Direct measurement of the hydrogen-bonding effect on the intrinsic redox potentials of [4Fe-4S] cubane complexes

To probe how H-bonding effects the redox potential changes in Fe-S proteins, we produced and studied a series of gaseous cubane-type analogue complexes, [Fe4S4(SEt)(3)(SCrH2n+1)](2-) and [Fe4S4(SEt)(3)(SCnH2nOH)](2-) (n = 4, 6, 11; Et = C2H5). Intrinsic redox potentials for the [Fe4S4](2+/3+) redox couple involved in these complexes were measured by photoelectron spectroscopy. The oxidation energies from [Fe4S4(SEt)(3)(SCnH2nOH)](2-) to [Fe4S4(SEt)(3)(SCnH2nOH)](-) were determined directly from the photoelectron spectra to be similar to130 meV higher than those for the corresponding [Fe4S4(SEt)(3)(SCnH2n+1)](2-) systems, because of the OH...S hydrogen bond in the former. Preliminary Monte Carlo and density functional calculations showed that the H-bonding takes place between the -OH group and the S on the terminal ligand in [Fe4S4(SEt)(3)(SC6H12OH)](2-). The current data provide a direct experimental measure of a net H-bonding effect on the redox potential of [Fe4S4] clusters without the perturbation of other environmental effects.