Effect of oxidation-reduction potential on light-induced cytochrome and bacteriochlorophyll reactions in chromatophores from the photosynthetic green bacterium Chlorobium

The reaction center bacteriochlorophyll of Chlorobium thiosulfatophilum has a midpoint oxidation-reduction potential (Em) of +330 mV. Its photooxidation is unaffected by oxidation-reduction potentials in the range from +260 mV to −70 mV but on further reduction is attenuated to zero in a one-electron transition with an Em of −130 mV. A c-type cytochrome with an Em of +220 mV and absorption maxima at 551–552 nm (α-band) and 420 nm (γ-band) is present in Chlorobium chromatophores and undergoes photooxidation. Cytocrome c photooxidation is attenuated to zero in two 1-electron steps with Em of +30 mV and −130 mV Possible roles for +30 mV and −130 mV components in photosynthetic electron transport in Chlorobium are discussed.