SINGLE-CRYSTAL PROTON ENDOR STUDIES OF THE [FE4S4](3+) CLUSTER - DETERMINATION OF THE SPIN POPULATION-DISTRIBUTION AND PROPOSAL OF A MODEL TO INTERPRET THE H-1-NMR PARAMAGNETIC SHIFTS IN HIGH-POTENTIAL FERREDOXINS

Proton ENDOR spectroscopy has been used in single crystals of the synthetic compound [N(C2D5)]2[Fe4S4(SCH2C6D5)4], which is a good biomimetic model of the active sites of many four-iron-four-sulfur proteins. The eight protons of the four thiolate CH2 groups have been used in order to probe in detail the distribution of the unpaired electron spin population in a paramagnetic [Fe4S4]3+ center created by gamma irradiation in the crystals. The thus obtained hyperfine tensors of the eight protons constitute an original, abundant, and precise source of information on this oxidation state. They have been analyzed in two separate parts. From their anisotropic parts, it is possible to deduce the distribution of the unpaired spin population on the different iron and sulfur atoms with the help of a point-dipole model. Within the limitations of the simple and symmetric vectorial spin coupling model which involves two equivalent mixed-valence iron atoms and two equivalent ferric iron atoms, we find that this paramagnetic center is close to the \7/2,3,1/2] state, the first number representing the spin state of the mixed-valence pair, the second one the spin state of the ferric pair, and the last one the resulting spin of the cluster This attribution is in contrast with recent proposals considering that the [Fe4S4]3+ Spin state is \9/2,4,1/2]. Finally, the analysis of the isotropic parts of the tensors leads us to propose a new quantitative model establishing the law existing between these isotropic couplings and two different parameters: a magnetic parameter which is the spin population on the adjacent iron and an angular parameter defining the orientation of each CH bond. This model seems indeed able to provide the basis of a quantitative interpretation of H-1 paramagnetic shifts in the NMR spectra of high-potential proteins in their oxidized state. Through the variety of results obtained, the interest of the present study is also that it gives the capacity to unify the interpretations of results concerning the [Fe4S4]3+ state in the proteins and in model compounds which have been derived from the EPR, ENDOR, Mossbauer, and NMR spectroscopies.