Rubredoxin peptide model complexes

III. Chelating Effects of Peptide Ligands A. Rubredoxin Peptide Model Complexes... 

The chelation effect also brings about a stabilization of the — 1 state of the peptide model complexes as indicated by the thermal stability and redox behavior. Only [Fe(Z-cys-Pro-Leu-cys-OMe)2] exhibits a relatively reversible redox couple in the cyclic voltammogram measurement, but the others do not (20). The bulkiness of side chains of the X and Y residues in Cys-X-Y-Cys probably restricts the adoption of the inherent by preferable conformation (ift = 0°), resulting in a more restricted orientation of Fe-S-C. In fact, the X-ray analysis of native rubredoxin shows that two of the Fe-S torsion angles are restricted and the other two are normal, i.e., conformationally more stable. 

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 redox stability of the various model complexes was examined in aqueous Triton X-100 solutions. Only [Fe(Z-cys1-Pro-Leu-cys-OMe)2]2 exhibits a quasireversible redox couple at — 0.37 V versus SCE, which is considered to simulate closely the value of native rubredoxin, although a small difference (0.1 V) still remains (7). Other model peptide complexes are rapidly decomposed by hydrolysis in an aqueous micellar solution and do not exhibit even an oxidation peak. The simple alkythiolate model [Fe(S2-o-xyl)2]2 exhibits a quasireversible redox couple at —1.0 V versus SCE in an aqueous micellar solution. Therefore, macro-ring peptide chelation with some hydrophobic side chains, is required in order to induce redox stability (reversibility of cyclic voltammogram measurements) of the Fe(III/II) couple. 

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