Electron paramagnetic resonance crystallography of 17 O-enriched oxycobaltomyoglobin: Stereoelectronic structure of the cobalt dioxygen system

An electron paramagnetic resonance crystallographic study was made on oxycobaltomyoglobin with the dioxygen ligand enriched to 19.1% in17 O. There are two spectroscopically distinct cobalt dioxygen species. The less abundant species, II (40%), has nonequivalent oxygen atoms with superhyperfine tensorsO Aα = (5, -67.5, 22.4)G andO Aβ = (5.4, -83.3, 30.3)G. Together with the previously reported59 Co hyperfine tensor [Chien, J. C. W. & Dickinson, L. C. (1972)Proc. Natl. Acad. Sci. USA 69, 2783-2787], the orbital spin densities are found to be Oα (p η ) = 0.48, Oα (p ζ ) = -0.11, Oβ (p η ) = 0.74, Oβ (p ζ ) = -0.16, Co(dxz ) = -0.01, Co(dyz ) = 0.06 for a total electron density of 1.01. The O—O axis is directed toward His-E7, suggesting a possible hydrogen bonding interaction which may contribute to the nonequivalency of the oxygen atoms; its projection approximately bisects N1 —Fe—N2 . Thez axis of theCo A tensor is tilted at an angle of 28° from the heme normal, resulting in a Co—O—O angle of 120°. The more abundant species, I (60%), has equivalent oxygen atoms withO Aγ = (12, -72.5, 20)G and orbital spin densities of Oγ (p η ) = 0.54, Oγ (p ζ ) = -0.05, Co(dxz ) = -0.02, Co(dyz ) = 0.09 for a total spin density of 1.10. Although the direction cosines for this molecule cannot be precisely determined, the projection of its O—O axis approximately bisects N2 —Fe—N3 and is parallel to the imidazole ring of His-F8. Increase of temperature changes g,Co A, andO A values, with the largest effect seen withO A. This temperature dependence indicates averaging of the two bond structures which are stabilized at 77 K.