THE ELECTRICAL AND SPECTROSCOPIC PROPERTIES OF PLANAR ASYMMETRICAL MEMBRANES INCORPORATING CHLOROPHYLL-A AND PLASTOQUINONE-9 - MODEL OF INCORPORATION OF CHLOROPHYLL-A AND PLASTOQUINONE-9

We have studied the incorporation of chlorophyll-alpha and plastoquinone-9 in Montal-Mueller membranes. In particular, we have been interested by the influence of both the lipid: chlorophyll-alpha ratio and the asymmetry of incorporation of the constituents on the electrical and fluorescence spectroscopic properties of the planar membranes built up from these constituents. The phospholipid matrix was made from phosphatidylethanolamine and phosphatidylserine. The monitoring of the fluorescence spectral properties of chlorophyll-alpha incorporated in various concentrations leads to the conclusion that chlorophyll-alpha is incorporated in the bilayers in monomeric form inside microdomains. It is shown that chlorophyll-alpha is positioned in these microdomains in such a way that the porphyric ring is interacting with the polar head of the lipid molecules where the interface polarity shows a dielectric constant varying between 25 and 35. The phytyl chain is embedded in the bilayer core, serving as an anchor, running parallel to the aliphatic chains of the phospholipids. We have also monitored the position of the plastoquinone-9 molecules within the bilayer. We found that plastoquinone-9 is incorporated in the center plane of the bilayer, increasing the thickness of the bilayer. This result confirms evidence, gathered in the literature from monolayer and differential scanning calorimetry studies, that long chain quinones and especially plastoquinone-9 are embedded deeply within the hydrophobic core of the bilayer. We also show that when chlorophyll-alpha and plastoquinone-9 are present together in the bilayer, the quinolic ring of the plastoquinone-9 molecule positions itself in the free volume created by the bulky porphyric ring of a chlorophyll-alpha molecule.