Heisenberg model for tetrameric iron clusters in high-potential iron-sulfur proteins. I. Exchange states, g-factors, hyperfine interactions

Single-crystal (57)Fe Q-band ENDOR study of the 4 iron-4 sulfur cluster in its reduced [4Fe-4S](1+) state

(57)Fe Q-band ENDOR has been used to study the [4Fe-4S](1+) state created by gamma irradiation of single crystals of the synthetic model compound [N(C(2)H(5))(4)](2)[Fe(4)S(4)(SCH(2)C(6)H(5))(4)] enriched in (57)Fe. This compound is an excellent biomimetic model of the active sites of many 4 iron-4 sulfur proteins, enabling detailed and systematic studies of its oxidized [4Fe-4S](3+) and reduced [4Fe-4S](1+) paramagnetic states. Taking advantage of the fact that Q-band ENDOR, in contrast with X-Band ENDOR, allows for a very good separation of the (57)Fe transitions from those of the protons, the complete hyperfine tensors of the four iron atoms for the [4Fe-4S](1+) species has been measured with precision. For each iron atom, the electron orbital and electron spin isotropic contributions have been determined separately. Moreover, it is remarkable that two (57)Fe hyperfine tensors attributed to the ferrous pair of iron atoms are very different. In effect, one tensor presents a much larger anisotropic part and a much smaller isotropic part than those of the other. This difference has been interpreted in terms of a differential electron orbital hyperfine interaction among the two ferrous ions.