the accessory [4Fe–4S] cluster relay (shown as dashed arrows).