Simultaneous interpretation of Mössbauer, EPR and 57Fe ENDOR spectra of the [Fe4S4] cluster in the high-potential iron protein I 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.

Protein–DNA charge transport: Redox activation of a DNA repair protein by guanine radical

“…The long-lived transient, with a maximum absorption at 405 nm, is attributed primarily to formation of [4Fe-4S] 3ϩ and possibly also [3Fe-4S] 1ϩ ; both absorb more in this region than does [4Fe-4S] 2ϩ (60)(61)(62)(63). The shape of the spectrum has some features that resemble that of a tyrosine radical, and several tyrosine residues surround the cluster in the enzyme (36, 37), but the extinction coefficient for [4Fe-4S] 3ϩ is expected to be significantly higher in this region, so that tyrosine radical or even guanine radical may not be distinguishable.…”

DNA repair glycosylases with a [4Fe–4S] cluster: A redox cofactor for DNA-mediated charge transport?

“…The peak at g = 2.02 is instead typical of the [3Fe-4S] 1+ cluster [58]. The signal at g = 2.08 and its accompanying secondary feature at g = 2.06 is assigned to the [4Fe-4S] 3+ cluster [58,68]. It is noteworthy that, with poly(dAT) and MutY, both signals are still observed, albeit at a much lower intensity.…”

Direct Observation of Quantum Coherence in Single-Molecule Magnets

“…Using pulsed electron-spin resonance (ESR) [16], spin-spin relaxation or coherence times (denoted T 2 ) were determined in several molecular magnets and metalloproteins with S = 1 2 ground states. In iron-sulfur clusters, for instance, T 2 is several hundreds of nanoseconds [17,18,19], while in the Cr 7 Ni and Cr 7 Mn antiferromagnetic rings, T 2 ≈ 400 ns at 4.5 K, increasing to 3.8 µs for deuterated Cr 7 Ni at 1.8 K [20]; and T 2 = 2.6 µs for an antiferromagnetic iron(III) trimer [21]. Recently, quan-tum oscillations were observed in the molecular magnet V 15 [22].…”