Electron transfer proteins, modeling

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Porphyrin complexes are particularly suitable cores to construct dendrimers and to investigate how the behavior of an electroactive species is modified when surrounded by dendritic branches. In particular, dendritic porphyrins can be regarded as models for electron-transfer proteins like cytochrome c [42, 43]. Electrochemical investigation on Zn-porphyrins bearing polyether-amide branches has shown that the first reduction and oxidation processes are affected by the electron-rich microenvironment created by the dendritic branches [42]. Furthermore, for the third generation compound all the observed processes become irreversible. 

Cytochromes are electron-transfer proteins having one or several haem groups. Cytochrome c binds to the protein by one or, more commonly two, thioether bonds involving sulphydryl groups of cysteine residues. The fifth haem iron ligand is always provided by a histidine residue. Cytochrome c has been proved to be a useful model system for studying the relationship between protein structure and thermostability due to the availability of its three-dimensional structure from a wide variety of organisms, both mesophiles and thermophiles. 

Rubredoxin is an electron-transfer protein with an Fe(IlI)/Fe(lI) redox couple at -0.31 V (SCE) in water (20). Our peptide model, [Fe( Cys-Pro-Leu-Cys-OMe)2] (Z = benzyloxycarbonyl) (21) exhibits its Fe(lll)/Fe(ll) redox couple at -0.50 V (SCE) in Mc2SO (9). This is similar to the value observed for the native protein when the difference of the solvent is taken into account. When the model complex is solubilized in water by formation of micelles with addition of the non-ionic detergent, Triton X-KX), we also observed a quasi-reversible redox couple at -0.37 V (SCE) (5). The Fe(lll) complexes of Cys-X-Y-Cys peptides also exhibit a characteristic MCD band at 350 nm due to ligand-to-metal charge transfer which has also been found in oxidized rubredoxin (4). 

The multinuclear tetrahedral iron clusters have the potential for additional formal oxidation states. Because not all of these states have been found in proteins or model compounds, it is possible that some oxidation states may be unstable. For a given Fe S protein only one redox couple is used the other possible states appear to be excluded by restrictions of the protein structure. This selection rule is illustrated with two 4Fe 4S low-molecular-weight electron transfer proteins ferredoxin and high-potential iron protein (HiPIP). The 4Fe 4S clusters in both proteins were shown by X-ray crystallography to be virtually identical. However, the redox potential and oxidation states for the two proteins are vastly... 

A number of situations may be visualized. Electron transfer may take place between a pair of redox proteins in solution. Certain reactions in the cytoplasm of the red blood cell fall into this category, such as that between hemoglobin and cytochrome b reductase. These reactions will probably occur by an outer-sphere mechanism, as was described earlier for model reactions between isolated electron-transfer proteins and also between these proteins and simple complexes. Interaction between such proteins probably utilizes specific charged areas on their surfaces. The possibility of inner-sphere reactions may have to be considered in a few cases. 

Diederich et al.11921 reported the divergent synthesis of dendrimers possessing porphyrin cores with the aim of modeling redox potentials of electroactive chromophores via environmental polarity modification. The dendrimers thus can be considered as electron-transfer protein mimics for such proteins as cytochrome c oxidation potentials for cytochrome c in aqueous solution are known to be 300-400 mV more positive than those reported for similarly ligated heme mimics lacking hydrophobic peptide encapsulation J193a ... 

Tetrahedral iron sulfide species that can have two or more charge states are involved in many electron transfer proteins.737 The core structures that have been identified in proteins include the single iron centres of rubredoxin, and the two iron/two sulfide and four iron/ four sulfide centres of various ferredoxins. A wide range of model complexes have been prepared and characterized, and the results of this work, as well as that on the natural system, have been extensively reviewed.737-744... 

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