Phase behaviour of lipid–cholesterol membranes (original) (raw)
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Phase diagrams of lipid mixtures relevant to the study of membrane rafts
Biochimica Et Biophysica Acta-molecular and Cell Biology of Lipids, 2008
The present paper reviews the phase properties of phosphatidylcholine-sphingomyelin-cholesterol mixtures, that are often used as models for membrane "raft" microdomains. The available data based on X-ray, microscopic and spectroscopic observations, surface pressure and calorimetric measurements, and detergent solubilization assays, are critically evaluated and rationalized in terms of triangular phase diagrams. The remaining uncertainties are discussed specifically and separately from the data on which a consensus appears to exist.
Evidence for Lipid/Cholesterol Ordering in Model Lipid Membranes
Biophysical Journal, 2006
It has been postulated that for a binary mixture of phospholipid and cholesterol, phospholipid/cholesterol complexes are formed. Using grazing incidence x-ray diffraction, we have obtained evidence for lipid/cholesterol ordering in model membranes. Scattering features consistent with the existence of lipid/cholesterol complexes persist to high surface pressures even though fluorescence microscopy suggests a homogeneously fluid phase. Contrary to pure phospholipid and cholesterol systems, the resulting lattice spacing, integrated scattering intensity, and coherence lengths of these complexes are almost independent of surface pressure. Furthermore, the single peak observed in these mixed systems is very broad, suggesting that the extent of order for a single scattering structure only persists over a few molecules. This observation is consistent with these complexes being dynamic structures.
Physical Review E, 2009
The influence of cholesterol on lipid bilayer structure is significant and the effect of cholesterol on lipid sorting and phase separation in lipid-raft-forming model membrane systems has been well investigated by microscopy methods on giant vesicles. An important consideration however is the influence of fluorescence illumination on the phase state of these lipids and this effect must be carefully minimized. In this paper, we show that synchrotron x-ray scattering on solution lipid mixtures is an effective alternative technique for the identification and characterization of the l o ͑liquid ordered͒ and l d ͑liquid disordered͒ phases. The high intensity of synchrotron x rays allows the observation of up to 5 orders of diffraction from the l o phase, whereas only two are clearly visible when the l d phase alone is present. This data can be collected in ϳ1 min/ sample, allowing rapid generation of phase data. In this paper, we measure the lamellar spacing in both the liquidordered and liquid-disordered phases simultaneously, as a function of cholesterol concentration in two different ternary mixtures. We also observe evidence of a third gel-phaselike population at 10-12 mol % cholesterol and determine the thickness of the bilayer for this phase. Importantly we are able to look at phase coexistence in the membrane independent of photoeffects.
Structure of Cholesterol in Lipid Rafts
Physical Review Letters, 2014
Rafts, or functional domains, are transient nano-or mesoscopic structures in the plasma membrane and are thought to be essential for many cellular processes such as signal transduction, adhesion, trafficking and lipid/protein sorting. Observations of these membrane heterogeneities have proven challenging, as they are thought to be both small and short-lived. With a combination of coarse-grained molecular dynamics simulations and neutron diffraction using deuterium labeled cholesterol molecules we observe raft-like structures and determine the ordering of the cholesterol molecules in binary cholesterol-containing lipid membranes. From coarse-grained computer simulations, heterogenous membranes structures were observed and characterized as small, ordered domains. Neutron diffraction was used to study the lateral structure of the cholesterol molecules. We find pairs of strongly bound cholesterol molecules in the liquid-disordered phase, in accordance with the umbrella model. Bragg peaks corresponding to ordering of the cholesterol molecules in the raft-like structures were observed and indexed by two different structures: a monoclinic structure of ordered cholesterol pairs of alternating direction in equilibrium with cholesterol plaques, i.e., triclinic cholesterol bilayers.
X-Ray Microscopy of Fluid Lipid Membranes
X-Ray Microscopy and Spectromicroscopy, 1998
Membranes may be envisaged as two-dimensional liquid crystals. The microscopic molecular arrangement of the involved lipid molecules results in complicated structures in the mesoscopic size. X-ray microscopy is a tool for direct imaging of such appearances. Here we report some new results obtained with the Göttingen X-ray microscope for POPC in pure water. Lipid membranes are observed both with amplitude and phase contrast setup of the microscope. Cryo experiments complete the observations done under normal and humid conditions. The effect of radiation damage is discussed.
Order Parameters and Areas in Fluid-Phase Oriented Lipid Membranes Using Wide Angle X-Ray Scattering
Biophysical Journal, 2008
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.
Exclusion of a cholesterol analog from the cholesterol-rich phase in model membranes
Biochimica et Biophysica Acta (BBA) - Biomembranes, 2001
Vesicles of phosphatidylcholine/cholesterol mixtures show a wide composition range with coexistence of two fluid phases, the`liquid disordered' (cholesterol-poor) and`liquid ordered' (cholesterol-rich) phases. These systems have been widely used as models of membranes exhibiting lateral heterogeneity (membrane domains). The distributions of two fluorescent probes (a fluorescent cholesterol analog, NBD-cholesterol, and a lipophilic rhodamine probe, octadecylrhodamine B) in dimyristoylphosphatidylcholine/cholesterol vesicles were studied, at 30³C and 40³C. The steady-state fluorescence intensity of both probes decreases markedly with increasing cholesterol concentration, unlike the fluorescence lifetimes. The liquid ordered to liquid disordered phase partition coefficients K p were measured, and values much less than unity were obtained for both probes, pointing to preference for the cholesterol-poor phase. Globally analyzed time-resolved energy transfer results confirmed these findings. It is concluded that, in particular, NBD-cholesterol is not a suitable cholesterol analog and its distribution behavior in phosphatidylcholine/cholesterol bilayers is in fact opposite to that of cholesterol. ß 2001 Elsevier Science B.V. All rights reserved.
Current Opinion in Colloid & Interface Science, 2004
The existence of relatively large and long-lived detergent-insoluble, sphingolipid-and cholesterol-enriched, liquid-ordered lipid raft domains in the plasma membranes of eukaryotic cells has become widely accepted. However, we believe that the evidence for their existence is not compelling despite extensive work on both lipid bilayer model and biological membranes. We review here the results of recent studies, which in our view call into question the existence of lipid rafts in membranes, at least in the form commonly depicted. ᮊ