Michio Nakata

Analogues of reduced rubredoxin: positive shifts of redox potentials of cysteine-containing peptide iron(II) complexes

Synthese des complexes de Fe(II) de Z-Cys-Thr-Val-Cys et Z-Cys-Pro-Leu-Cys-OMe. Potentiels redox du couple Fe(II). Fe(III) dans ces complexes, dans DMSO

Chemical simulation of rubredoxin by iron(II)/tetrapeptide complexes in aqueous triton X-100 micelle solution

The iron(II) complexes of the type Fe(L-L)2 (L=anion of Z-Cys-Thr-Val-Cys-OMe, Z-Cys-Pro-Leu-Cys-OMe, Z-Cys-Ala-Ala-Cys-OMe, or Z-Ala-Cys-OMe (Z=benzyloxycarbonyl)) were synthesized in aqueous 10% Triton X-100 solution and were characterized by the absorption, CD and MCD spectra. The spectra indicated a core structure similar to that of native rubredoxin. Among the complexes, only the iron(II) complex of Z-Cys-Pro-Leu-Cys-OMe provides a quasi-reversible redox couple at −0.37 V vs. standard calomel electrolyte in aqueous Triton X solution (cf. −0.30 V vs. standard calomel electrode for native rubredoxin in aqueous solution). The iron(II) complex of Z-Cys-Pro-Leu-Cys-OMe exhibits a remarakable electron-transfer activity in the system of NADPH/ferredoxin:NADP + oxidoreductase/FeII peptide complex/ferricytochrome c.

Binuclear molybdenum(V) complexes of L-cysteine-containing dipeptides as catalysts for the reduction of azobenzene

Abstract Doubly sulfur-bridged binuclear Mo(V) complexes of cysteine-containing dipeptides, Mo2O2S2(Cys-containing peptide anion)2, and related ligands were prepared and examined as catalysts for the reduction of azobenzene with NaBH4 in protic media. The complexes having 5-membered chelation of cysteine residue, e.g. Mo2O2S2(Cys/3-Phe/3-OMe)2 were active for formation of hydrazobenzene. A small initial activity for the reductive cleavage to aniline was found for complexes involving larger chelate rings with peptide ligands, e.g. Pro/3-Cys/3-OMe.

Mo(IV) Complexes of cysteine-containing peptides and their interaction with 4Fe ferredoxin model complexes

Abstract Mo(Ac-cys-OH)4, Mo(S-tBu)2 (Z-Ala-cys-OMe)2 (Z = carbobenzoxy), and Mo(Z-cys-Ala-Ala-cys-OMe)2 were prepared from Mo(S-tBu)4 and the corresponding peptides using a ligand exchange method. The Mo(IV) complexes were characterized by 1H NMR, visible, and CD spectra. Reaction of these complexes with 4Fe ferredoxin model complexes, [Fe4S4(SR)4]2−, was examined by CD spectroscopy to reveal ligand exchange between these complexes. Formation of a MoFe mixed cluster was implied by the Cd results.

Redox potentials of 1Fe- and 4Fe-ferredoxin model complexes of cysteine-containing peptides in micelle

Abstract Active centers of ironsulfur proteins and rubredoxin are surrounded with cysteine thiolates and peptide bonds in a hydrophobic environment, where specific interactions, such as NH⋯S hydrogen bonds, exist between FeS cores and peptide bonds. The active site of rubredoxin consist of one iron and two specific sequences, Cys–X–Y–Cys, as shown in Fig. 1. Thus, Z–Cys–Thr–Val–Cys–OMe and Z–Cys–Pro–Leu–Cys–OMe were synthesized. Z–Cys–Ala–Ala–Cys–OMe was also examined as a chelating ligand for reference. A dipeptide, Z–Ala–Cys–OMe, was examined as non-chelating peptide. 1Fe and Fe 4 S 4 complexes of t-Boc GlyCysGly 4 NH 2 were reported by Rydon [1] and Holm [2], respectively. However, the -Gly–Gly- sequence between the two Cys residues is not preferable for turn conformation which is essential for chelation and for formation of the NH⋯S hydrogen bonds. We reported that Fe(III)/Z–Cys–Ala–Ala–Cys–OMe is a good spectral model of rubredoxin in Me 2 SO [3]. The electrochemical properties of 1Fe and Fe 4 S 4 complexes are important in aqueous solutions. The redox potentials of ferredoxins are influenced by the core as well as the surrounding environments. In native proteins, the core is non-polar and surrounded by polar aqueous environments. Therefore, we examined these model complexes spectrally and electrochemically in micelle by using 2–10% solutions of Triton X-100. The redox potential values obtainable by cyclic voltammogram in micelle are compared with the values of native rubredoxin or ironsulfur proteins in aqueous solution. Fe(II)/Z–Cys–Pro–Leu–Cys–OMe (1:2) complex in aqueous micelle exhibited CD extrema at 309 nm (Δϵ: −24.2) and 332 nm (Δϵ: 10.9), similar to those of reduced rubredoxin [4]. A redox couple of Fe(II)/Fe(III) was observed for Fe(II)/Z–Cy–Pro–Leu–Cys–OMe (1:2) at −0.37 V(SCE) in aqueous micelle, which is very close to −0.30 V(SCE) reported for rubredoxin. Such a positive shift was observed for the first time in synthetic model complexes of rubredoxin. No redox couple was found for Fe(II)/Z–Cys–Thr–Val–OMe (1:2) or Fe(II)/Z–Cys–Ala–Ala–Cys–OMe (1:2) in aqueous micelle, whereas Fe(II)/Z–Ala–Cys–OMe (1:4) decomposed gradually in micelle. Observation of the redox couple of [Fe(S 2 - o -xyl) 2 ] 2− [5] at −0.64 V(SCE) in aqueous micelle reveals that the potentials of the Fe(II) complexes having two specific peptide ligands (Cys–X–Y–Cys) shift extraordinarily to the positive side. CD and visible spectra of Fe 4 S 4 -type complexes of Cys-containing peptides, Z–Cys–Gly–OMe, Z–Cys–Gly–Ala–OMe, and Z–Cys–Gly–Ala–Cys–OMe, in aqueous micelle were found to be very similar to native 4Fe4S proteins. These complexes provided redox couples (2 – /3 –) except for the Fe 4 S 4 complex of Z–Cys–Gly–OMe.