Near-infrared spectra of hemoglobin and Fe-Mn hybrid hemoglobins have been obtained at cryogenic temperatures. The charge-transfer (a2u(π)→dzy) transition at ~760 nm (band III) has been found to be a conformationally sensitive indicator of the heme-pocket geometry in these species. Temperature, protein tertiary and quaternary structure, chain heterogeneity, and ligand rebinding subsequent to CO photolysis all affect the line width and position of this transition. We conclude that the overall line shape of band III is derived from both subunit heterogeneity and conformational disorder within each subunit. A model is suggested that relates the observed pH dependence of the kinetic hole burning due to ligand rebinding to specific structural parameters of the proximal heme pocket that influence both the peak position and the inhomogeneous line shape of band III.
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