PERMEABILITY AND ELECTRICAL-PROPERTIES OF PLANAR LIPID-MEMBRANES FROMTHYLAKOID LIPIDS

Electrical measurements were carried out on planar lipid membranes fro m thylakoid lipids. The specific capacitance of membranes formed from decane-containing monogalactosyldiacylglycerol (MGDG), which accounts for 57% of the total lipid content of thylakoids, showed that it adopt ed a bilayer structure. Solvent-free bilayers of MGDG were not formed, with very rare exceptions, indicating that decane is required to stab ilize the planar conformation. However, this cone-shaped lipid produce s bilayer structures in combination with other cylindrical thylakoid l ipids even in the absence of organic solvent. We compared the properti es of solvent-free and decane-containing bilayers from MGDG, soybean l ecithin, and the quaternary mixture of lipids similar to that found in vivo. The conductance of decane-MGDG was 26 times higher than that of decane-lecithin. The flux through the decane-lecithin bilayer was fou nd to be slightly dependent on pH, whereas the decane-MGDG membrane wa s not. The specific conductance of bilayers formed from the quaternary mixture of lipids was 5 to 10 times larger than lecithin (with alkane or not). Further experiments with bilayers made in the presence of a KCl gradient showed that decane-MGDG, decane-MGDG/DGDG/SQDG/PG, and so lvent-free MGDG/DGDG/SQDG/PG were cation-selective. The permeability c oefficient for potassium ranged from 4.9 to 8.3 x 10(-11) cm s(-1). Th e permeability coefficient for protons in galactolipids, however, was determined to be about six orders of magnitude higher than the value f or potassium ions. The HCl permeation mechanism through the lipid memb ranes was determined from diffusion potentials measured in HCl gradien ts. Our results suggest that HCl was not transported as neutral molecu les. The data is discussed with regard to the function of galactolipid s in the ion transport through thylakoid membranes.