X-RAY-ABSORPTION SPECTROSCOPIC STUDIES OF THE BINDING OF LIGANDS TO FEMOCO OF NITROGENASE FROM KLEBSIELLA-PNEUMONIAE

The binding of ligands to the iron-molybdenum cofactor (FeMoco) from K lebsiella pneumoniae nitrogenase has been studied by XANES and EXAFS a t the Fe, Mo and Se k-edges and by EPR. Ligands investigated include t he anions derived from thiophenol, 2-bromophenylthiol, phenylselenol, a trithiolate ligand, 1, and the cyanate and thiocyanate anions. Evide nce has been obtained directly that phenylselenol, and indirectly that thiophenol, 2-bromophenylthiol and ligand 1, are bound by iron. For F eMoco plus phenylselenol, an iron-selenium distance of 2.36 Angstrom w as determined from the Fe k-edge data and 2.38 Angstrom from the Se k- edge data. The results suggest that one selenol (and by analogy thioph enol and 2-bromophenylthiol) is bound in a non-bridging mode by FeMoco . Binding of ligand 1 led to a large splitting of the Fe shell at the Mo k-edge. At the Pe k-edge, binding of ligand 1 led to large decrease s in the intensity of the iron-iron contacts. No evidence was found fo r cyanate or thiocyanate binding to Fe. (C) 1998 Elsevier Science S.A. All rights reserved.