Synthetic Analogs of the Active Sites of Iron-Sulfur Proteins. VI. Spectral and Redox Characteristics of the Tetranuclear Clusters [Fe₄S₄(SR)₄]^2-

The electronic spectra (250-700 nm) and redox properties of a series of tetrameric complexes [Fe₄S₄-(SR)₄]²⁻ (R = alkyl, aryl, Ac-Cys-NHMe) have been examined in nonaqueous and aqueous DMSO solutions. [Fe₄S₄(S-Cys(Ac)NHMe)₄]²⁻ and [Fe₄S₄(SC₆H₄NMe₃)₄]²⁺ were generated in solution by ligand substitution reactions of the free thiols and [Fe₄S₄(S-t-Bu)₄]²⁻. Alkyl-substituted tetramers possess two principal absorption bands whose positions (297-308, 417-421 nm) and intensities reveal a relationship with the two prominent absorptions in the spectra of the reduced “high-potential” (HP_red) and oxidized ferredoxin (Fd_ox) proteins. Aryl-substituted tetramers have maxima at 455-510 nm. Polarographic results reveal the electron transfer series [Fe₄S₄(SR)₄]^z with z = 4–, 3 –.2 –, and 1 –; the 2 –/3 – process is reversible or nearly reversible in all cases. Half-wave potentials for this process involving tetramers with R = alkyl and p-C₆H₄X [X = H, Me, ⁺NMe₃(2+/1+ couple)] are linearly correlated with the σ^* and σ_p substituent constants, respectively. The relationship between the total oxidation levels, z, of the synthetic analogs and those of the proteins is summarized. Spectra and 2-/3-potentials of [Fe₄S₄(S-Cys(Ac)NHMe)₄]²⁻ in DMF, DMSO, and DMSO–H₂O are somewhat solvent dependent, and most closely resemble the corresponding properties of HP_red and Fd_ox in aqueous DMSO or water solution. Half-wave potentials in DMSO–H₂O after correction for apparent liquid junction potentials are near the values estimated for reduction of HP_red to HP_s-red in this solvent but are estimated to be ca. 0.2−0.6 V more negative than E_0^′ values for Fd_ox/Fd_red in aqueous solution. These and other results indicate that the protein structure and environment make a significant contribution to the redox potentials of the iron-sulfur clusters.