Simultaneous interpretation of Mossbauer, EPR and Fe-57 ENDOR spectra of the [Fe4S4] cluster in the high-potential iron protein I from Ectothiorhodospira halophila

Mossbauer spectra of the oxidized [Fe4S4](3+) and the reduced Fe4S4](2+) cl usters in the high-potential iron protein I from Ectothiorhodospira halophi la were measured in a temperature range from 5 K to 240 K. EPR measurements and Fe-57 electron-nuclear double resonance (ENDOR) experiments were carri ed out with the oxidized protein. In the oxidized state the cluster has a n et spin S=1/2 and is paramagnetic. As common in [Fe4S4](3+) clusters, the M ossbauer spectrum was simulated with two species contributing equally to th e absorption area: two Fe3+ atoms couple to the "ferric-ferric" pair, and o ne Fe2+ and one Fe3+ atom give the "ferric-ferrous pair". For the simulatio n of the Mossbauer spectrum, g-values were taken from EPR measurements. A-t enser components were determined by Fe-57 ENDOR experiments that turned out to be a necessary source of estimating parameters independently. In order to obtain a detailed agreement of Mossbauer and ENDOR data, electronic rela xation has to be taken into account. Relaxing the symmetry condition in a w ay that the electric field gradient tensor does not coincide with g- and A- tensors yielded an even better agreement of experimental and theoretical Mo ssbauer spectra. Spin-spin and spin-lattice 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 Mossbauer fit were compared and yield nearly identical values. The reduced cluster h as one additional electron and has a diamagnetic (S=0) ground state. All th e four irons are indistinguishable in the Mossbauer spectrum, indicating a mixed-valence state of Fe2.5+ for each.