LIGHT GRADIENTS IN SPHERICAL PHOTOSYNTHETIC VESICLES

Light-gradient photovoltage measurements were performed on EDTA-treate d thylakoids and on osmotically swollen thylakoids (blebs), both of sp herical symmetry but of different sizes. In the case of EDTA vesicles, a negative polarity (due to the normal light gradient) was observed i n the blue range of the absorption spectrum, and a positive polarity, corresponding to an inverse light gradient, was observed at lambda = 5 30 and lambda = 682 nm. The sign of the photovoltage polarity measured in large blebs (swollen thylakoids) is the same as that obtained for whole chloroplasts, although differences in the amplitudes are observe d. An approach based on the use of polar coordinates was adapted for a theoretical description of these membrane systems of spherical symmet ry. The light intensity distribution and the photovoltage in such syst ems were calculated. Fits to the photovoltage amplitudes, measured as a function of light wavelength, made it possible to derive the values of the dielectric constant of the protein, epsilon(s) = 3, and the ref ractive index of the photosynthetic membrane for light propagating per pendicular and parallel to the membrane surface, n(t) = 1.42 and n(n) = 1.60, respectively. The latter two values determine the birefringenc e of the biological membrane, Delta n = n(n) - n(t) = 0.18.