Hydrophobic Effect of Peptide and Related Ligands

A hydrophobic environment around the iron site has been found to exist on the inside of many globular proteins. In the case of HiPIP, an Fe4S4 is buried in the hydrophobic cavity of the protein. The local structure within 5.5 A of the core shows the situation more clearly, as shown in Fig. 16 (53). Especially noticeable here are the Trp side chains, which originate from a characteristic peptide sequence of Trp-Cys or Trp-Cys-Ala at the coordinating cysteine residues. We have examined the effect of Trp by preparation and spectroscopic and electrochemical measurements of its Fe4S4 complexes. 

Peptide Model and Simple Thiolate Complexes of High-Potential Iron-Sulfur Proteins 

Recently, aconitase has been proposed to have a unique Fe4S42 + core at the active site, as shown in Fig. 17 (57). The cluster consists of one reactive Fe ion in one corner of the cubane cluster and three inert Fe ions occupying the three other corners (42). The heterogeneity of the Fe4S42+ core may be caused by the unusual peptide coordination. 

Chemical activation of one of the Fe ions for redox reaction has been postulated. The biochemical significance of this type of enzymatic activation should be pursued in the future. 

Reductions of acetylene, ketone, isonitrile, or nitrile have been carried out using the —3/ —2 redox couple of many model complexes (58-61), but not using the peptide model complex. A proton transfer function has been observed in studies on the ferredoxin model complex, [Fe4S4(SPh)4]2, in toluene/water (62). 

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