LIGHT GRADIENT IN PHOTOSYNTHETIC SYSTEMS - THEORY AND EXPERIMENT

The macroscopic electric polarization resulting from the primary charg e separation in reaction centers of photosynthetic membranes is observ ed in media containing chloroplasts, photosynthetic bacteria, and orie nted membrane fragments. Photovoltages elicited by a short nonsaturati ng flash, resulting from the so-called ''light-gradient'' effect in ch loroplast suspensions, were measured almost 20 years ago by Fowler and Kok [1] and Witt and Zickler [2]. Because of the antiparallel orienta tion of reaction centers in the opposing thylakoid membranes and the s hadowing of the ''lower'' membrane by the ''upper'' one, a net dipole moment appears, which is a source of a small potential difference. It was thought that the polarity of this potential difference could be de duced from the known position of electron carriers in the photosynthet ic reaction center. However, the observed polarity of the signal induc ed at some flash wavelengths was opposite to that predicted by this cl assical model assumption. In addition, the measured photovoltage ampli tudes were not quantitatively related to experimental parameters. Here , it is shown that the light propagation and interference in pigmented multilayers of the sample are responsible for the light-gradient effe ct. A model calculation is carried out for a pair of membranes, simula ting stroma lamellae of chloroplasts. A wavelength-dependent light dis tribution and photovoltage, as well as the polarity, is thus predicted . For low intensities, the amplitude is found to be proportional to th e intensity of the incoming light, to the optical density, and to the reciprocal of the dielectric constant of the sample. When the membrane s contain no chromophores or when the absorption coefficient is low, t he predicted polarity is opposite to that expected from the original m acroscopic picture. The model is tested with a set of experimental pho tovoltage data obtained at different wavelengths.