G. Paillotin et al., WHY DOES THE LIGHT-GRADIENT PHOTOVOLTAGE FROM PHOTOSYNTHETIC ORGANELLES SHOW A WAVELENGTH-DEPENDENT POLARITY, Biophysical journal, 65(1), 1993, pp. 379-385
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.