The lack of a solvent accessible hydroxide or water ligand to iron at the 3Fe center of Azotobacter vinelandii ferredoxin I.

The X-ray crystal structure of Azotobacter vinelandii ferredoxin I (FdI) describes a planar 3Fe-3S center in which one of the iron atoms is ligated to a solvent accessible oxo ligand, presumably from water or hydroxide (Ghosh et al., (1982) J. Mol. Biol. 158, 73-109). Efforts to displace the proposed oxo ligand with cyanide were unsuccessful, even in 80% dimethylsulfoxide. In addition, comparison of the electron spin echo envelopes for H2O- and D2O-equilibrated samples of FdI showed only a slight deuterium modulation, far less than would be expected were water to be bound as an iron ligand. These results do not support the presence of a solvent accessible oxo ligand to the 3Fe center as described in the X-ray crystal structure.