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

B. V. Depamphilis, B. A. Averill, T. Herskovitz, L. Que, R. H. Holm

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Abstract

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” (HPred) and oxidized ferredoxin (Fdox) 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 σp 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 HPred and Fdox 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 HPred to HPs-red in this solvent but are estimated to be ca. 0.2−0.6 V more negative than E0 values for Fdox/Fdred 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.

Original languageEnglish (US)
Pages (from-to)4159-4167
Number of pages9
JournalJournal of the American Chemical Society
Volume96
Issue number13
DOIs
StatePublished - Jun 1 1974

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