Lipid Lateral Diffusion in Ordered and Disordered Phases in Raft Mixtures

Abstract

Lipid lateral diffusion coefficients in the quarternary system of dioleoylphosphatidylcholine (DOPC), sphingomyelin, cholesterol, and water were determined by the pulsed field gradient NMR technique on macroscopically aligned bilayers. The molar ratio between dioleoylphosphatidylcholine and sphingomyelin was set to 1:1, the cholesterol content was varied between 0 and 45 mol %, the water content was 40 wt %, and the temperature was varied between 293 and 333 K. The diffusion coefficients were separated into fast and slow spectral components by using the CORE method for global analysis of correlated spectral data. A large two-phase region, tentatively assigned to the liquid disordered (l(d)) and the liquid ordered (l(o)) phases, was present in the phase diagram. The l(d) phase was enriched in dioleoylphosphatidylcholine and exhibited diffusion coefficients that were about three to five times larger than for the l(o) phase. Both the diffusion coefficients and the apparent activation energies for the quarternary systems were compatible with earlier reports on ternary phospholipid/cholesterol/water systems. However, in contrast to the latter ternary systems, the exchange of lipids between the l(o) and the l(d) phases was slow on the timescale for the diffusion experiment for the quarternary ones. This means that on the millisecond timescale fluid, ordered domains are floating around in a sea of faster diffusing lipids, assigned to consist of mainly dioleoylphosphatidylcholine.

Assessing the Nature of Lipid Raft Membranes

“…This suggests that bilayer S C is close to the liquid disordered state of a POPC bilayer. This is also supported by the finding that small CHOL [33] or SM [35] concentrations have minor effects on D values of PC above melting temperatures.…”

Bacteria May Cope Differently from Similar Membrane Damage Caused by the Australian Tree Frog Antimicrobial Peptide Maculatin 1.1

“…In particular, SM and Chol that exist in high concentration in mammalian cells are known to increase lipid membrane order and to segregate and form more rigid domains (i.e. "raft"-like domains) in fluid cell membranes (30). Lytic activity was considerably reduced in the presence of Chol, which supports a likely role in protection of the host cell.…”

Free Cholesterol Alters Lipid Raft Structure and Function Regulating Neutrophil Ca2+ Entry and Respiratory Burst: Correlations with Calcium Channel Raft Trafficking

“…The mole fraction of cholesterol in rafts has been assessed at ϳ25% (4,52). Further ordering of rafts can occur at mole fraction of ϳ33%, but at higher concentrations still, cholesterol is incorporated into rafts without complexing to sphingolipids (51).…”