PHASE-EQUILIBRIA OF CHOLESTEROL DIPALMITOYLPHOSPHATIDYLCHOLINE MIXTURES - H-2 NUCLEAR MAGNETIC-RESONANCE AND DIFFERENTIAL SCANNING CALORIMETRY

Deuterium nuclear magnetic resonance spectroscopy and differential scanning calorimetry are used to map the phase boundaries of mixtures of cholesterol and chain-perdeuteriated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine at concentrations from 0 to 25 mol % cholesterol. Three distinct phases can be identified: the Lα or liquid-crystalline phase, the gel phase, and a high cholesterol concentration phase, which we call the ß phase. The liquid-crystalline phase is characterized by highly flexible phospholipid chains with rapid axially symmetric reorientation; the gel phase has much more rigid lipid chains, and the motions are no longer axially symmetric on the 2H NMR time scale; the ß phase is characterized by highly ordered (rigid) chains and rapid axially symmetric reorientation. In addition, we identify three regions of two-phase coexistence. The first of these is a narrow Lα/gel-phase coexistence region lying between 0 and about 6 mol % cholesterol at temperatures just below the chain-melting transition of the pure phospholipid/water dispersions, at 37.75 °C. The dramatic changes in the 2H NMR line shape which occur on passing through the phase transition are used to map out the boundaries of this narrow two-phase region. The boundaries of the second two-phase region are determined by 2H NMR difference spectroscopy, one boundary lying near 7.5 mol % cholesterol and running from 37 down to at least 30 °C; the other boundary lies near 22 mol % cholesterol and covers the same temperature range. Within this region, the gel and ß phases coexist. As the temperature is lowered below about 30 °C, the phospholipid motions reach the intermediate time scale regime of 2H NMR so that spectral subtractions become difficult and unreliable. The third two-phase region lies above 37 °C, beginning at a eutectic point somewhere between 7.5 and 10 mol % cholesterol and ending at about 20 mol %. In this region, the Lα and ß phases are in equilibrium. The boundaries for this region are inferred from differential scanning calorimetry traces, for the boundary between the Lα- and the two-phase region, and from a dramatic sharpening of the NMR peaks on crossing the boundary between the two-phase region and the β-phase region. In this region, the technique of difference spectroscopy fails, presumably because the diffusion rate in both the Lα- and β-phase domains is so rapid that phospholipid molecules exchange rapidly between domains on the experimental time scale. © 1990, American Chemical Society. All rights reserved.