Significant sequestration by the pro tein may be

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proteins this unit requires strong binding. Significant sequestration by the pro- tein may be needed to stabilize the Fe3S4 unit against oligomerization through sulfide bridges or, alternatively, rearrangement to the stable Fe4S4 center. K. Fe-S Chemistry: Comments and New Structures The first successful model system for an iron-sulfur protein was an analogue of the Fe4S4 system, i.e., the system with the largest presently established biolog- ical Fe cluster. The reactions used to synthesize the cluster shown in Figure 7.13 are said to involve self-assembly, meaning that starting materials are sim- ply mixed together, and thermodynamic control causes the cluster to assemble in its stable form. Interestingly, the Fe4S4-containing proteins, such as those of C. pasteurianum, are considered to be among the most ancient of proteins. Perhaps on the anaerobic primordial Earth, Fe-S clusters self-assembled in the presence of protein ligands to form the progenitors of the modem ferredoxins. Much progress has been made in synthetic chemistry, and it is clear that both understanding and control of Fe-S chemistry are continuing to grow. New
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398 7 I FERREDOXINS, HYDROGENASES, AND NITROGENASES: METAL-SULFIDE PROTEINS preparations for known clusters continue to be found, and new clusters continue to be synthesized, Although many of the new clusters appear to be abiological, we should not ignore them or their potential. They add to our understanding of Fe-S chemistry in general, and serve as starting points in the study of hetero- nuclear clusters, There is also the distinct possibility that one or more of these synthetic clusters represent an existing biological site that has not yet been iden- tified in an isolated system. Among the "nonbiological" structures that have been synthesized are com- plexes with Fe6S63+/2+ cores 177,178 including the thioprismanes, 108,109,177~179 the octahedron/cube Fe6S83+ cores,180,181 the Fe6Sl- cores 176a,b discussed above, the adamantane-like Fe6(SR)10 4- complexes related to Zn and Cu structures in metallothioneins,182 basket Fe6S62+ /Fe6S6 + cores,182a,b,c,d monocapped pris- matic Fe7S63+ cores,183 the cube/octahedron Fe8S65 + cores,179 and the circular Na+-binding Fe18S301O- unit. 183a ,b Representative ions are shown in Figure 7.19. Some of these cores are stabilized by distinctly nonbiological phosphine ligands. Nevertheless, one should not a priori eliminate any of these structures from a possible biological presence. Indeed, recently a novel, apparently six-iron pro- tein from Desulfovibio gigas has been suggested 183c to have the thioprismane core structure first found in model compounds. L. Detection of Fe-S Sites Several recent reviews have concentrated on the ways in which the various Fe- S centers can be identified in newly isolated proteins. 5,6, 184 It is instructive to summarize the central techniques used in the identification of active sites. Op- tical spectra are usually quite distinctive, but they are broad and of relatively low intensity, and can be obscured or uninterpretable in complex systems. MCD spectra can give useful electronic information, especially when the temperature
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