Paramagnetic States of Four Iron−Four Sulfur Clusters. 2. Proton ENDOR Study of a 1+ State in an Asymmetrical Cluster

Abstract

Proton ENDOR spectroscopy on γ-irradiated single crystals of the synthetic model compound (Et4N)2[Fe4S4(SC6H4-o-OH)4] has been used to study a [4Fe−4S]+ center that has properties similar to those of the reduced states of some particular ferredoxins and of the enzyme aconitase. The interesting point about this model compound is that one iron atom is five-coordinate, with an extra phenolic oxygen attached to one of its thiolate ligand. The complex thus represents an example of an asymmetrical [4Fe−4S] cluster in the solid crystalline state. From the study of the angular dependence of the proton ENDOR lines, it has been possible to derive eight hyperfine tensors of protons occupying different positions in the near vicinity of the iron atoms, the main bearers of the spin population of the paramagnetic anion. Some protons belong to thiolate ligands of the anion whereas others belong to close (Et4N)+ counterions. From the anisotropic parts of the tensorsand by using a multicentric point-dipole modelit has been...