Nature of photochemical reactions in chromatophores of Chromatium D. II. Quantum yield of photooxidation of cytochromes in Chromatium chromatophores

Initial rates of the light-induced absorption decrease in Chromatium chromatophores due to the oxidation of cytochromes were measured under various experimental conditions. The initial rate in the presence of 10 mM potassium ferrocyanide and 50 μM potassium ferricyanide was about one-half to two-thirds of that in the presence of 30 mM ascorbate or in a medium with a redox potential (E_h) of − 78 mV.

Light-minus-dark difference spectrum indicated that, in the presence of 10 mM ferrocyanide and 50 μM ferricyanide, only cytochrome c-555 was photooxidized. In the presence of 30 mM ascorbate or at E_h values lower than about 0 mV, both cytochrome c-555 and cytochrome c-552 were photooxidized. The quantum yield of cytochrome c-555 photooxidation was calculated to be about 0.4.

The results obtained in the present study are compared with other investigators' and the possibility of the presence of two types of associations between the cytochromes and reaction-center bacteriochlorophyll is discussed.