WHY DOES THE LIGHT-GRADIENT PHOTOVOLTAGE FROM PHOTOSYNTHETIC ORGANELLES SHOW A WAVELENGTH-DEPENDENT POLARITY

The light-gradient photovoltage from photosynthetic organisms and orga nelles is thought to arise from the primary charge separation in the r eaction centers. The current explanation of the effect is the stronger excitation of the membrane side of a vesicle facing the light source than the one on the opposite side. Together with the known orientation of reaction centers, this explanation predicts unequivocally the pola rity of the photovoltage. However, a polarity opposite to the one expe cted has often been reported. A dependence of the polarity on the wave length has been published but no explanation was given (Graber, P., an d H.-W. Trissl. 1981. FEBS Lett. 123:95-99) Here we report on a theore tical treatment of light propagation and interference in pigmented and nonpigmented multilayers. A model calculation is carried out for a pa ir of membranes, demonstrating the wavelength-dependent light distribu tion as well as the relative photovoltage and its polarity. When the m embranes contain no chromophores or when the absorption coefficient is low, the predicted polarity is opposite to that expected from a simpl e macroscopic absorption behavior. The model is tested by comparing ne w photovoltage data obtained at 532 nm as well as in the blue and red absorption bands of chlorophyll in chloroplasts. It is concluded that outside the main absorption bands the amplitude and polarity of the ph otovoltage is determined by the ratio of the refractive indices of the membrane and the medium.