The electrical and spectroscopic properties of planar asymmetrical membranes incorporating chlorophyll a and plastoquinone-9. Influence of surface charges on electron transfer.

Abstract

We studied the influence of surface charges on the efficiencies of electron transfer between a donor molecule, chlorophyll a (Chla), and an acceptor molecule, plastoquinone-9 (PQ-9), asymmetrically incorporated into a phospholipid matrix built from phosphatidylethanolamine, phosphatidylserine, and dimethyldistearylammonium bromide. Membrane conductance and capacitance measurements, as well as fluorescence emission experiments, were performed on bilayers containing positive or negative surface charges. The conductance of the bilayers showed important increases upon illumination of the Chla, this effect being observed only when both the donor and the acceptor molecules were present within the bilayer. This suggested that an electron transfer between Chla and PQ-9 occurred. The same kind of behaviour was observed with the membrane capacitance, but the amplitude of the effect was smaller. The results showed that an electric field gradient favorably oriented to promote electron transfer from Chla to PQ-9 maximized the electron transfer between these two molecules. However, both the membrane resistance and capacitance were permanently modified when illumination was stopped. On the other hand, the fluorescence results showed that for the range of surface charges covered, the position of the maximum of absorption was found unchanged around 675 nm and the intensity of fluorescence was almost constant, of the order of 7 x 10(6)-8 x 10(6) photons.s-1. This suggested that Chla was embedded as microdomains within the bilayer. The results presented here were also compared with what is known on the organization of the donor and acceptor molecules within the reaction centres of photosynthetic bacteria.