Photoelectronic effects in organic materials. II. Variation of photovoltage with pH in chlorophyll-quinone solutions

[Investigations concerning the correlation between the light-dependent and the redox-dependent change of the membrane potential in Griffithsia setacea]

1. With the red alga Griffithsia setacea in the dark the addition of reduced DCPIP effects a depolarization of the membrane potential. The kinetics of this redox-dependent depolarization is in good conformity with that of the light-dependent depolarization. Ascorbate as well as cystein are efficient as reducing substance. 2. By simultaneous application of light and reduced DCPIP it can be demonstrated that the light-dependent and the redox-dependent change of the membrane potential may be involved in the same system. 3. The redox-dependent membrane potential change is a function of the redox potential difference between oxidized and reduced DCPIP. In the linear range of this relation the simultaneously measured light-dependent membrane potential change corresponds to a redox potential difference of 124mV. 4. The redox-dependent membrane potential change, like the light-dependent membrane potential change, is dependent on the pH of the medium. Between pH 9.0 and pH 9.5a transition from a depolarization to a hyperpolarization of the membrane potential can be observed. 5. The results support the hypothesis that the redox level of a component in the photosynthetic electron transport chain may steer the light-dependent and the redox-dependent change of the membrane potential.