Effects of surface potential and membrane potential on the midpoint potential of cytochrome c-555 bound to the chromatophore membrane of Chromatium vinosum.

The values of midpoint potential (Em) of cytochrome c-555 bound to the chromatophore membranes of a photosynthetic bacterium Chromatium vinosum was determined under various pH and salt conditions. After a long incubation at high ionic concentrations in the presence of carbonylcyanide m-chlorophenylhydrazone, which was added to abolish electrical potential difference between the inner and outer bulk phases of chromatophore, the Em value was almost constant at pH values between 4.0 and 8.4. With the decrease of salt concentration, the pH dependence of the Em value became more marked. Under low ionic conditions, Em became more positive with the decrease of pH. Addition of salt made the value more positive or negative at pH values higher or lower than 4.5, respectively. Divalent cation salts were more effective than monovalent cation salts in producing the positive shift of Em at pH 7.8. The Em value became more positive when the electrical potential of the inner side of the chromatophore was made more positive by the diffusion potential induced by the K+ concentration gradient in the presence of valinomycin. These results were explained by a change of redox potential at the inner surface of the chromatophore membrane, at which the cytochrome is assumed to be situated, due to the electrical potential difference with respect to the outer solution induced by the surface potential or membrane potential change. The values for the surface potential and the net surface charge density of the inner surface of the chromatophore membrane were estimated using the Gouy-Chapman diffuse double layer theory.