CONTINUOUS FLOW-RESONANCE RAMAN SPECTROSCOPY OF AN INTERMEDIATE REDOX STATE OF CYTOCHROME c

ABSTRACT An intermediate redox state of cytochrome c at alkaline pH, generated upon rapid reduction by sodium dithionite, has been observed by resonance Raman (RR) spectroscopy in combination with the continuous flow technique. The RR spectrum of the intermediate state is reported for excitation both in the (a, f3) and the Soret optical absorption band. The spectra of the intermediate state are more like those of the stable reduced form than those of the stable oxidized form. For excitation at 514.5 nm, the most prominent indication of an intermediate state is the wave-number shift of one RR band from 1,562 cm- ' in the stable oxidized state through 1,535 cm- ' in the intermediate state to 1,544 cm- ' in the stable reduced state. For excitation at 413.1 nm, a band, present at 1,542 cm-' in the stable reduced state but not present in the stable oxidized state, is absent in the intermediate state. We interpret the intermediate species as the state where the heme iron is reduced but the protein remains in the conformation of the oxidized state, with methionine-80 displaced as sixth ligand to the heme iron, before relaxing to the conformation of the stable reduced state, with methionine-80 returned as sixth ligand