Resonance Raman spectroscopy of cytochrome oxidase using Soret excitation: selective enhancement, indicator bands, and structural significance for cytochromes a and a3

Resonance Raman studies of oxidized and reduced cytochrome oxidase and liganded derivatives of the oxidized enzyme have been performed by using direct-Soret excitation at 413.1 and 406.7 nm, as well as near-Soret excitation (457.9 nm) and alpha-band excitation (604.6 nm). The Soret results clearly show selective enhancement of Raman modes of the hemes of cytochromes a and a3, depending upon the excitation wavelength chosen. For the preparations employed in this study, photoreduction of cytochrome oxidase in the laser beam was not a significant problem. Resonance Raman frequencies sensitive to oxidation state and spin state or core expansion of the a and a3 hemes are identified and correlated with those previously identified for other heme proteins. An unusual low-frequency (less than 500 cm(-1)) spectrum is observed for oxidized high-spin cytochrome a3, which may be due to axial nonheme structures in this cytochrome.