Effects of cholesterol and unsaturated DOPC lipid on chain packing of saturated gel-phase DPPC bilayers.
Wide angle x-ray scattering (WAXS) from oriented lipid multilayers was used to study the effect of adding cholesterol (Chol) or 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) to gel-phase 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers. Small quantities (X < 0.10 mole fraction) of both molecules disrupt the tight packing of tilted chains of pure gel-phase DPPC, forming a more disordered, untilted phase. The addition of larger quantities of DOPC causes the sample to phase-separate into a gel-phase, characterized by a narrow WAXS peak, and liquid disordered phase, characterized by wide, diffuse WAXS scattering. In contrast, two WAXS peaks indicative of two coexisting phases were not observed in Chol/DPPC mixtures (X(Chol) = 0.07 to 0.40). Instead, Chol caused a gradual increase in the width of the WAXS peak, consistent with a gradual change from a more gel-like to a more liquid-like state rather than passing through a region of two phase coexistence. Our WAXS data include a huge amount of information. A new method of analysis suggests that WAXS data may provide definitive results relating to the disagreements between previously published phase diagrams for Chol/DPPC.
Mills
Thalia T
TT
Department of Physics, Carnegie Mellon University, Pittsburgh, Pensylvania 15213, U.S.A.
Huang
Juyang
J
Feigenson
Gerald W
GW
Nagle
John F
JF
D015060:1,2-Dipalmitoylphosphatidylcholine / Q000737:chemistry*; D002784:Cholesterol / Q000737:chemistry*; D008051:Lipid Bilayers / Q000737:chemistry*; D010713:Phosphatidylcholines / Q000737:chemistry*; D012542:Scattering, Radiation; D013997:Time Factors; D014965:X-Rays
D008051:Lipid Bilayers; D010713:Phosphatidylcholines; D015060:1,2-Dipalmitoylphosphatidylcholine; D002784:Cholesterol; C017251:1,2-oleoylphosphatidylcholine