Polyelectrolyte-induced domains in lipid bilayer membranes: the deuterium NMR perspective

Domain formation in lipid bilayer membranes can occur through electrostatic interactions between charged lipids and oppositely charged polyelectrolyte s, such as proteins or polynucleic acids. This review describes a novel met hod for examining such domains in lipid bilayers, based on H-2 NMR spectros copy. The H-2 NMR spectrum of choline-deuterated phosphatidylcholine is sen sitive to, and reports on, lipid bilayer surface charge. When a charged lip id bilayer is exposed to an oppositely charged polyelectrolyte, the latter binds electrostatically to the bilayer surface and attracts charged lipids into its vicinity. The resulting inhomogeneous charge distribution produces overlapping H-2 NMR subspectra arising from phosphatidylcholine within cha rge-enriched versus charge-depleted regions. Such spectral details as the q uadrupolar splittings and the relative intensities of the subspectra permit a complete analysis of the domain composition, size, and, within limits, l ifetime. Using H-2 NMR, domain formation in lipid bilayer membranes can be observed with both cationic and anionic polyelectrolytes, whether of natura l or synthetic origin. Domain size and composition prove to be sensitive to the detailed chemical structure of both the polyelectrolyte and the charge d lipids. Within the domains there is always a stoichiometric anion/cation binding ratio, indicating that the polyelectrolyte lies flat on the membran e surface. The amount of phosphatidylcholine within the domain varies as a function of its statistical availability, in accordance with the prediction s of a recent thermodynamic model of domain formation. When the molecular w eight of the polyelectrolyte is varied, the domain size alters in accordanc e with the predictions of classical polymer physics. As expected for a pred ominantly electrostatic phenomenon, the observed domains dissipate at high ionic strength.