Abstract
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.
Highlights
A wealth of research has indicated that cell membrane ‘‘rafts’’, small domains containing cholesterol and rich in sphingolipids, provide platforms for protein and lipid sorting and play important roles in cellular processes such as signal transduction and membrane transport [1,2,3]
The I(q) plots in Fig. 1, B and C, show that these samples are liquid-like because the peak has a large halfwidth at half-maximum (HWHM; ;0.16 A À1), similar to liquid phase DOPC/Chol and DPPC/Chol mixtures [31]
The angular (f, as defined in Fig. 1 C) distribution of scattering in oriented Wide angle x-ray scattering (WAXS) data gives additional information, which we explore in this work in regard to liquid-liquid phase coexistence
Summary
A wealth of research has indicated that cell membrane ‘‘rafts’’, small domains containing cholesterol and rich in sphingolipids, provide platforms for protein and lipid sorting and play important roles in cellular processes such as signal transduction and membrane transport [1,2,3]. Detection of these lipid domains in cells relies on indirect methods such as detergent extraction and cholesterol depletion because they are too small to be observed with optical microscopy [4]. These include confinement of nanoscopic lipid microenvironments by corrals in the cytoskeleton network [9] and nanoscale fluc-
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