PNNL

Abstract

To probe how H-bonding effects the effects the redox potential changes in Fe-S proteins, we produced and studied a series of gaseous cubane-type analogue complexes, [Fe4S4(SEt)3(SCnH2n+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 photo-electron spectra to be ~130 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.