Electron Spin Resonance Monitoring of Ligand Ejection Reactions Following Solid-State Reduction of Cobalt Globin and Cobalt Protoporphyrin Complexes

Article abstract of DOI:10.1021/j100395a016

Cobaltihemoglobin, isolated α and β chains, and cobaltimyoglobin in aqueous solution at neutral pH were irradiated at 77 K with 3 Mrd of 60Co γ-rays.These diamagnetic Co(III) species are converted to paramagnetic Co(II) species in high yield.The EPR spectra are identical with those of authentic six-coordinate cobalt(II) porphyrins.Upon partial annealing of the species, the EPR spectrum transforms irreversibly to that of a five-coordinate species, indicating that at 77 K these cobaltiglobins are cobaltichromes in analogy to the hemichromes of the native iron species.Differences are seen among all of the six-coordinate, reduced protein ligated species.This ejection of the sixth ligand with thermal annealing after addition of one electron to the d_z2 orbital of the cobalt porphyrin also occurs in aqueous glasses of cobalt protoporphyrin IX in pyridine, n-butylamine, or quinuclidine.The five-coordinate species in aqueous media are stable with annealing to room temperature.