title: Antisymmetric exchange in [2Fe-2S]1+ clusters: EPR of the Rieske protein from Thermus thermophilus at pH 14.
abstract: [2Fe-2S] clusters found in the xanthine oxidase family of proteins exhibit an S = 1/2 EPR feature, called signal II, for which one g-value is significantly above g = 2.0. The g-values of signal II cannot be explained with the standard spin coupling model that has been so successful in describing the g = 1.94 signals of [2Fe-2S] ferredoxins. We have studied the EPR spectra of the Rieske protein from Thermus thermophilus at pH 14 and observed a signal II-type EPR spectrum, with g-values at 1.81, 1.94, and 2.14. It is shown that the g-values of signal II can be explained by including an antisymmetric exchange term, d.S1xS2, in the spin Hamiltonian. The presence of this term is sensed by EPR if the isotropic exchange coupling constant J is sufficiently small. For the Rieske protein we determined J = 43 cm-1 which is at least 4 times smaller than the J values reported for [2Fe-2S] clusters that yield standard g = 1.94 signals.
authors.affiliation: Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA.
authors.forename: Filipe
authors.initials: F
authors.lastname: Tiago de Oliveira
authors.forename: Emile L
authors.initials: EL
authors.lastname: Bominaar
authors.forename: Judy
authors.initials: J
authors.lastname: Hirst
authors.forename: James A
authors.initials: JA
authors.lastname: Fee
authors.forename: Eckard
authors.initials: E
authors.lastname: Münck
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D014450:Electron Transport Complex III / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000737:chemistry*; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D013455:Sulfur / Q000737:chemistry*; D016963:Thermus thermophilus / Q000737:chemistry*
chemical_list: D007506:Iron-Sulfur Proteins; C037989:Rieske iron-sulfur protein; D013455:Sulfur; D007501:Iron; D014450:Electron Transport Complex III