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Liquid-Liquid Domains in Bilayers Detected by Wide Angle X-Ray Scattering

Thalia T. Mills ^* , Stephanie Tristram-Nagle , Frederick A. Heberle , Nelson F. Morales , Jiang Zhao , Jing Wu , Gilman E. S. Toombes ^* ¶, John F. Nagle  and Gerald W. Feigenson 

^* Department of Physics, Cornell University, Ithaca, New York 14853; Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213; Field of Biophysics, and Department of Chemistry, Cornell University, Ithaca, New York 14853; and ^¶ Unités mixtes de recherche 168 Centre National de la Recherche Scientifique/Institut Curie, 75005 Paris, France

Correspondence: Address reprint requests to Gerald W. Feigenson, Tel.: 607-255-4744; Fax: 607-255-6249; E-mail: gwf3{at}cornell.edu.

Wide angle x-ray scattering (WAXS) from oriented lipid multilayers is used to examine liquid-ordered (Lo)/liquid-disordered (Ld) phase coexistence in the system 1,2-dioleoyl-sn-glycero-3-phosphocholine/1,2-dipalmitoyl-sn-glycero-3-phosphocholine/cholesterol (DOPC/DPPC/Chol), which is a model for the outer leaflet of the animal cell plasma membrane. Using the method of analysis developed in the accompanying work, we find that two orientational distributions are necessary to fit the WAXS data at lower temperatures, whereas only one distribution is needed at temperatures higher than the miscibility transition temperature, T_mix = 25–35°C (for 1:1 DOPC/DPPC with 15%, 20%, 25%, and 30% Chol). We propose that the necessity for two distributions is a criterion for coexistence of Lo domains with a high S_x-ray order parameter and Ld domains with a lower order parameter. This criterion is capable of detecting coexistence of small domains or rafts that the conventional x-ray criterion of two lamellar D spacings may not. Our T_mix values tend to be slightly larger than published NMR results and microscopy results when the fluorescence probe artifact is considered. This is consistent with the sensitivity of WAXS to very short time and length scales, which makes it more capable of detecting small, short-lived domains that are likely close to T_mix.

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				T. T. Mills, G. E. S. Toombes, S. Tristram-Nagle, D.-M. Smilgies, G. W. Feigenson, and J. F. Nagle Order Parameters and Areas in Fluid-Phase Oriented Lipid Membranes Using Wide Angle X-Ray Scattering Biophys. J., July 15, 2008; 95(2): 669 - 681. [Abstract] [Full Text] [PDF]