Chromophore interactions in flavocytochrome c552: a resonance Raman investigation

Resonance Raman spectra with both Soret and visible excitation have been obtained for Chromatium flavocytochrome c552 and its isolated diheme subunit under varying conditions of pH and inhibitor binding. The spectra are generally consistent with previously established classification schemes for porphyrin ring vibrations. The presence of covalently bound flavin in the protein was apparent in the fluorescent background it produced and in flavin-mediated photoeffects observed in heme Raman spectra obtained at high laser power. No flavin modes were present in the Raman spectra, nor was any evidence of direct heme-flavin interaction found by using this technique; however, a systematic perturbation of heme B1g vibrational frequencies was found in the oxidized holoprotein. The heme vibrational frequencies of c552 are compared to those of the diheme peptide and of other c-type cytochromes. They are consistent with an interpretation that involves pH-dependent changes in axial ligation and treats the hemes and flavin as isolated chromphores communicating via protein-mediated interactions.