H-2 NMR and polyelectrolyte-induced domains in lipid bilayers

H-2 NMR studies of polyelectrolyte-induced domain formation in lipid bilaye r membranes are reviewed. The H-2 NMR spectrum of choline-deuterated phosph atidylcholine (PC) reports on any and all sources of lipid bilayer surface charge, since these produce a conformation change in the choline head group of PC, manifest as a change in the H-2 NMR quadrupolar splitting. In addit ion, homogeneous and inhomogeneous surface charge distributions are differe ntiated. Adding polyelectrolytes to lipid bilayers consisting of mixtures o f oppositely charged and zwitterionic lipids produces H-2 NMR spectra which are superpositions of two Fake sub-spectra: one corresponding to a polyele ctrolyte-bound lipid population and the other to a polyelectrolyte-free lip id population. Quantitative analysis of the quadrupolar splittings and spec tral intensities of the two sub-spectra indicate that the polyelectrolyte-b ound populations is enriched with oppositely charged lipid, while the polye lectrolyte-free lipid population is correspondingly depleted. The same doma in-segregation effect is produced whether cationic polyelectrolytes are add ed to anionic lipid bilayers or anionic polyelectrolytes are added to catio nic lipid bilayers. The H-2 NMR spectra permit a complete characterization of domain composition and size. The anion:cation ratio within the domains i s always stoichiometric, as expected for a process driven by Coulombic inte ractions. The zwitterionic lipid content of the domains is always statistic al, reflecting the systems tendency to minimize the entropic cost of demixi ng charged lipids into domains. Domain formation is observed even with rath er short polyelectrolytes, suggesting that individual polyelectrolyte chain s aggregate at the surface to form "superdomains". Overall, the polyelectro lyte bound at the lipid bilayer surface appears to lie flat along the surfa ce and to be essentially immobilized through its multiple electrostatic con tacts. (C) 2000 Elsevier Science B.V. All rights reserved.