Characterization of the pH-Dependent Resonance Raman Transitions of Archaeal and Bacterial Rieske [2Fe−2S] Proteins

The pH-dependent resonance Raman (RR) spectral changes of the cytochrome bc 1-associated, high-potential Rieske proteins have frequently been invoked to explain the redox-linked ionization behavior. We report herein RR spectral data of archaeal and bacterial Rieske proteins that directly demonstrate the pH-dependent changes near and above pK a,ox2, but not around pK a,ox1, of the visible circular dichroism (CD) transitions. The RR spectral changes are attributed to modification of the immediate [2Fe−2S] cluster environment due to deprotonation of some exchangeable amide groups in the polypeptide backbone, rather than previously assumed simple changes of the Fe−Nimid stretching vibrations.