Lipid phase behaviour under steady state conditions

Åberg, Christoffer; Sparr, Emma; Wennerström, Håkan

doi:10.1039/c2fd20079a

Cited by 8 publications

(21 citation statements)

References 40 publications

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“…However, the outer phase also becomes thicker and the overall permeability to water of the membrane thus decreases (8). This constitutes a feedback loop in which the increase in the evaporation driving force is compensated by a decrease in the permeability to water (9). This mechanism is consistent with observations of A B phases transitions in the stratum corneum between solid and fluid structures (8), which possess very different permeabilities (6).…”

Section: Discussionsupporting

confidence: 75%

“…The key is the ability of the system to form different self-assembled structures, with different transport properties, depending on the water content. This implies the existence of a feedback loop on the water loss across the interface through structural changes in the interfacial layer in response to changes in boundary conditions (9). We designed an experimental setup to study the nonequilibrium system of the air-liquid interfacial layer through the monitoring of the following: (i) the water loss with gravimetric measurements, (ii) the water composition gradient with infrared microscopy, (iii) the sequence of phases with polarized optical microscopy, and (iv) the nanostructure with small-angle X-ray scattering (SAXS) profiles (Fig.…”

Section: Resultsmentioning

confidence: 99%

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Controlling water evaporation through self-assembly

Roger

Liebi

Heimdal

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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Water evaporation concerns all land-living organisms, as ambient air is dryer than their corresponding equilibrium humidity. Contrarily to plants, mammals are covered with a skin that not only hinders evaporation but also maintains its rate at a nearly constant value, independently of air humidity. Here, we show that simple amphiphiles/water systems reproduce this behavior, which suggests a common underlying mechanism originating from responding self-assembly structures. The composition and structure gradients arising from the evaporation process were characterized using optical microscopy, infrared microscopy, and smallangle X-ray scattering. We observed a thin and dry outer phase that responds to changes in air humidity by increasing its thickness as the air becomes dryer, which decreases its permeability to water, thus counterbalancing the increase in the evaporation driving force. This thin and dry outer phase therefore shields the systems from humidity variations. Such a feedback loop achieves a homeostatic regulation of water evaporation.T he evaporation of water from an aqueous medium to a dry gas phase is a ubiquitous phenomenon in nature. Evaporation can occur freely, as from oceans into the air, or be hindered by membranes or barrier films. Land-living organisms face the challenge of adjusting to the relative humidity (RH) of ambient air, which varies from a few percent to saturation at 100%, whereas the living-cell water chemical potential corresponds to a RH of above 99%. This difference drives water transport from the cells to the ambient air, exposing life to a drying-out threat. Different strategies have emerged to counter this threat. Plant leaves are covered with a waxy cuticle layer composed of polymers and associated lipids (1), whereas animals like mammals are protected by a skin composed of dead cells embedded in a lipid matrix (2), and a lipid film on the tear liquid of their eyes (3). Water transport across an inert diffusional barrier is proportional to the difference in water chemical potential between the inside and the outside. Total water loss through an inert membrane would thus vary in response to changes in the environmental humidity, with the risk of massive water loss in dry conditions. This phenomenon is typically observed in the plant cuticular film that coats the leaves (1, 4), as displayed in Fig. 1. On the contrary, several studies show that, for healthy human stratum corneum, the outermost layer of skin, the evaporation rate increases with lowering RH at high humidities, whereas it is virtually constant and independent of the outside humidity for RH < 85% (5-7) (Fig. 1). The stratum corneum is a thin and dry layer composed of dead keratin-filled cells embedded in a lipid multilamellar matrix, which realizes the barrier function of the skin (2). This membrane responds to drier conditions by decreasing its water permeability and can thus not be described as an inert barrier membrane. Sparr and Wennerström (8) previously pointed out a mechanism for this responsive behavior of ...

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Section: Discussionsupporting

confidence: 75%

Section: Resultsmentioning

confidence: 99%

Controlling water evaporation through self-assembly

Roger

Liebi

Heimdal

et al. 2016

Proc. Natl. Acad. Sci. U.S.A.

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“… 36 It is likely that for a multicomponent system, such as lung surfactant, more than one composition gradient builds up in the interfacial multilayer film, as previously predicted for ternary systems. 53 In such situations, the lipid distribution within the multilayer film will not be uniform, and the outer layer of the film can then be enriched in certain lipid components. This nonuniform distribution of lipid components can be explained by differences in diffusional properties of the various components within the interfacial mesoscopic structures.…”

Section: Resultsmentioning

confidence: 99%

The Impact of Nonequilibrium Conditions in Lung Surfactant: Structure and Composition Gradients in Multilamellar Films

et al. 2018

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The lipid–protein mixture that covers the lung alveoli, lung surfactant, ensures mechanical robustness and controls gas transport during breathing. Lung surfactant is located at an interface between water-rich tissue and humid, but not fully saturated, air. The resulting humidity difference places the lung surfactant film out of thermodynamic equilibrium, which triggers the buildup of a water gradient. Here, we present a millifluidic method to assemble multilamellar interfacial films from vesicular dispersions of a clinical lung surfactant extract used in replacement therapy. Using small-angle X-ray scattering, infrared, Raman, and optical microscopies, we show that the interfacial film consists of several coexisting lamellar phases displaying a substantial variation in water swelling. This complex phase behavior contrasts to observations made under equilibrium conditions. We demonstrate that this disparity stems from additional lipid and protein gradients originating from differences in their transport properties. Supplementing the extract with cholesterol, to levels similar to the endogenous lung surfactant, dispels this complexity. We observed a homogeneous multilayer structure consisting of a single lamellar phase exhibiting negligible variations in swelling in the water gradient. Our results demonstrate the necessity of considering nonequilibrium thermodynamic conditions to study the structure of lung surfactant multilayer films, which is not accessible in bulk or monolayer studies. Our reconstitution methodology also opens avenues for lung surfactant pharmaceuticals and the understanding of composition, structure, and property relationships at biological air–liquid interfaces.

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“…86 (Note that quite different diagrams for other ternary mixtures occur in this Discussion . 87 ) The most famous of the canonical raft diagrams was built from fluorescence imaging and NMR data. 88 Recently, we added X-ray data that modified the boundaries of the three-phase and low cholesterol regions; we also made sure that our diagram conformed to Schreinemakers’ equilibrium thermodynamic rules that have often been violated by previous diagrams.…”

Section: Towards More Complex Membranesmentioning

confidence: 99%

Introductory Lecture: Basic quantities in model biomembranes

2013

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One of the many aspects of membrane biophysics dealt with in this Faraday Discussion regards the material moduli that describe energies at a supramolecular level. This introductory lecture first critically reviews differences in reported numerical values of the bending modulus KC, which is a central property for the biologically important flexibility of membranes. It is speculated that there may be a reason that the shape analysis method tends to give larger values of KC than the micromechanical manipulation method or the more recent X-ray method that agree very well with each other. Another theme of membrane biophysics is the use of simulations to provide exquisite detail of structures and processes. This lecture critically reviews the application of atomic level simulations to the quantitative structure of simple single component lipid bilayers and diagnostics are introduced to evaluate simulations. Another theme of this Faraday Discussion was lateral heterogeneity in biomembranes with many different lipids. Coarse grained simulations and analytical theories promise to synergistically enhance experimental studies when their interaction parameters are tuned to agree with experimental data, such as the slopes of experimental tie lines in ternary phase diagrams. Finally, attention is called to contributions that add relevant biological molecules to bilayers and to contributions that study the exciting shape changes and different non-bilayer structures with different lipids.

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Lipid phase behaviour under steady state conditions

Cited by 8 publications

References 40 publications

Controlling water evaporation through self-assembly

Controlling water evaporation through self-assembly

The Impact of Nonequilibrium Conditions in Lung Surfactant: Structure and Composition Gradients in Multilamellar Films

Introductory Lecture: Basic quantities in model biomembranes

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