Q-BAND RESONANCE RAMAN ENHANCEMENT OF FE-CO VIBRATIONS IN FERROUS CHLORIN COMPLEXES - POSSIBLE MONITOR OF AXIAL LIGANDS IN D-CYTOCHROMES

Resonance Raman (rRaman) spectroscopy has been used extensively in the studies of the heme chemistry of carbon monoxy adducts. In porphyrin systems, the axial ligand vibrational modes of the CO, v(Fe-CO) and v( CO), are enhanced with Soret excitation via an A-term (Franck-Condon) mechanism, but are not expected with Q excitation (B-term or vibronic mechanism). For the first time, these modes have been obtained with Q, as well as Soret excitation in rRaman spectra of CO complexes of ferr ous chlorins. The enhancement with Q,excitation arises from an A-term mechanism of Raman scattering for these chlorins owing to their reduce d molecular symmetry. Thus, in iron chlorins or other heme systems wit h reduced molecular symmetry, axial ligand vibrational modes may be en hanced with a, excitation if they are observed with Soret excitation i n the corresponding iron porphyrins. These findings show rRaman spectr oscopy to be exceptionally valuable in the study of chlorin chromophor es with a, enhancement using red or orange-red excitation. Furthermore , the method appears to be selective for chlorin cofactors in proteins containing multiple heme centers such as cytochrome bd oxidase (see, for example, Sun; et al. Biochemistry 1995, 35, 2403-2412). It has bee n known that v(Fe-CO) and v(CO) frequencies of CO complexes of iron po rphyrins and heme proteins exhibit linear correlations, falling into d istinct sets for complexes possessing the same fifth ligand (for examp le, Ray; et al. J. Am. Chem. Sec. 1994, 126, 162-176). In this work, w e have found that v(Fe-CO) and v(CO) of iron-chlorin-CO complexes also respond to the nature of the opposite axial ligand and fellow the sam e correlations derived from porphyrin systems. Thus, the reduction of one of the pyrrole rings of porphyrins has little effect on v(Fe-CO) a nd v(CO) frequencies, and their correlation behavior may perhaps be us ed to ascertain the identity of the proximal ligand of the chlorin in a protein system of unknown coordination, as in cytochrome bd oxidase.