Simultaneous interpretation of Mössbauer, EPR and 57Fe ENDOR spectra of the [Fe4S4] cluster in the high-potential iron protein I from Ectothiorhodospira halophila.
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.
Dilg
A W
AW
Physik-Department E17, Technische Universität München, Garching, Germany.
Mincione
G
G
Achterhold
K
K
Iakovleva
O
O
Mentler
M
M
Luchinat
C
C
Bertini
I
I
Parak
F G
FG
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
D001426:Bacterial Proteins; D007506:Iron-Sulfur Proteins; D045322:Photosynthetic Reaction Center Complex Proteins; D011994:Recombinant Proteins; C007943:high potential iron-sulfur protein