Abstract
The condensing effect and the ability of cholesterol (CHOL) to induce ordering in lipid films is a question of relevance in biological membranes such as the milk fat globule membrane (MFGM) in which the amount of CHOL influences the phase separation and mechanical resistance to rupture of coexisting phases relevant to emulsified food systems. Here, we study the effect of different salts (NaCl, CaCl2, MgCl2, LaCl3) on monolayers made of a model mixture of lipids (DPPC:DPPS 4:1) and CHOL. To this end, we apply Langmuir Film Balance to report a combined analysis of surface pressure-area (π-A) and surface potential-area (ΔV–A) isotherms along with Micro-Brewster Angle Microscopy (Micro-BAM) images of the monolayers in the presence of the different electrolytes. We show that the condensation of lipid by CHOL depends strongly on the nature of the ions by altering the shape and features of the π-A isotherms. ΔV–A isotherms provide further detail on the ion specific interactions with CHOL. Our results show that the condensation of lipids in the presence of CHOL depends on the combined action of ions and CHOL, which can alter the physical state of the monolayer.
Highlights
Cholesterol (CHOL) is an important component in cell membranes of most vertebrates, being an essential component of brain and nerve cells and of bile, which helps the body absorb fats and fat-soluble vitamins [1]
Lipid monolayers made of mixtures of lipids DPPC:DPPS:CHOL (Table 1) were formed at the air–water interface in a Langmuir Film Balance equipped with paper Wilhelmy plate surface pressure measuring system (KSV)
Surface pressure-area (π-A) isotherm of a lipid monolayer provides a fundamental characterization of the properties of the interfacial film including information on the lateral interactions and the surface
Summary
Cholesterol (CHOL) is an important component in cell membranes of most vertebrates, being an essential component of brain and nerve cells and of bile, which helps the body absorb fats and fat-soluble vitamins [1]. CHOL participates in several membrane trafficking and transmembrane signaling processes [2,3] and plays an important role in the cellular processes such as endocytosis and exocytosis [4]. The fundamental functions of the CHOL include cellular processes by interacting with other lipids and proteins in the membrane. In contrast to phospholipids (formed by a larger hydrophilic head and a longer, flexible hydrocarbon tails), CHOL presents a peculiar structure. CHOL contains a short, thermally flexible hydrocarbon tail with a rigid hydrophobic structure containing four hydrocarbon rings, which is attached to a small polar headgroup consisting of only one hydroxyl group
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