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

We have examined the bacteriochlorophyll reaction-center complex of Chlorobium lirnicola 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 mY. There are two cytochrome c-553 hemes per reaction center, and the light-induced oxidation of each is biphasic (t,/2 of <= 5 ~s and ~ 50 ps), We believe 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.