Fluorine-19 chemical shifts as structural probes of metal-sulfur clusters and the cofactor of nitrogenase.

Several properties of the FeMo-cofactor (co) of nitrogenase in N-methylformamide solution at ambient temperature have been investigated by means of 19F NMR spectroscopy. With C6H5CF3 reference signals the magnetic moment per Mo atom was found to be approximately equal to 3.9 BM, consistent with S = 3/2 ground state identified by other spectroscopic methods at low temperature. Reaction of FeMo-co with 1.0 eq of RFS- (RF = p-C6H4CF3, p-C6H4F) afforded isotropically shifted signals indicative of binding to a paramagnetic cluster. By comparison with the spectra of Fe-S and Fe-Mo-S species derivatized with RFS-, including the cubane-type MoFe3S4 clusters with S = 3/2 ground states, it was concluded that the essential FeMo-co cluster structure remains intact and a Fe atom is the probable thiolate binding site. An interaction of FeMo-co with C6H5S- had been detected earlier by low temperature EPR spectroscopy. The binding site assignment is based on large observed isotropic shifts (ca. -12ppm) compared to the much smaller values found for Mo-SRF ligands in MoFe3S4 clusters and anticipated in FeMo-co on the basis of recent spectroscopic results. Isotropic 19F shifts have proven extremely sensitive to electronic and structural features of Fe-S and Fe-Mo-S clusters. The inclusion of a 19F NMR label in FeMo-co should prove of utility in further investigation of cofactor properties and reactions.