Intermediate and stable redox states of cytochrome c studied by low temperature resonance Raman spectroscopy.

Stabilized intermediate redox states of cytochrome c are generated by radiolytic reduction of initially oxidized enzyme in glass matrices at liquid nitrogen temperature. In the intermediate states the heme group is reduced by hydrated electrons, whereas the protein conformation is restrained close to its oxidized form by the low-temperature glass matrix. The intermediate and stable redox states of cytochrome c at neutral and alkaline pH are studied by low-temperature resonance Raman spectroscopy using excitations in resonance with the B (Soret) and Q1 (beta) optical transitions. The assignments of the cytochrome c resonance Raman bands are discussed. The observed spectral characteristics of the intermediate states as well as of the alkaline transition in the oxidized state are interpreted in terms of oxidation-state marker modes, spin-state marker modes, heme iron--axial ligand stretching modes, totally symmetric in-plane porphyrin modes, nontotally symmetric in-plane modes, and out-of-plane modes.