X-ray diffraction of sphingomyelin-dihydrocholesterol (SM-DChol) monolayers revealed short-ranged ( approximately 25 A) 2D ordering. These nanoclusters show two distinct regions: below the cusp point of the phase diagram (35 mol% DChol), a constant d spacing was observed; above the cusp, the d spacing increases linearly with DChol in accordance to Vegard's law for binary alloys. The components in this lipidic alloy are thus a 65ratio35 SM-DChol entity and excess DChol. Reflectivity data further support the emergence above the cusp of an uncomplexed DChol population with greater vertical mobility.
It has been postulated that for a binary mixture of phospholipid and cholesterol, phospholipid/cholesterol complexes are formed. Using grazing incidence x-ray diffraction, we have obtained evidence for lipid/cholesterol ordering in model membranes. Scattering features consistent with the existence of lipid/cholesterol complexes persist to high surface pressures even though fluorescence microscopy suggests a homogeneously fluid phase. Contrary to pure phospholipid and cholesterol systems, the resulting lattice spacing, integrated scattering intensity, and coherence lengths of these complexes are almost independent of surface pressure. Furthermore, the single peak observed in these mixed systems is very broad, suggesting that the extent of order for a single scattering structure only persists over a few molecules. This observation is consistent with these complexes being dynamic structures.
Adding cholesterol to monolayers of certain phospholipids drives the separation of liquid-ordered from liquid-disordered domains. The ordered phases appear to contain stoichiometric complexes of cholesterol and phospholipid. Furthermore, it has been suggested that the cholesterol in these complexes has a low chemical activity compared to that of the free sterol; i.e., that in excess of the phospholipid binding capacity. We have now tested the hypothesis that the membrane intercalator 1-hexadecanol (HD) similarly associates with phospholipids and thereby displaces the complexed cholesterol. HD introduced into monolayers of pure dimyristoylphosphatidylcholine generated highly condensed (stable and solid) domains. In contrast, the phase behavior of mixed monolayers of the phospholipid, sterol, and alcohol suggested that HD could substitute for cholesterol mole for mole in promoting liquid-ordered domains. We also found that the transfer of cholesterol from mixed monolayers to aqueous cyclodextrin was greatly stimulated by the presence of HD, but only at levels sufficient to competitively displace the sterol from the phospholipid. This enhanced efflux was interpreted to reflect an increase in uncomplexed cholesterol. We conclude that HD forms complexes with dimyristoylphosphatidylcholine that are surprisingly similar to those of cholesterol. HD competitively displaces cholesterol from the phospholipid and thereby increases its chemical activity.
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