Some thermodynamic and kinetic properties of the primary photochemical reactants in a complex from a green photosynthetic bacterium

We have examined the bacteriochlorophyll reaction-center complex of Chlorobium limicola f. thiosulfatophilum, strain Tassajara. Our results indicate that the midpoint potential of the primary electron donor bacteriochlorophyll of the reaction center is +250 mV at pH 6.8, while that of cytochrome c-553 is +165 mV. There are two cytochrome c-553 hemes per reaction center, and the light-induced oxidation of each is biphasic (t1/2 of less than 5 mus and approximately 50 mus). We belive that this indicates a two state equilibrium with each cytochrome heme being either close to, or a little removed from, the reaction-center bacteriochlorophyll. We have also titrated the primary electron acceptor of the reaction center. Its equilibrium midpoint potential at pH 6.8 is below -450 mV. This is very much lower than the previous estimate for green bacteria, and also substantially lower than values obtained for purple bacteria. Such a low-potential primary acceptor would be thermodynamically capable of direct reduction of NAD+ via ferredoxin in a manner analagous to photosystem I in chloroplasts and blue-green algae.

Quantum Efficiency of Cytochrome Oxidation in a Photosynthetic Bacterium

Illumination of Chromatium with light absorbed by bacteriochlorophyll causes the oxidation of intracellular cytochrome. The quantum efficiency of this reaction approaches one electron per photon at 589 millimicrons and at wavelengths between 862 and 908 millimicrons. The efficiency of converting excitation energy to chemical energy is estimated to be about 30 percent.