Abstract
We 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 1,2-dioleoyl- sn-glycero-3-phosphocholine (DOPC)/cholesterol and 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (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 does not yield information about orientational order, and the scattering from the Lo and Ld phases looks similar. An analytical model to describe the relationship between the chain orientational distribution and WAXS data was used to obtain an average orientational order parameter, S x-ray. When 40% cholesterol is added to either DOPC or DPPC, S x-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 extended a commonly used method for calculating areas for gel-phase lipids to fluid-phase lipids and obtained agreement to within 5% of literature values.
Highlights
Incorporation of cholesterol into a phospholipid bilayer causes dramatic changes in the ordering of the lipid chains, as many investigations have shown
For the fluid-phase samples, we found that wide angle x-ray scattering (WAXS) was insensitive to a in the range 0–0.5°, some subtle differences were observed for gel-phase DPPC when a was smaller than the critical angle ac 1⁄4 0.12° for lipids at l 1⁄4 1.2 A
The I(q) plots defined in Fig. 3 B report the position of the lipid WAXS peak, q0, which gives the average spacing for molecular packing (d 1⁄4 2p/q0), dominated largely by the chain spacing, and the width of the peak gives information about disorder in the packing
Summary
Incorporation of cholesterol into a phospholipid bilayer causes dramatic changes in the ordering of the lipid chains, as many investigations have shown. Increased chain order leads to increased hydrophobic thickness and a decrease in the area per lipid, the well-known ‘‘cholesterol-condensing effect’’ [1,2,3,4,5]. Protein sorting into ‘‘rafts’’, cholesterol-rich regions in the plasma membrane, may be related to differences in hydrophobic thickness and chain order properties [6]. The liquid-ordered (Lo) phase in model membranes has been linked to these cell membrane rafts [7]. Thorough structural characterization of different lamellar lipid phases in model membranes provides a foundation for understanding the structure-function relationships in cell membrane processes such as protein sorting
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