Charges in phospholipid layers

Abstract

The interfacial properties of a membrane are determinant for interaction among bio-membranes / lipid bilayers, or for establishing contact among layer surfaces and substrates approaching from the bulk. The access to the bilayer and its local structural modifications upon interaction with an adsorbing guest molecule are influenced significantly by the presence of charges, and local changes in surface charge density. Results are being presented on model mono- and bilayers prepared from zwitterionic POPC (DPPC) (1-palmitoyl-2-oleoyl-sn-glycero-3-ethylphosphocholine) and its cationic sibling lipids E-POPC (E-DPPC) (1-palmitoyl-2-oleoyl-sn-glycero-3-ethyl-phosphocholine/ di-1,2-palmitoyl-sn-glycero-3-ethyl-phosphocholine) that served to inoculate charge densities at different mol percentages. Monolayer compression isotherms obtained for the mixtures are compared with isotherms of pure POPC as a reference system. The presence of layer charges is manifested in an earlier onset of interaction, the range of interaction is increased. POPC bilayers with the same charge densities as the monolayers studied were then investigated by single molecule tracking using the fluorophore DiI-C18 for diffusion tracing. Initial results indicate a linear decrease of the lateral diffusion coefficient with increasing charge density. In the course of the study indications for domain formation in pure POPC layers were observed as novel peculiarities; these will be presented and discussed. Preliminary results about the adsorption of partially charged phospholipid layers onto hydrogel polyelectrolyte cushions on solid supports will be presented.