Surface charge dependence of polyelectrolyte-induced domain size and composition in lipid bilayer membranes

H-2 NMR spectroscopy was used to investigate the response of specifically choline-deuterated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) to changes in surface electrostatic charge in lipid bilayer membranes consisting of mixtures of POPC with the anionic lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG), plus the cationic polyelectrolyte poly(vinylbenzyltrimethylammonium chloride) (PVTA). The 2H NMR spectra consisted of two overlapping Fake powder patterns, indicating the coexistence of two lipid domains with only slow exchange of lipids between the two. There was no evidence of any nonbilayer lipid arrangements. Analysis of the quadrupolar splittings of the two H-2 NMR spectral components for POPC-alpha-d(2) versus POPC-beta-d(2) demonstrated that the two domains differ with respect to their overall surface charge environments and that the PVTA-bound domain was enriched with respect to anionic lipids while the PVTA-free domain was depleted. When the initial bilayer POPG content was varied, the number of POPG molecules associated with each bound PVTA molecule remained constant at a 1:1 POPG anion: PVTA cation equivalents ratio. However, the number of POPC molecules associated with each bound PVTA molecule increased with decreasing initial POPG content. Thus, the surface area occupied per PVTA domain expanded or contracted as the available oppositely charged POPG decreased or increased. This indicates that the PVTA/POPG complex is behaving at the surface like a two-dimensional random-coil polymer undergoing swelling by a solvent, POPC. Within the PVTA-bound domain, the 2H NMR quadrupolar splittings suggest that the PVTA/POPG complex penetrates more deeply into the lipid bilayer than does POPG alone, as might be expected for a neutralized salt pair.