The effects of cholesterol on magnetically aligned phospholipid bilayers: a solid-state NMR and EPR spectroscopy study

Abstract

This paper presents the first time that both solid-state NMR spectroscopy and EPR spectroscopy are used to study the effects of cholesterol on magnetically aligned phospholipid bilayers (bicelles). Solid-state deuterium NMR spectroscopy was carried out using both chain perdeuterated 1,2-dimyristoyl- sn-glycero-3-phosphatidylcholine (DMPC-d 54) and a partially deuterated β-[2,2,3,4,4,6- 2H 6]cholesterol (cholesterol-d 6). Also, EPR spectroscopy was carried out utilizing a 3β-doxyl-5α-cholestane (cholestane) spin probe incorporated into magnetically aligned bilayers to provide a more complete picture about the ordering and dynamics of the phospholipid and cholesterol molecules in the bicelle membrane system. The results demonstrate that cholesterol was successfully incorporated into the phospholipid bilayers. The molecular order parameters extracted directly from the 2H NMR spectra of both DMPC-d 54 and cholesterol-d 6 were compared to that from the EPR study of cholestane. The order parameters indicate that the sterol was motionally restricted, and that the DMPC had high order and low motion for the hydrocarbon segments close to the head groups of the phospholipids and less order and more rapid motion toward the terminal methyl groups. Both methods clearly indicate an overall increase in the degree of ordering of the molecules in the presence of cholesterol and a decrease in the degree of ordering at higher temperatures. However, EPR spectroscopy and 2H NMR spectroscopy exhibit different degrees of sensitivity in detecting the phospholipid molecular motions in the membrane. Finally, cholesterol increases the minimum alignment temperature necessary to magnetically align the phospholipid bilayers.