Resonance Raman Interrogation of the Consequences of Heme Rotational Disorder in Myoglobin and its Ligated Derivatives
Resonance Raman spectroscopy is employed to characterize heme site structural changes arising from conformational heterogeneity in deoxyMb and ligated derivatives, i.e., the ferrous CO (MbCO) and ferric cyanide (MbCN) complexes. The spectra for the reversed forms of these derivatives have been extracted from the spectra of reconstituted samples. Dramatic changes in the low-frequency spectra are observed, where newly observed RR modes of the reversed forms are assigned using protohemes that are selectively deuterated at the four methyl groups or at the four methine carbons. Interestingly, while substantial changes in the disposition of the peripheral vinyl and propionate groups can be inferred from the dramatic spectral shifts, the bonds to the internal histidyl imidazole ligand and those of the Fe−CO and Fe−CN fragments are not significantly affected by the heme rotation, as judged by lack of significant shifts in the ν(Fe−NHis), ν(Fe−C), and ν(C−O) modes. In fact, the apparent lack of an effect on these key vibrational parameters of the Fe−NHis, Fe−CO, and Fe−CN fragments is entirely consistent with previously reported equilibrium and kinetic studies that document virtually identical functional properties for the native and reversed forms.