Studies on Cobalt Myoglobins and Hemoglobins

Abstract

Abstract Artificial myoglobins and hemoglobins containing cobaltous proto-, meso-, and deuteroporphyrins and their interaction with oxygen are examined by X- and K-band electron paramagnetic resonance (EPR) spectroscopy in a range from 4.2°K to room temperature. Deoxy cobalt porphyrin-substituted myoglobin (CoMb) and cobalt porphyrin-substituted hemoglobin (CoHb) exhibit an EPR absorption of axial symmetry (g⊥ = 2.3 and g|| = 2.0) with 59Co hyperfine and 14N superhyperfine splittings at the g|| extreme. These features are consistent with a penta-coordinated, square-pyramidal configuration of the low spin cobaltous ion (3d7, S = ½) in deoxy CoMb and deoxy CoHb. Upon oxygenation, the deoxy EPR absorption is replaced by another EPR absorption of a free radical type with axial symmetry (g⊥ = 2.00 and g|| = 2.08), indicating a substantial transfer of the unpaired spin density from the cobaltous ion to the bound oxygen. The oxy EPR absorption is sharpened upon measurements in D2O, suggesting a possible hydrogen bonding of the paramagnetic center (the bound oxygen) to the distal histidine residue. Oxy CoMb and oxy CoHb are readily photolyzed at 4.2°K. The reversion of the photolyzed products to the original oxy state is essentially retarded below 20°K. Visible absorption spectra of the photolyzed products are indistinguishable from those of the corresponding deoxy compounds. However, the photolyzed products exhibit new EPR extrema at g = 3.87 and ∼ 1.9, which may be attributable to a new intermediate of photolysis containing a high spin cobaltous ion (3d7, S = 3/2).