Cyclodextrin-induced lipid lateral separation in DMPC membranes: (2)H nuclear magnetic resonance study

Cholesteryl cyclodextrins, obtained by grafting a cholesterol moiety on the oligosaccharide core, combine the size selectivity of the cyclodextrin cavity with the carrier properties of model membrane systems such as micelles or liposomes. The cholesteryl cyclodextrins were incorporated as guests in chain perdeuterated dimyristoyl phosphatidylcholine (DMPC-d54) membranes. The deuterium nuclear magnetic resonance (NMR) spectra obtained with the A form of cholesteryl-beta-cyclodextrin (beta CC(A)), with a succinyl spacer inserted between the cholesterol moiety and the cyclodextrin headgroup, indicated that this compound induces a lateral phase separation of DMPC-d54, into a pure lipid phase and a cholesteryl cyclodextrin-rich phase. The lipid exchange rate between the two phases was slow on the NMR timescale (>10(-5) s), and two well-resolved spectral components could be detected. The laterally segregated mixed phase was observed at various membrane concentrations of cholesteryl cyclodextrin, even with dispersions containing only 5% of the derivative. The dePaked spectra allowed the determination of the relative amount of DMPC-d54 molecules contained in each phase, giving approximately 1 to 1.5 DMPC molecules per unit of beta CC(A). This ratio was found to be independent of the total membrane concentration of beta CC(A). The cholesteryl cylodextrin-rich phase was detected on a large range of temperature from -12 degrees C to 25 degrees C and exhibits a smooth transition from a fluid environment to a more ordered state, occurring approximately 0 degrees C. A boundary phase between the pure lipid and cyclodextrin-rich phase was detected at 19 degrees C just below the fluid-to-gel transition. The average orientational order was reduced in the cholesteryl cyclodextrin-rich phase, and quasi-independent of temperature, as opposed to the order parameters measured for the NMR signals of the pure lipid phase. However, the NMR data obtained with beta CC(A) deuterated on the cyclodextrin headgroup indicated that the latter was quasistatic, with very large order parameters (approximately 120 kHz) at all temperatures, suggesting strong interactions between neighboring cyclodextrin headgroups. The interactions of DMPC-d54 membranes with the B form of cholesteryl-beta-cyclodextrin, lacking the succinyl spacer, was also investigated in a parallel study. No lateral phase separation was found with this compound, indicating that the spatial location and a precise positioning (allowed by the spacer) of the cyclodextrin headgroup at the membrane interface was crucial for the stability of the cholesteryl cyclodextrin lamellar phase.