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

Publisher Website
Google Scholar
Abstract
An EPR crystallographic study was made on oxycobaltomyoglobin with the dioxygen ligand enriched to 19.1% in 17O. There are 2 spectroscopically distinct cobalt dioxygen species. The less abundant species, II (40%), has nonequivalent O atoms with superhyperfine tensors OA.alpha. = (5, -67.5, 22.4)G and OA.beta. = (5.4, -83.3, 30.3)G. Together with the previously reported 59Co hyperfine tensor, the orbital spin densities are O.alpha.(p.eta.) = 0.48, O.alpha.(p.zeta.) = -0.11, O.beta.(p.eta.) = 0.74, O.beta.(p.zeta.) = -0.16, Co(dxz) = -0.01, Co(dyz) = 0.06 for a total electron density of 1.01. The O.sbd.O axis is directed toward His-E7, suggesting a possible H-bonding interaction which may contribute to the nonequivalency of the O atoms: its projection approximately bisects N1.sbd.Fe.sbd.N2. The z axis of the CoA tensor is tilted at an angle of 28.degree. from the heme normal, resulting in a Co.sbd.O.sbd.O angle of 120.degree.. The more abundant species, I (60%), has equivalent O atoms with OA.gamma. = (12, -72.5, 20)G and orbital spin densities of O.gamma.(p.eta.) = 0.54, O.gamma.(p.zeta.) = -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.sbd.O axis approximately bisects N2.sbd.Fe.sbd.N3 and is parallel to the imidazole ring of His-F8. Increase of temperature changes g, CoA, and OA values, with the largest effect seen with OA. This temperature dependence indicates averaging of the 2 bond structures which are stabilized at 77.degree. K.