Freestanding black films of phospholipids and phospholipid with proteins

Free black films formed from aqueous solutions of phospholipids and phospholipids and proteins mixtures have been studied by a X-ray reflectivity method. From the reflectivity experiments one can extract the electron density profile perpendicular to the films. By an appropriate stratified model of the film we determined its structure (i.e. the thickness, the roughness and the densities of the different slabs). Additional surface tension measurements of all the solutions were performed in order to obtain complementary information about the state of the monolayers. The experiments were performed with mixtures of two types of lipids, dimyristoyl phosphatidyl choline (DMPC) which is a noncharged lipid and dimyristoyl phosphatidyl glicerol (DMPG) which is a negatively charged. Aqueous suspensions of unilamellar vesicles were prepared by sonication. Bovine serum albumin (BSA) which is a globular soluble protein was added to the above mixture. All the solutions were studied in the presence or absence of NaCl. The experiments were carried out at temperatures above the phase transition temperature of the lipids and of the mixture. The X-ray experiments show a very strong influence of DMPG on the thickness and on the structures of the phospholipid films. The addition of a protein in the DMPC/DMPG mixture results in the formation of thicker films due to the strong lipid/protein interactions. The film thickness increases with the BSA concentration. This result can be explained by the presence of protein molecules inserted between the two film layers.