INTERACTIONS OF OXYSTEROLS WITH A LIPID-MEMBRANE MODEL - SOLID-STATE H-2-NMR STUDIES

The effect of oxysterols on model membranes has been studied by deuter ium nuclear magnetic resonance (H-2 NMR) spectroscopy. Oriented bilaye rs were prepared from 1-myristoyl-2-[H-2(27)] myristoyl-sn-glycero-3-p hosphocholine (DMPC-d(27)) and cholesterol + oxysterols. Order paramet er profiles were determined and showed that, contrary to intuitive pre dictions, when 7 alpha-, 7 beta-, 22S-hydroxycholesterol and 25-hydrox ycholesterol were used at 15 mol% with 15 mol% cholesterol, they had a very strong ordering effect on the acyl chains of DMPC. The H-2 NMR s pectrum bf oriented bilayers and the H-2 NMR spectrum of aqueous dispe rsions of DMPC-d(27) + 30 mol% 25-hydroxycholesterol at 40 degrees C s how two different patterns, probably corresponding to two populations of DMPC which differ by their 25-hydroxycholesterol/DMPC ratio. This h ypothesis was confirmed by P-31 NMR spectroscopy and differential scan ning calorimetry (DSC). Liposomes of DMPC containing 25-hydroxycholest erol form microdomains with 25-hydroxycholesterol-rich and -poor regio ns. This is probably due to the tendency of the 25-hydroxycholesterol to form clusters with itself.