Resonance Raman spectra of cytochrome a in cytochrome oxidase: Excitation in the 605‐nm absorption region

Abstract

AbstractVisible excitation resonance Raman difference spectra, in resonance with the 605‐nm absorption maximum of fully reduced and mixed‐valence cyanide‐bound cytochrome oxidase, were recorded. Under these conditions, the vibrations of ferrocytochrome a are markedly enhanced owing to its dominant contribution to the 605‐nm α‐band absorption of reduced cytochrome oxidase. The effect of H/D exchange in the Raman spectra of cytochrome a was also investigated as a way to establish formyl‐ and vinyl‐peripheral substituent sensitive modes. Measurements of the depolarization ratio dependence of cytochrome a vibrational modes resulted in some isotope‐sensitive modes with characteristic depolarized behavior; however, it was observed that the majority of the visible excitation vibrations exhibit polarized values. This is interpreted as being due to a significant lowering in the molecular symmetry of the heme a porphyrin macrocycle caused by the unusual peripheral substituent pattern of heme a and, presumably, by the strong cytochrome a‐protein interactions. Both structural effects appear to result in a selective enhancement of cytochrome a Eu substituent‐sensitive modes as detected by the low‐frequency resonance Raman spectrum.