Liquid-liquid domains in bilayers detected by wide angle x-ray scattering
Thalia T. Mills 1, Stephanie Tristram-Nagle 2, Frederick A. Heberle 3, Nelson F. Morales 3, Jiang Zhao 3, Jing Wu 3, Gilman E. S. Toombes 4, John F. Nagle 1 and Gerald W. Feigenson 3*
1 Carnegie Mellon University
2 Carnegie Mellon Univ.
3 Cornell University
4 Institut Curie
* To whom correspondence should be addressed. E-mail: gwf3{at}cornell.edu.
Submitted on December 16, 2007
Revised on February 5, 2008
Accepted on 12 March 2008
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Abstract |
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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 DOPC/DPPC/cholesterol, which is a model for the outer leaflet of the animal cell plasma membrane. Using the method of analysis developed in the accompanying paper, 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, Tmix= 25 to 35°C (for 1:1 DOPC/DPPC with 15%, 20%, 25%, and 30% cholesterol). We propose that the necessity for two distributions is a criterion for coexistence of Lo domains with a high Sx-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 Tmix= 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 Tmix=.
Key Words:
chain order, cholesterol, liquid disordered, liquid ordered, model membrane, rafts
