title: Direct measurement of the hydrogen-bonding effect on the intrinsic redox potentials of [4Fe-4S] cubane complexes.
abstract: 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, [Fe(4)S(4)(SEt)(3)(SC(n)H(2n+1))](2-) and [Fe(4)S(4)(SEt)(3)(SC(n)H(2n)OH)](2-) (n = 4, 6, 11; Et = C(2)H(5)). Intrinsic redox potentials for the [Fe(4)S(4)](2+/3+) redox couple involved in these complexes were measured by photoelectron spectroscopy. The oxidation energies from [Fe(4)S(4)(SEt)(3)(SC(n)H(2n)OH)](2-) to [Fe(4)S(4)(SEt)(3)(SC(n)H(2n)OH)](-) were determined directly from the photoelectron spectra to be approximately 130 meV higher than those for the corresponding [Fe(4)S(4)(SEt)(3)(SC(n)H(2n+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 [Fe(4)S(4)(SEt)(3)(SC(6)H(12)OH)](2-). The current data provide a direct experimental measure of a net H-bonding effect on the redox potential of [Fe(4)S(4)] clusters without the perturbation of other environmental effects.
authors.affiliation: Department of Physics, Washington State University, 2710 University Drive, Richland, Washington 99352, USA.
authors.forename: Xin
authors.initials: X
authors.lastname: Yang
authors.forename: Shuqiang
authors.initials: S
authors.lastname: Niu
authors.forename: Toshiko
authors.initials: T
authors.lastname: Ichiye
authors.forename: Lai-Sheng
authors.initials: LS
authors.lastname: Wang
mesh_terms: D001665:Binding Sites; D003198:Computer Simulation; D006860:Hydrogen Bonding; D007501:Iron / Q000737:chemistry*; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D008958:Models, Molecular; D009010:Monte Carlo Method; D010084:Oxidation-Reduction; D013057:Spectrum Analysis; D013455:Sulfur / Q000737:chemistry*
chemical_list: D007506:Iron-Sulfur Proteins; D013455:Sulfur; D007501:Iron