Polyelectrolyte molecular weight and electrostatically-induced domains in lipid bilayer membranes

Polyelectrolyte-induced domain formation in charged lipid bilayer membranes was investigated as a function of polyelectrolyte molecular weight using 2H nuclear magnetic resonance (NMR) spectroscopy. Lipid bilayers consisting of mixtures of alpha- or beta-choline-deuterated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC-alpha-d2 or POPC-beta-d2) plus the cationic amphiphile 1,2-dioleoyl-3-(dimethylamino)propane (DODAP) were exposed to the anionic polyelectrolyte poly(sodium 4-styrenesulfonate) (PSSS) of various molecular weights. Regardless of molecular weight, PSSS produced dual component 2H NMR spectra, indicating two distinct POPC populations, corresponding to PSSS-bound and PSSS-free lipid, in slow exchange with one another. Analysis of the 2H NMR subspectra quadrupolar splittings and intensities showed the PSSS-bound domain to be enriched in DODAP, with the PSSS-free domain correspondingly depleted. At polyelectrolyte loadings below global charge equivalence, PSSS bound DODAP stoichiometrically for all PSSS molecular weights, indicating that the polyelectrolyte chain lies flat upon the membrane surface. At higher PSSS loadings the domains dissipated, leading to single component 2H NMR spectra. At high NaCl concentrations PSSS dissociated from the bilayer surface. Domain size on a per PSSS chain basis increased while the degree of enrichment with DODAP decreased progressively as the PSSS chain length decreased. Such molecular weight-dependent domain characteristics have not been predicted theoretically and need to be taken into account in future refinements of domain models.