The complex [Fe4S4(S-z-Bu)4]2- in DMSO solution reacts readily with Ac-L-Cys-NHMe and the lcysteinyl peptides z-BOC-Gly-Cys-Gly-Gly-Cys-Gly-Gly-Cys-Gly-NH2 and z-BOC-Gly-Cys-Gly-Gly-Cys-Gly-Gly-Cys-Gly-Gly-Cys-Gly-NH2, to afford the species [Fe4S4(S-Cys(Ac)NHMe)4]2- (2), [Fe4S4(9-peptide)(S-z-Bu)]2- (3), and [Fe4S4(9-peptide)]2- (5) isolated as their tetraphenylarsonium salts. Reaction of 3 with Ac-L-Cys-NHMe affords the salt of [Fe4S4(9-peptide)(S-Cys(Ac)NHMe)]2- (4). The pmr and electronic spectra and voltammetry of 2-5 were determined in DMSO or 80% DMSO-20% H2O solutions. Contact shifted cysteinyl methylene proton resonances were observed at ca. 10-14 ppm downfield of TMS and provide further confirmation of signal assignments in oxidized ferredoxin (Fdox) and reduced high-potential (HPred) proteins. Electronic spectra and 2-/3-redox potentials of 2-5 more closely resemble those of the proteins in aqueous solution than is the case for simple alkylthiolate tetramers such as [Fe4S4(SEt)4]2-. However, potentials for the synthetic species are estimated to be ≳ 300 mV more negative than that usually found for FdOX/Fdred. Significantly, spectra of 2-5 and that of the unfolded form of Chromatium HProd in 80% DMSO are very similar and their redox potentials approach the upper limit estimated for the protein. These results indicate that denaturation of the protein affords the active site in a condition such that features inherent to isolated [Fe4S4(S-Cys)4] clusters, as exemplified by 2-5, are developed.