Abstract
One of the main known effects of cholesterol is to rigidify the cell membrane throughout the so-called condensing effect. Although many studies have been done in mixtures of cholesterol with different membrane lipids, there are not many studies in a wide concentration range of cholesterol or at physiological conditions. In this work, we studied mixtures of DMPC/Cholesterol monolayers to determine the effect of cholesterol, from very low to physiological concentrations and two pHs. We use a Langmuir balance and Brewster angle microscopy to study their thermodynamic behavior at 37.0 ± 0.1°C at the air/solution interface. From the analysis of the (π−A) isotherms, we determined the excess area and the compressibility elastic modulus to determine the monolayers mechanical properties. Surprisingly, we found three main effects of cholesterol: The first one is a fluidization effect of the monolayer at all cholesterol concentrations. The second effect is the so-called condensing effect that appears due to the non-ideality of the mixture. The third effect is a stiffness of the monolayer as the cholesterol concentration increases. These effects are stronger in pure water, pH ≈ 6.6, than on buffer at physiological pH = 7.4. We also found that all mixtures are thermodynamically stable at all concentrations at a surface pressure of 30.1 ± 1.6 and 27.4 ± 3.2 mN/m in pure water and buffer, respectively. Furthermore, we compared this stability with a fatty acid monolayer that shows a much lower surface pressure equilibrium value that DMPC or its mixtures with cholesterol, indicating a possibly reason why double chain lipids are better than single chain lipids to made up the cell membrane.
Highlights
Cholesterol is a very important component in all membranes of mammalian cells and it is critical to human health: It is known that cholesterol is responsible for the modulation of physical properties of cell membranes, because the bulky molecular structure of cholesterol interferes with the movement of the phospholipid tails [1]
This difference implies that cholesterol interacts strongly with the liquid expanded (LE) DMPC phase, disrupting its formation and making the gas phase to disappear at much lower areas per molecule
At the higher concentration of cholesterol in the mixture, we notice that the gas phase disappeared at about 5–8 mN/m and at lower cholesterol concentrations the gas phase disappear at even lower surface pressure; we are taking the take-off pressure area as a reference for this case
Summary
Cholesterol is a very important component in all membranes of mammalian cells and it is critical to human health: It is known that cholesterol is responsible for the modulation of physical properties of cell membranes, because the bulky molecular structure of cholesterol interferes with the movement of the phospholipid tails [1]. Other effects of adding cholesterol include changes in the lipid molecule cross sectional area, the thickness of the bilayer, the orientational order of the lipids and the motion of the hydrocarbon chains [1, 3, 5]. Cholesterol mixed with phospholipids can form oligomeric chemical complexes with a fundamental stoichiometry 3:2 and 6:1 phospholipids per cholesterol molecule [8, 9] The formation of these phospholipid/cholesterol complexes produce the socalled cholesterol condensing effect [4, 7, 10, 11] where the area occupied by the molecules is decreased. As stated by the umbrella model, the lipid acyl chains and the nonpolar cholesterol part become densely packed as they share the limited space below the phospholipid head groups. The solubility limit for cholesterol in phosphatidilcholines (PC) bilayers is known to be around 66% [7, 10, 12]
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