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Originally published as Biophys J. BioFAST on April 4, 2008.
doi:10.1529/biophysj.107.127845
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Biophysical Journal 95:669-681 (2008)
© 2008 The Biophysical Society

Order Parameters and Areas in Fluid-Phase Oriented Lipid Membranes Using Wide Angle X-Ray Scattering

Thalia T. Mills * {dagger}, Gilman E. S. Toombes * {ddagger}, Stephanie Tristram-Nagle {dagger}, Detlef-M. Smilgies §, Gerald W. Feigenson ¶ and John F. Nagle {dagger}

* Department of Physics, Cornell University, Ithaca, New York 14853; {dagger} Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213; {ddagger} UMR 168 Centre National de la Recherche Scientifique/Institut Curie, 75005 Paris, France; and § Cornell High Energy Synchrotron Source and Field of Biophysics, Cornell University, Ithaca, New York 14853

Correspondence: Address reprint requests to John F. Nagle, Tel.: 412-268-2764; Fax: 412-681-0648; E-mail: nagle{at}cmu.edu.

We used wide angle x-ray scattering (WAXS) from stacks of oriented lipid bilayers to measure chain orientational order parameters and lipid areas in model membranes consisting of mixtures of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/cholesterol and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/cholesterol in fluid phases. The addition of 40% cholesterol to either DOPC or DPPC changes the WAXS pattern due to an increase in acyl chain orientational order, which is one of the main properties distinguishing the cholesterol-rich liquid-ordered (Lo) phase from the liquid-disordered (Ld) phase. In contrast, powder x-ray data from multilamellar vesicles does not yield information about orientational order, and the scattering from the Lo and Ld phases looks similar. An analytical model to describe the relationship between the chain orientational distribution and WAXS data was used to obtain an average orientational order parameter, Sx-ray. When 40% cholesterol is added to either DOPC or DPPC, Sx-ray more than doubles, consistent with previous NMR order parameter measurements. By combining information about the average chain orientation with the chain-chain correlation spacing, we extended a commonly used method for calculating areas for gel-phase lipids to fluid-phase lipids and obtained agreement to within 5% of literature values.




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T. T. Mills, S. Tristram-Nagle, F. A. Heberle, N. F. Morales, J. Zhao, J. Wu, G. E. S. Toombes, J. F. Nagle, and G. W. Feigenson
Liquid-Liquid Domains in Bilayers Detected by Wide Angle X-Ray Scattering
Biophys. J., July 15, 2008; 95(2): 682 - 690.
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