Synthetic analogs of active sites of iron-sulfur proteins: bis (o-xylyldithiolato) ferrate (III) monoanion, a structurally unconstrained model for the rubredoxin Fe-S4 unit

To complete the set of synthetic analogs of the three recognized types of active sites in iron-sulfur redox proteins, the compound (Et4N)[Fe((SCH2)2C6H4)2], derived from o-xylyl-alpha,alpha'-dithiol, has been prepared and its structure has been determined by x-ray diffraction. The bischelate anion contains a near-tetrahedral Fe(III)-S4 coordination unit with small rhombic distortions and all Fe-S bond distances in the range 2.252-2.279 A. Its electronic properties have been partially characterized by measurement of electronic absorption, paramagnetic resonance, Mössbauer spectra, and magnetic susceptibility. The analog, as the protein, exhibits the Fe(III)/Fe(II) redox couple. These results substantiate designation of [Fe((SCH2)2C6H4)2]- as a synthetic analog of the Fe(III)(S-Cys)4 center in oxidized rubredoxin proteins. Comparison of the analog structure with that of the Clostridium pasteurianum rubredoxin active site shows that the former is substantially less distorted from idealized tetrahedral symmetry, and is considered to represent an essentially unconstrained structural model of the latter. Provided the grossly distorted tetrahedral stereochemistry of the protein site persists through final structural refinement, the analog-protein structural comparison supports an entatic state description of oxidized rubredoxin.