title: Simultaneous interpretation of Mössbauer, EPR and 57Fe ENDOR spectra of the [Fe4S4] cluster in the high-potential iron protein I from Ectothiorhodospira halophila.
abstract: Mössbauer spectra of the oxidized [Fe4S4]3+ and the reduced [Fe4S4]2+ clusters in the high-potential iron protein I from Ectothiorhodospira halophila were measured in a temperature range from 5 K to 240 K. EPR measurements and 57Fe electron-nuclear double resonance (ENDOR) experiments were carried out with the oxidized protein. In the oxidized state the cluster has a net spin S = 1/2 and is paramagnetic. As common in [Fe4S4]3+ clusters, the Mössbauer spectrum was simulated with two species contributing equally to the absorption area: two Fe3+ atoms couple to the "ferric-ferric" pair, and one Fe2+ and one Fe3+ atom give the "ferric-ferrous pair". For the simulation of the Mössbauer spectrum, g-values were taken from EPR measurements. A-tensor components were determined by 57Fe ENDOR experiments that turned out to be a necessary source of estimating parameters independently. In order to obtain a detailed agreement of Mössbauer and ENDOR data, electronic relaxation has to be taken into account. Relaxing the symmetry condition in a way that the electric field gradient tensor does not coincide with g- and A-tensors yielded an even better agreement of experimental and theoretical Mössbauer spectra. Spin-spin and spinlattice relaxation times were estimated by pulsed EPR; the former turned out to be the dominating mechanism at T = 5 K. Relaxation times measured by pulsed EPR and obtained from the Mössbauer fit were compared and yield nearly identical values. The reduced cluster has one additional electron and has a diamagnetic (S = 0) ground state. All the four irons are indistinguishable in the Mössbauer spectrum, indicating a mixed-valence state of Fe2.5+ for each.
authors.affiliation: Physik-Department E17, Technische Universität München, Garching, Germany.
authors.forename: A W
authors.initials: AW
authors.lastname: Dilg
authors.forename: G
authors.initials: G
authors.lastname: Mincione
authors.forename: K
authors.initials: K
authors.lastname: Achterhold
authors.forename: O
authors.initials: O
authors.lastname: Iakovleva
authors.forename: M
authors.initials: M
authors.lastname: Mentler
authors.forename: C
authors.initials: C
authors.lastname: Luchinat
authors.forename: I
authors.initials: I
authors.lastname: Bertini
authors.forename: F G
authors.initials: FG
authors.lastname: Parak
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D020607:Halorhodospira halophila / Q000737:chemistry*; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D045322:Photosynthetic Reaction Center Complex Proteins; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry; D015204:Spectroscopy, Mossbauer
chemical_list: D001426:Bacterial Proteins; D007506:Iron-Sulfur Proteins; D045322:Photosynthetic Reaction Center Complex Proteins; D011994:Recombinant Proteins; C007943:high potential iron-sulfur protein