The Reduction of Ferric o-Phenanthroline Complexes by Reduced Diphosphopyridine Nucleotide

The binuclear complex, bisdiferric tetra-o-phenanthrolinediol, is a two-electron acceptor. The two adjoining ferric ions are bridged by the two hydroxyl groups. The magnetic properties of this molecule suggest overlapping of d orbitals between the iron atoms (Gaines, A., Jr., Hammett, L. P., and Walden, G. H., Jr. (1936), J. Am. Chem. Soc. 58,1668). The ferric dimer is reduced in slightly acid solution by reduced diphosphopyridine nucleotide. In the reactions 1 mole of reduced diphosphopyridine nucleotide reduces 1 mole of the dimer forming 2 moles of ferrous tri-o-phenanthroline. The redox reaction is of first-order kinetics when both reactants are in nearly equimolar concentrations. This indicates that a mixed complex of ferric dimer and reduced diphosphopyridine nucleotide is formed at a rate higher than that of the electron transfer. The overall reaction velocity decreases with increase of pH, the concentration of ferric dimer needed to saturate the reduced diphosphopyridine nucleotide increases with increasing pH. These two phenomena are well correlated with the relative concentration of hydroxyl and oxo forms of the dimer. This is explained by assuming that the hydroxyl-iron bond is broken and replaced by a reduced diphosphopyridine nucleotide-iron bond, thus forming the mixed complex. The oxo-iron bond is much more resistant to such displacement. By saturating the reduced diphosphopyridine nucleotide molecules with the ferric dimer, both K_diss of the mixed complex and the rate constant of the electron transfer step were estimated. We suggest that the redox reaction is carried by two one-electron transfers, both of them from the reduced nicotinamide moiety to the ligated ferric ion. Because of the overlapping d orbitals the first newly gained electron is rapidly transferred to the adjoining ferric ion, thus enabling the ligated iron to accept the second electron.