⁵⁷Fe Hyperfine Coupling Tensors of the FeMo Cluster in Azotobacter vinelandii MoFe Protein: Determination by Polycrystalline ENDOR Spectroscopy

³⁷Fe ENDOR spectra obtained from frozen-solution samples of Azotobacter vinelandii molybdenum—iron protein (Avl) have been interpreted through use of a method we have devised for analyzing and simulating the ENDOR spectra of a randomly oriented polycrystalline (powder) paramagnet that has g and hyperfine tensors of arbitrary symmetry and relative orientation. The hyperfine tensor principal values and orientation relative to the g tensor (zero-field splitting tensor) have been determined for five distinct iron sites of the Avl FeMo-cofactor cluster by examining the variations in the spectrum as the observing g value (static field) is moved across the EPR envelope. Along with the recent description of the molybdenum site of the cofactor, this brings to six the number of individual metal sites of the protein-bound FeMo-co cluster whose magnetic properties have been described by the ENDOR technique. The protein-bound cofactor cluster has a remarkably complex structure: No two of the five resolved Fe sites have equivalent hyperfine tensor components or orientation. The results are used to discuss the cluster's composition, the degree of intrinsic versus protein-imposed structural complexity, and the Fe-site properties.