Three-iron clusters

Gayda, J.-P., Bertrand, P., and Theodule, F.-X. 1982. Three-iron clusters in iron-sulfur proteins An EPR study of the exchange interactions. Journal of Chemical Physics 77 3387-3391. 

Hagen, W.R., Dunham, W.R., Johnson, M.K., and Fee, J.A. 1985a. Quarter field resonance and integer-spin/half-spin interaction in the EPR of Thermus thermophilus ferredoxin. Possible new fingerprints for three iron clusters. Biochimica et Biophysica Acta 828 369-374. 

Kent, T.A., Huynh, B.H., and Miinck, E. 1980. Iron-sulfur proteins Spin-coupling model for three-iron clusters. Proceedings of the National Academy of Sciences of the USA 77 6574-6576. 

Three-iron clusters. Despite the availability of an X-ray crystallographic structure determination of Azotobacter vinelandii ferredoxin I (27), which contains both a normal 4Fe-4S cluster and a 3Fe cluster, the fundamental properties and even the structures... 

Figure 2. Zero-field MSssbauer spectra of dithionite reduced three-iron clusters of (A) D. gigas ferredoxin n, (B) A. vinelandii ferredoxin I, and (C) aconitase isolated from beef heart mitochondria. (Reproduced with permission fixim Ref. 38. Copyright 1982 Elsevier.)...

Many reviews are available. Older work is well summarized in a series on the iron-sulfur proteins,736 while reviews are available on general aspects,737 738 structural features,739 bacterial iron-sulfur proteins740 and three-iron clusters.741... 

In contrast to the well-defined situation described in the previous sections for the one-, two-and four-iron clusters in iron-sulfur proteins, there is still much uncertainty about the structural and biochemical aspects of the three-iron clusters.73 741... 

Conversion of four-iron clusters into three-iron clusters by treatment with ferricyanide has been demonstrated for ferredoxin from Clostridium pasteurianum797 and from Bacillus stearother-mophilus.798 The reverse process, the conversion of a three-iron cluster into a [4Fe-4S] cluster, has been achieved by incubation with Fe2+ and S2 in the presence of dithiothreitol for ferredoxin from Desulfovibrio gigas.799 The reconstitution of [4Fe-4S] clusters allows the possibility of incorporating an isotopic label into the cluster. 

A range of techniques, i.e. Mossbauer, magnetic circular dichroism, ESR and resonance Raman spectroscopy together with EXAFS results on two 3Fe proteins, have been applied to the problem of the structure of these three-iron clusters. These results have been comprehensively and critically reviewed.741... 

The three-iron cluster [Fe3S(S2-o-xyl)3]2-, where S2-o-xyl = o-xylene-a,a-dithiolate, has a pyramidal [Fe3(ju3-S)]4+ central unit.809... 

It is still uncertain whether any of these three-iron clusters have any physiological role at all. It is evident that 3Fe and [4Fe-4S] clusters are interconverted readily, and a 3Fe cluster may lie on the biosynthetic route that leads to the formation of [4Fe-4S] clusters. There is some evidence that the 3Fe cluster of Fdll can participate in electron transfer between redox proteins, but again this has no ultimate significance, as a variety of added mediator complexes are able to carry out such a function. It has been suggested recently that the 3Fe centre in aconitase is capable of promoting partial catalytic activity of the enzyme.813... 

Some of the earlier approaches to the characterization of iron-sulfur clusters have proved to be inadequate in the presence of mixtures of clusters and, particularly, three-iron clusters. Thus the technique of core extrusion has worked well for [2Fe-2S] and [4Fe-4S] cores, but led to confusing results for aconitase and Fdl from A vinelandii as [2Fe-2S] cores were extruded. 

The [Fe3S4] complex occurs naturally in a completely oxidized (3Fe ) and mixed Fe /Fe° oxidation states. Fully reduced forms of the three-iron clusters in either synthetic or natural systems have not been isolated. In mixed-valence clusters, electron delocahzation and antiferromagnetic pairing... 

The Zn complex is of particular interest because it stabilizes the more reduced forms of three-iron clusters but does not affect the spin coupling problem since it is diamagnetic. 

Broken Symmetry Analysis for a Three-Iron Cluster. 

We consider first a three iron cluster with three equivalent metal sites in the "singly reduced", Fe(II)/Fe(III)/Fe(III) formal oxidation state. In the high spin configurations, the first five d-electrons on each site are aligned in a parallel fashion, say spin-up. We form three basis configurations by allowing the final d-electron (which must be spin-down) to reside in turn on each of the three sites. We have worked out the matrix elements of the... 

Coordinates and sphere radii for the linear three-iron cluster are given in our earlier paper (9) based on synthetic model clusters (16). 

Figure 1. Orbital energies (in Rydberg atomic units) for the high spin form of the cubane three-iron cluster. Solid lines show spin-up orbital energies, dashed lines show spin-down. Numbers beside each line give the percent iron d-character in that orbital. The two orbitals marked with an asterisk (of a and e symmetry) are the potential locations for the final electron. The plot corresponds to the state with the final electron in the a orbital marked with an asterisk.

Table I summarizes the results of our calculations, along with previous work on two- and three-iron clusters. As we have noted before, the general tendency of the Xa scattered wave method is to overestimate J for example, the experimental estimate of J for the oxidized linear three-iron cluster (18) is 300 cm, about 45% lower than the value we estimate. Estimates for J in oxidized two-iron clusters are smaller than our estimates by similar amounts (6). We expect the behavior to be about the same for the reduced species, but that the qualitative trends should be correct. We have discussed the results of the oxidized three-iron clusters in our previous paper (9), showing that the Xa calculations predict a ground state for the...

The results reported here use the Xa exchange-correlation function, which has historical interest and can be compared to past calculations. Within the local density approximation, parametrizations that include the correlation effects found in a uniform electron gas often give a better account of spin-dependent properties (19). Since correlation effects generally stabilize low spin species more than high-spin states (20), one would expect correlation effects to increase J over the values reported here, and this was indeed found in our earlier studies of oxidized three-iron clusters (9). Calculations on the reduced species using improved exchange-correlation potentials are in progress. 

I) a three-iron cluster with a non-identified number of sulfur atoms27 35), arranged as a unique center or participating in some more complex arrangement ... 

---