Carbon-13 solid-state NMR studies on synthetic model compounds of [4Fe-4S]clusters in the 2(+) state

Carbon-13 solid-state NMR is used to determine, via the temperature depende nce of paramagnetic shifts, the spin ladder of magnetic levels and the rela ted spin exchange parameters in [4Fe-4S](2+) clusters. This study has been carried out between 180 and 330 K in three model compounds of the active si tes of 4Fe-4S proteins: [(C2D5)(4)N](2)[Fe4S4((SCD2C6D5)-C-13)(4)], [(C2H5) (4)N](2)[Fe4S4(SC(CH3)(3))(4)], and [(C2H5)(4)N](2)[Fe4Se4(SC-(CH3)(3))(4)] The temperature dependencies of both the isotropic and anisotropic parts o f the C-13 paramagnetic shift tensors have been measured and analyzed. Anti ferromagnetic exchange coupling constants among iron atoms of J approximate to 430, 380, and 370 cm(-1) have been deduced from fits of their temperatu re dependencies. Additional contributions of the double-exchange term B fav oring electronic delocalization and of a DeltaJ term associated with the co mpression of the cubane geometry are also discussed. We also show that conn ections between these measurements and those of magnetic susceptibility are especially close in the case of the [Fe-4-4S](2+) odor state.