Order parameters and areas in fluid-phase oriented lipid membranes using wide angle x-ray scattering
Thalia T. Mills 1, Gilman E. S. Toombes 2, Stephanie Tristram-Nagle 1, Detlef-M. Smilgies 3, Gerald W. Feigenson 4 and John F. Nagle 1*
1 Carnegie Mellon Univ.
2 Institut Curie
3 CHESS Cornell Univ.
4 Cornell Univ.
* To whom correspondence should be addressed. E-mail: nagle{at}cmu.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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We have 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 DOPC/cholesterol and 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 (MLVs) does not yield information about orientational order, and the scattering from the Lo and Ld phases look similar. An analytical model to describe the relationship between the chain orientational distribution and WAXS data is 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 extend a commonly used method for calculating areas for gel-phase lipids to fluid-phase lipids and obtain agreement to within 5% of literature values.
Key Words:
chain packing, cholesterol condensation, lipid bilayers, liquid disordered, liquid ordered
