Comparative Calorimetric and Spectroscopic Studies of the Effects on Lanosterol and Cholesterol on the Thermotropic Phase Behavior and Organization of Dipalmitoylphosphatidylcholine Bilayer Membranes

¹ University of Alberta

^* To whom correspondence should be addressed. E-mail: rmcelhan{at}ualberta.ca.

Submitted on March 2, 2006
Revised on March 28, 2006
Accepted on 26 July 2006

	Abstract

We carried out comparative DSC and FTIR spectroscopic studies of the effects of cholesterol and lanosterol on the thermotropic phase behaviour and organization of DPPC bilayers. Lanosterol is the biosynthetic precursor of cholesterol and differs in having three rather than two axial methyl groups projecting from the -face of the planar steroid ring system and one axial methyl group projecting from the -face, whereas cholesterol has none. Our DSC studies indicate that the incorporation of lanosterol is more effective than cholesterol is in reducing the enthalpy of the pretransition. Lanosterol is also initially more effective than cholesterol in reducing the enthalpies of both the sharp and broad components of main phase transition. However, at sterol concentrations of 50 mol %, lanosterol does not abolish the cooperative hydrocarbon chain-melting phase transition as does cholesterol. Moreover, at higher lanosterol concentrations (~ 30-50 mol %), both sharp and broad low-temperature endotherms appear in the DSC heating scans, suggestive of the formation of lanosterol crystallites,and of the lateral phase separation of lanosterol-enriched phospholipid domains, respectively, at low temperatures, whereas such behaviour is not observed with cholesterol at comparable concentrations. Our FTIR spectroscopic studies demonstrate that lanosterol incorporation produces a less tightly packed bilayer than does cholesterol which is characterized by increased hydration in the glycerol backbone region of the DPPC bilayer. These and other results indicate that lanosterol is less miscible in DPPC bilayers than is cholesterol but perturbs their organization to a greater extent, probably due primarily to the rougher faces and larger cross-sectional area of the lanosterol molecule and perhaps secondarily to its decreased ability to form hydrogen bonds with adjacent DPPC molecules. Nevertheless, lanosterol does appear to produce a lamellar liquid-ordered phase in DPPC bilayers, although this phase is not as tightly packed as in the case of cholesterol.

Key Words: FTIR spectroscopy, cholesterol, differential scanning calorimetry, lanosterol, lipid bilayers, sterol-lipid interactions