VIBRATIONAL STRUCTURE OF THE FORMYL GROUP ON HEME-A - IMPLICATIONS ON THE PROPERTIES OF CYTOCHROME-C-OXIDASE

Resonance Raman spectra have been recorded for heme a derivatives in which the oxygen atom of the formyl group has been isotopically labeled and for Schiff base derivatives of heme a in which the Schiff base nitrogen has been isotopically labeled. The N-14-N-15 isotope shift in the C = N stretching mode of the Schiff base is close to the theoretically predicted shift for an isolated C = N group for both the ferric and ferrous oxidation states and in both aqueous and nonaqueous solutions. In contrast, the O-16-O-18 isotope shift of the C = O stretching mode of the formyl group is significantly smaller than that predicted for an isolated C = O group and is also dependent on whether the environment is aqueous or nonaqueous. The differences between the theoretically predicted shifts and the observed shifts are attributed to coupling of the C = O stretching mode to as yet unidentified modes of the heme. The complex behavior of the C = O stretching vibration precludes the possibility of making simple interpretations of frequency shifts of this mode in cytochrome c oxidase.