Salt-mediated interactions between vesicles of the thylakoid lipid digalactosyldiacylglycerol

Abstract Interbilayer interactions between large unilamellar vesicles of the plant thylakoid galactolipid digalactosyldiacylglycerol in aqueous salt solutions have been examined by light scattering, freeze-fracture electron microscopy, and X-ray diffraction. When suspended in aqueous salt solutions, vesicle of 100 nm diameter were found to aggregate in a rapid and reversible manner to yield aggregates greater than 1000 nm in diameter. Freeze-fracture electron microscopy showed these aggregates to consist of appressed, but not fused, vesicles. Quasi-elastic light scattering and turbidity experiments showed that aggregation was not due to charged impurities of the lipid behaving in accordance with electrostatic double double layer theory. Experiments testing the efficacies of various chloride salts indicated a strong correlation existed between ionic radius and ability of the salt to promote aggregation. Similar experiments examining the effect of sodium salts, glycerol, and pH on vesicle aggregation implicate an interaction between the digalactosyldiacylglycerol head group and structured water as underlying the aggregation process. The results suggest that digalactosyldiacylglycerol may contribute to close membrane approach of thylakoids in higher plant chloroplasts.