Resonance Raman spectroscopy of dioxygen adducts of cobalt-substituted hemoproteins and model compounds
Abstract
The $\nu$(O-O) of dioxygen adducts of cobalt protoporphyrin IX reconstituted $\alpha$ and $\beta$ chains, CoHb tetramer, CoMb and FeCo hybrids were measured for the dioxygen isotopomers ($\sp{16}$O$\sb2$, $\sp{18}$O$\sb2$ and $\sp{16}$O$\sp{18}$O) in H$\sb2$O and D$\sb2$O buffered solutions. The RR spectra of the $\sp{16}$O$\sb2$ adducts in H$\sb2$O for all proteins studied were dominated by the presence of a strong band near 1135 cm$\sp{-1}$ that experienced 3-5 cm$\sp{-1}$ upshifts to $\sim$1140 cm$\sp{-1}$ in D$\sb2$O. Another feature near 1150 cm$\sp{-1}$ was present as a prominent shoulder for some systems (CoHb, $\beta\sb{\rm Co}$ and $(\rm\alpha\sb{Fe}\beta\sb{Co})\sb2)$, a weak shoulder for others ($\rm \alpha\sb{Co}$ and $(\rm \alpha\sb{Co}\beta\sb{Fe})\sb2)$ and absent for another (CoMb). The RR spectra of the $\sp{18}$O$\sb2$ adducts in H$\sb2$O were dominated by a large band at $\sim$1065 cm$\sp{-1}$ that upshifted 2-9 cm$\sp{-1}$ to $\sim$1070 cm$\sp{-1}$ in D$\sb2$O and weak feature near 1095 cm$\sp{-1}$ that was H$\sb2$O/D$\sb2$O insensitive. The RR spectra of all proteins with $\sp{16}$O$\sp{18}$O exhibited a strong band near 1098 cm$\sp{-1}$ that was insensitive to H$\sb2$O/D$\sb2$O exchange and a weak feature near 1080 cm$\sp{-1}$ in H$\sb2$O that disappeared in D$\sb2$O. Model complexes which utilized the histidine models imidazole and 4-methylimidazole exhibited spectral patterns similar to those found with the proteins. In the region where one $\nu$(O-O) was expected, two bands were often observed. The presence of two bands was attributed to vibrational coupling between the $\nu$(O-O) and an internal mode of the axial ligand (imidazole or 4-methylimidazole). Changes in the RR spectra were induced by deuterating the axial ligand which changed its internal vibrations and altered the vibrational coupling mechanism. Additional influences on the vibrational behavior of bound dioxygen included solvent interaction, porphryrin interaction and axial ligand self association. Previous research on cobalt substituted hemoproteins noted the observation of multiple bands in the $\nu$(O-O) region and attributed their presence to multiple Co-O$\sb2$ conformers due to distal side hydrogen bonding. The present study suggests that the most plausible explanation for the complicated vibrational behavior of bound dioxygen in the protein systems is vibrational coupling between $\nu$(O-O) and internal modes of HisF8.
This paper has been withdrawn.