Differential modulation of membrane structure and fluctuations by plant sterols and cholesterol
Aden Hodzic 1, Michael Rappolt 1, Heinz Amenitsch 1, Peter Laggner 1 and Georg Pabst 1*
1 Austrian Academy of Sciences
* To whom correspondence should be addressed. E-mail: georg.pabst{at}oeaw.ac.at.
Submitted on October 2, 2007
Revised on October 26, 2007
Accepted on 11 December 2007
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Abstract |
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We have studied the concentration and temperature dependent influence of cholesterol, stigmasterol and sitosterol on the global structure and the bending fluctuations of fluid dimyristoyl phosphatidylcholine and palmitoyl oleoyl phosphatidylcholine bilayers applying small-angle x-ray scattering, as well as dilatometry and ultrasound velocimetry. Independent of the lipid matrix, cholesterol was found to be most efficient in modulating bilayer thickness and elasticity, followed by sitosterol and stigmasterol. This can be attributed to the additional ethyl groups and double bond at the C17 alkyl side-chain of the two plant sterols. Hence, it appears that some flexibility of the sterol hydrocarbon chain is needed in order to accommodate within the lipid bilayer. In addition, we did not observe two populations of membranes within the putative liquid-ordered/liquid-disordered phase coexistence regime of binary sterol/lipid mixtures. Instead, the diffraction patterns could be interpreted in terms of a uniform phase. This lends further support to the idea of compositional fluctuations of unstable sterol rich domains recently brought up by fluorescence microscopy experiments, which contrasts the formation of stable domains within the miscibility gap of binary lipid/sterol mixtures.
Key Words:
binary lipid/sterol mixtures, combined dilatometry and ultrasound velocimetry, interactions, phase diagrams, phytosterols, small-angle x-ray scattering
