Electron Paramagnetic Resonance of Single Crystal Oxycobaltmyoglobin and Deoxycobaltmyoglobin

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Abstract
Single crystals of oxycobaltmyoglobin and deoxycobaltmyoglobin have been prepared and found to be isomorphous. The paramagnetic resonance spectra of deoxycobaltmyoglobin yield g xx = 2.33 0 , g yy = 2.32 3 , g zz = 2.02 8 with the z-axis parallel to the “heme” normal. The A Co and g tensors share the same principal axes with| A Co ∥| = 79 G and| A Co [unk]| = 6 G. A value of A N = 17.5 G was also obtained for the ε-N atom of the F8 histidine. There are two paramagnetic species in oxycobaltmyoglobin having apparently identical g-tensors ( g ξξ = 2.08 3 , g ηη = 2.00 6 , and g ζζ = 1.98 9 ) but different A Co -tensors. Furthermore, g values A Co do not share the same principal axes. The A Co -values for one of the species are ( A xx = 16.7 G, A yy = 5.95 G, A zz = 9.3 G), they are all 40% larger for the other species. The two species are oriented 90° to each other in the crystal. The results from electron paramagnetic resonance studies are consistent with a π-bounded structure for Co MbO 2 .