Abstract
Diverse variations in membrane properties are observed in binary phosphatidylcholine/cholesterol mixtures. These mixtures are nonideal, displaying single or phase coexistence, depending on chemical composition and other thermodynamic parameters. When compared with pure phospholipid bilayers, there are changes in water permeability, bilayer thickness and thermomechanical properties, molecular packing and conformational freedom of phospholipid acyl chains, in internal dipolar potential and in lipid lateral diffusion. Based on the phase diagrams for DMPC/cholesterol and DPPC/cholesterol, we compare the equivalent polarity of pure bilayers with specific compositions of these mixtures, by using the Py empirical scale of polarity. Besides the contrast between pure and mixed lipid bilayers, we find that liquid-ordered (ℓ o) and liquid-disordered (ℓ d) phases display significantly different polarities. Moreover, in the ℓ o phase, the polarities of bilayers and their thermal dependences vary with the chemical composition, showing noteworthy differences for cholesterol proportions at 35, 40, and 45 mol%. At 20 °C, for DMPC/cholesterol at 35 and 45 mol%, the equivalent dielectric constants are 21.8 and 23.8, respectively. Additionally, we illustrate potential implications of polarity in various membrane-based processes and reactions, proposing that for cholesterol containing bilayers, it may also go along with the occurrence of lateral heterogeneity in biological membranes.
Highlights
Hydrated pure phospholipid bilayers exist almost in either a highly ordered gel phase (Lβ or so) or a liquid–crystalline (Lα) depending on whether the temperature is below or above the main phase transition temperature (Tm)
Given that pyrene is essentially insoluble in water, but fairly soluble in aliphatic hydrocarbons, it is readily incorporated into the hydrophobic acyl chain region close to the glycerol moiety and first methylenic groups in phospholipid bilayers [49,50,51,52], probing this way the average polarity corresponding to its location inside the bilayer
Our studies demonstrate variations in the equivalent polarity as a function of cholesterol proportion in the lipidic mixture, in the more ordered region of the methylenic palisade, since this is the observable location of pyrene inserted parallel to the ordered portion of acyl chains, from direct NMR and molecular dynamics findings [49,51,52] and by means of indirect arguments from two-dimensional kinetics analysis [50]
Summary
Hydrated pure phospholipid bilayers exist almost in either a highly ordered gel phase (Lβ or so) or a liquid–crystalline (Lα) depending on whether the temperature is below or above the main phase transition temperature (Tm). There is a large diversity of fluid lipid bilayer properties that change upon mixing cholesterol into pure phospholipid membranes, such as reduction of water permeability [16], reduction by a factor of about 2–3 in the lipid lateral diffusion [17,18], higher conformational ordering of aliphatic chains of phospholipids [3,19], which influences the modulation of the lateral pressure in depth–manner [20] and thermomechanical and elasticity properties [21,22], as well as increase in bilayer thickness [23], and a general increase in the internal dipole potential [24] Much of these observations are founded essentially in the condensing effect of cholesterol on phosphatidylcholines, and a phosphatidylcholine/cholesterol mixture occupies a smaller area than that expected from the sum of the molecular areas of both constituents [15]. These authors established the steepest sigmoidal-like transmembrane polarity profile upon the addition of cholesterol to phospholipid bilayers, due to augmentation of polarity in the interface and ordered methylene portion, in parallel with a decrease of polarity in the central section
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