Metal-substituted hemeproteins are an excellent means of studying the role of the heme iron in oxygen-binding and release. When the iron center in myoglobin is replaced by cobalt, cobalt-substituted myoglobin has been shown to reversibly bind oxygen. However, the oxygen affinity for cobaltous myoglobin (CoMb) is lower than ferrous myoglobin (Mb). EPR studies on CoMb have shown that, unlike native Mb where deoxy Mb is high spin (S=2) and oxy Mb is low spin (S=0), the oxy- and deoxy- states of CoMb are both low spin (S==). This implies structural/electronic differences in the deoxy- states of the two species, which may contribute to the lower oxygen affinity for CoMb. At Beamline U4B, we have obtained the L-edge spectra of cobaltous myoglobin in the oxy- and deoxy- states. The experimental data demonstrate that the L-edge spectra of Deoxy CoMb and Oxy CoMb are noticeably different, corresponding to differences cobalt oxidation states and site symmetries between these two species. In the future, we will examine the Oxy CoMb photoproduct, which is generated by photolysis of Oxy CoMb at 15 K, and has been shown to be electronically different than deoxy CoMb by EPR. Finally, we will continue to examine cobaltous heme model compounds in order to assist in our multiplet simulations of the protein species.
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