Critical pressures in multicomponent lipid monolayers

Hagen, John P.; McConnell, Harden M.

doi:10.1016/0005-2736(96)00009-0

Cited by 18 publications

(22 citation statements)

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“…Previous work has reported that the miscibility transition pressure of monolayers containing phospholipid and 20 mol% Dchol is linearly, systematically altered by the addition of a second phospholipid (Hagen and McConnell, 1996). This conclusion is true for the cases previously studied.…”

Section: Discussionsupporting

confidence: 69%

“…4). The phase diagram for this mixture does not differ significantly from the shape of common binary phase diagrams of phospholipid and Dchol (Hagen and McConnell, 1996;Keller et al, 2000). At transition points far from the critical point, stripes are not seen (open symbols, Fig.…”

Section: Resultsmentioning

confidence: 74%

“…In contrast, the unsaturated lipids di(14:1)PC and di(18:1) PC act as an average lipid in the presence of 33% Dchol. The phase diagrams were made by plotting data along the P1-Dchol and P2-Dchol axes (data from Hagen and McConnell, 1996;Hagen and McConnell, 1997;Keller et al, in press), along points A-B (data from Fig. 3), and along C-D (data from Fig.…”

Section: Calculationsmentioning

confidence: 99%

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Miscibility Critical Pressures in Monolayers of Ternary Lipid Mixtures

Keller

Anderson

McConnell

2000

Biophysical Journal

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When phospholipids are mixed with cholesterol in a monolayer at an air-water interface, coexisting 2-dimensional liquid phases can be observed if the surface pressure, pi, is lower than the miscibility critical pressure, pi(c). Ternary mixtures of two phospholipid species with dihydrocholesterol have been reported to have critical pressures that are linearly proportional to the relative composition of the phospholipids. However, we report here that, if the acyl chains of the two phospholipids differ significantly in length or unsaturation, the behavior is markedly different. In this case, the critical pressure of the ternary mixture can be remarkably high, exceeding the critical pressures of the corresponding binary mixtures. High critical pressures are also seen in binary mixtures of phospholipid and dihydrocholesterol when the two acyl chains of the phospholipid differ sufficiently in length. Using regular solution theory, we interpret the elevated critical pressures of these mixtures as an attractive interaction between the phospholipid components.

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Section: Discussionsupporting

confidence: 69%

Section: Resultsmentioning

confidence: 74%

Section: Calculationsmentioning

confidence: 99%

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Miscibility Critical Pressures in Monolayers of Ternary Lipid Mixtures

Keller

Anderson

McConnell

2000

Biophysical Journal

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“…Mixtures of phospholipids alone have not generally been found to exhibit liquid-liquid immiscibility in monolayers. However, immiscibility is seen in binary mixtures of cholesterol with the phospholipids phosphatidylcholine [1][2][3][4], phosphatidylethanolamine [10], or phosphatidylserine [11]. Cholesterol and egg sphingomyelin also exhibit immiscibility (data not shown).…”

mentioning

confidence: 99%

“…At the putative critical composition, the pressure is changed until the critical pressure p c is found. Conducting experiments at physiological temperature (37 ± C) rather than room temperature (23 ± C) is expected to lower the critical pressure by only 1-3 dyn͞cm, assuming the average values of the a ij are similar [10,14]. Figure 3(a) gives a schematic ternary phase diagram for two ideally mixing phospholipids (components 1 and 2) and cholesterol (component 3).…”

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confidence: 99%

Red Blood Cell Lipids Form Immiscible Liquids

et al. 1998

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Monolayers at the air-water interface were prepared from lipids extracted from human red blood cells. Epifluorescence microscopy was used to show that monolayers simulating the inner and outer leaflets of the red cell membrane form immiscible liquid phases with critical points at surface pressures of 21 and 29 dyn͞cm. At these pressures the monolayer lipid density is comparable to that in the red cell membrane. This suggests that lipid bilayers of a red blood cell are near a miscibility critical point, which should significantly affect the biophysical properties of the red cell membrane.[S0031-9007(98)07922-8] 64.75. + g, 87.22.Bt, Epifluorescence microscope studies have shown that mixtures of cholesterol and phospholipids form immiscible liquid phases in monolayers at the air-water interface [1][2][3][4]. There is indirect evidence that immiscibility also occurs in bilayers of these lipids [5][6][7]. These results raise the question of whether immiscible liquid domains exist in biological membranes. To this end, we extracted the lipids from membranes of human red blood cells (erythrocytes). Red blood cell membranes contain over 250 lipid species [8], most of which are arranged asymmetrically across the membrane [9]. The major headgroup classes of the extracted erythrocyte lipids were separated by thin layer chromatography, then reconstituted with 50 mol % cholesterol so as to mimic compositions of the inner and outer erythrocyte membrane leaflets. Monolayers of these lipids at the air-water interface are found to form immiscible liquid phases. Figures 1 and 2 show epifluorescence micrographs of monolayers of reconstituted lipids simulating the inner and outer leaflets. Contrast between phases is achieved by 0.2 mol % of a fluorescent probe that is preferentially excluded from the cholesterol-rich phase [4]. Two immiscible liquid phases coexist to surface pressures greater than 20 dyn͞cm. Domain shapes characteristic of proximity to a critical point are observed at 21 dyn͞cm in the simulated inner leaflet [ Fig. 1(b)] and at 29 dyn͞cm in the outer leaflet [Figs. 2(c) and 2(d)]. The fingering of the domains in Fig. 2(f) indicates the monolayer is close to the critical composition. "Fingered" domains are circular domains from which stripes emanate. At higher pressures the monolayers are homogeneous as in Figs. 1(c) and 2(e). The domain shape changes are reversible.It is helpful to consider these results in terms of a simple thermodynamic model. Mixtures of phospholipids alone have not generally been found to exhibit liquid-liquid immiscibility in monolayers. However, immiscibility is seen in binary mixtures of cholesterol with the phospholipids phosphatidylcholine [1-4], phosphatidylethanolamine [10], or phosphatidylserine [11]. Cholesterol and egg sphingomyelin also exhibit immiscibility (data not shown).Thus, all of the red cell's major lipid components can potentially contribute to the observed immiscibility. This leads to the question of how binary properties manifest themselves in a multicomponent mixture. C...

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Liquid–liquid immiscibility in model membranes activates secretory phospholipase A2

Wagner

Desbat

Brezesinski

2008

Biochimica et Biophysica Acta (BBA) - Biomembranes

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Secretory phospholipase A2 (sPLA2) hydrolyzes phosphatidylcholines (PC) within lipid bilayers to produce lyso-PC and a fatty acid, which can act as signaling molecule in biological membranes. The activity of sPLA2 depends on the membrane structure. Bilayer defects, curvature, and gel-fluid micro-heterogeneity are known to activate sPLA2. Here, we investigate if liquid-liquid immiscibility within model membranes is sufficient for sPLA2 activation. The onset of the hydrolytic activity of cobra-venom sPLA2 towards mixed monolayers of dimyristoyl-PC (DMPC)/cholesterol 2:1 (mol/mol) has been determined using infrared reflection-absorption spectroscopy (IRRAS) and polarization-modulated (PM-) IRRAS. The lag phase of sPLA2 activity increases exponentially with rising surface pressures starting at 12 mN/m. This indicates that enzyme activation is hampered at higher surface pressures. Below 12 mN/m, no lag phase is observed, and sPLA2 is efficiently activated. The surface pressure that is critical for sPLA2 activation correlates with the critical miscibility pressure according to the phase diagram of DMPC and cholesterol. Thus, coexisting, liquid-phase domains provide sufficient boundaries to activate sPLA2. Moreover, liquid-liquid immiscibility is an activating mechanism for sPLA2 that also applies to biological membranes under physiological conditions because the corresponding bilayer structure is associated with that of membrane rafts.

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Critical pressures in multicomponent lipid monolayers

Cited by 18 publications

References 23 publications

Miscibility Critical Pressures in Monolayers of Ternary Lipid Mixtures

Miscibility Critical Pressures in Monolayers of Ternary Lipid Mixtures

Red Blood Cell Lipids Form Immiscible Liquids

Liquid–liquid immiscibility in model membranes activates secretory phospholipase A2

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