Sterol orientations in phosphatidylcholine liposomes as determined by deuterium NMR

Abstract

Deuterium magnetic resonance spectra (55.26 MHz) of cholesterol-3.alpha.-d1 and epicholesterol-3.beta.-d1 in dipalmitoylglycerophosphocholine (DPPC) liposomes were measured as a function of sterol-to-phospholipid ratio below (24.degree. C) and above (60.degree. C) the phase transition temperature of DPPC. From the quadrupolar splittings .DELTA..nu.q, the molecular order parameters S describing the motions of the sterols in the bilayer were calculated, and the most probable angle of tilt .alpha.0 of the molecular axis of the sterols relative to the bilayer normal was determined. We observed that the molecular axis of cholesterol in DPPC liposomes at both 24 and 60.degree. C is tilted at an angle of 16-19.degree. with the 3.beta.-hydroxyl group projecting parallel to the bilayer normal into the aqueous interface. In contrast, at 24.degree. C, epicholesterol is aligned parallel (0.degree.) to the bilayer normal, placing the 3.alpha.-hyroxyl group essentially perpendicular to the bilayer normal along the aqueous interface. At 60.degree. C, the average angle of epicholesterol (16-18.degree.) is similar to that of cholesterol, which can project the 3.alpha.-hydroxyl group into the hydrophobic bilayer region. On the basis of the observed tilt angles of the two isomeric sterols in DPPC liposomes, a model is proposed that can rationalize the differential effects of cholesterol and epicholesterol on membrane properties. The near-parallel alignment, at both 24 and 60.degree. C, of cholesterol relative to the fatty acyl chains in phospholipid bilayers (16-25.degree.), with the sterol 3.beta.-hydroxyl group projecting into the aqueous interface, would promote strong lipophilic interactions as well as hydration of the hydroxyl function. The nonparallel arrangement, at 24.degree. C, of epicholesterol and the fatty acyl chains, again assuming an acyl tilt of 16-25.degree., would reduce lipophilic interactions as compared to those observed for cholesterol, while the hydroxyl function would remain relatively accessible to hydration. At 60.degree. C, even though epicholesterol would be oriented nearly parallel to the acyl chains, effective lipophilic interactions as well as hydration would be hindered by the 3.alpha.-hydroxyl group rotating into the hydrophobic bilayer region. The differential sterol effects of cholesterol and epicholesterol are discussed are in light of the model proposed.