COMPARATIVE EFFECTS OF CHOLESTEROL AND CHOLESTEROL SULFATE ON HYDRATION AND ORDERING OF DIMYRISTOYLPHOSPHATIDYLCHOLINE MEMBRANES

The comparative effect of cholesterol (CH) versus cholesterol sulfate (CS) on dimyristoylphosphatidylcholine (DMPC) membranes has been inves tigated by optical microscopy, freeze-fracture electron microscopy, x- ray diffraction, and solid state H-2 and P-31 nuclear magnetic resonan ce (NMR). The sulfate analogue extends the lamellar phase domain towar d high water contents, and substitution of 30 mol % CH by CS in DMPC l amellae induces the trapping of 30 wt % additional water. The greater swelling of the CS-containing systems is evidenced by determination of lamellar repeat distances at maximal hydration: 147 +/- 4 Angstrom an d 64 +/- 2 Angstrom in the presence of CS and CH, respectively. H-2-NM R of heavy water demonstrates that CS binds similar to 12 more water m olecules at the interface than CH whereas NMR of deuterium-labeled DMP C chains reveals that 30 mol % CS orders the membrane as 15 mol % CH a t high temperature and disorders much more than CH at low temperatures . The various effects of CS versus CH are discussed by taking into acc ount attractive Van der Waals forces and repulsive steric/electrostati c interactions of the negatively charged sulfate group.