DEMONSTRATION OF THE GENERALITY OF THE [FE4S4(SR)4]2 (COMPRESSED D2D) [FE4S4(SR)4]3 (ELONGATED D2D) STRUCTURAL-CHANGE IN ELECTRON-TRANSFER REACTIONS OF FERREDOXIN 4-FE SITE ANALOGS - MODEL FOR UNCONSTRAINED STRUCTURAL-CHANGES IN FERREDOXIN PROTEINS

The 57Fe Mössbauer, magnetic susceptibility, magnetization, and EPR properties of an extensive series of (R4'-N)3[Fe4S4(SR)4] compounds (R = Ph. o-, m-, p-C6H4Me, p-C6H4-i-Pr, CH2Ph, CH2-p-C6H4OMe) have been examined in the solid state and in frozen acetonitrile solutions. The cluster trianions serve as analogues of the sites of Fdre(j proteins. Based on similarities and differences in their properties in the solid state the compounds divide into two categories: those whose [Fe4S4(SR)4]3- clusters contain tetragonal or nontetragonal Fe4S4 core structures. The lack of core structural uniformity is attributed to perturbing influences in the solid state. Irrespective of their solid-state category all compounds in frozen solution exhibit essentially coincident properties (most thoroughly documented by EPR and Mössbauer spectral results), which are indicative of a single core structure of a set of closely related core structures. From a previous demonstration of the similarity of properties of [Fe4S4(SPh)4]3- salts in the solid and solution state and the X-ray structure of this cluster, the solution core structure of the set of cluster trianions is identified as elongated tetragonal. These findings, together with the previously established high degree of core structural uniformity in the Fdox analogues [Fe4S4(SR)4]2-, provide substantial experimental support for two proposals: (1) an elongated (idealized) D2d core structure is the intrinsically stable configuration of [Fe4S4(SR)4]3-; (2) the unconstrained idealized core structural change accompanying electron transfer is [Fe4S4(SR)4]2- (compressed D2d) ⇌ [Fe4S4(SR)4]3- (elongated D2d). This process serves as a representation of protein site structural changes in a Fdox/Fdred electron transfer couple in the absence of extrinsic constraints such as might be imposed by protein structural features. © 1979, American Chemical Society. All rights reserved.