CHEMICAL AND SPECTROSCOPIC STUDIES OF THE MIXED-VALENT DERIVATIVES OF THE NONHEME IRON PROTEIN HEMERYTHRIN

The electronic transitions of the mixed-valent [Fe(II), Fe(III)] form of the binuclear non-heme iron protein hemerythrin are assigned using absorbance, circular dichroism, and low-temperature magnetic circular dichroism (MCD) spectroscopies, 1/2Met(r) (prepared by reduction of met) and the ligand bound forms are found to have both irons in octahedral ligand geometries, while for 1/2Met(o) (prepared by oxidation of deoxy) the Fe2+ is five-coordinate and the Fe3+ is six-coordinate. Variable-temperature MCD and EPR spectroscopies are used to probe the ground-state magnetic properties; all 1/2met forms are found to have J almost-equal-to -8 cm-1, consistent with an endogenous bridging OH-. 1/2Met(r) and 1/2Met(0) are found to be in a pH-dependent equilibrium, reflecting binding of OH- as an exogenous ligand. The differences in geometric and electronic structure between 1/2met(r) and 1/2met(o) are related to the redox reactivity of this active site.