Complex roles of hybrid lipids in the composition, order, and size of lipid membrane domains.

Hybrid lipids (HL) are phospholipids with one saturated chain and one unsaturated chain. HL are hypothesized to act as linactants (i.e., 2D surfactants) in cell membranes, reducing line tension and creating nanoscopic lipid domains. Here we compare three hybrid lipids of different chain unsaturation (16:0-18:1PC (POPC), 16:0-18:2PC (PLPC), and 16:0-20:4PC (PAPC)) in their abilities to alter the composition, line tension, order, and compactness of lipid domains. We found that the liquid-ordered (Lo) and liquid-disordered (Ld) lipid domains in PAPC/di18:0PC(DSPC)/cholesterol and PLPC/DSPC/cholesterol mixtures are micrometer-sized, and only the POPC/DSPC/cholesterol system has nanoscopic domains. The results indicate that some HLs with polyunsaturated chains are not linactants, and the monounsaturated POPC displays both properties of weak linactants and "Ld-phase" lipids such as di18:1PC (DOPC). The obtained phase boundaries from giant unilamellar vesicles (GUV) show that both POPC and PLPC partition well in the Lo phases. Our MD simulations reveal that these hybrid lipids decrease the order and compactness of Lo domains. Thus, hybrid lipids distinguish themselves from other lipid groups in this combined "partitioning and loosening" ability, which could explain why the Lo domains of GUVs, which often do not contain HL, are more compact than the raft domains in cell membranes. Our line tension measurement and Monte Carlo simulation both show that even the monounsaturated POPC is a weak linactant with only modest ability to occupy domain boundaries and reduce line tension. A more important property of HLs is that they can reduce physical property differences of Lo and Ld bulk domains, which also reduces line tension at domain boundaries.

Model membrane thermodynamics and lateral distribution of cholesterol: from experimental data to Monte Carlo simulation.

Thermodynamic analysis and Monte Carlo simulation techniques were used to study cholesterol-lipid interactions in lipid membranes. Experimental data, including the maximum solubility of cholesterol in lipid bilayers, the 1-to-1 displacement of cholesterol by ceramide, and the cholesterol chemical activity with cholesterol oxidase (COD), were systematically analyzed using thermodynamic principles. A conceptual model, the umbrella model, is presented to describe the key cholesterol-lipid interaction in lipid membranes. In a lipid membrane, nonpolar cholesterol relies on polar phospholipid headgroup coverage to avoid the unfavorable free energy of cholesterol contact with water. This coverage requirement leads to cholesterol's strong tendency not to clustering in a bilayer, its preferential association with large headgroup lipids with saturated acyl chains, and its competition with ceramide for large headgroup lipids. The umbrella model was parameterized into a form of multibody (i.e., nonpairwise) interaction for Monte Carlo simulation, and the measured chemical potentials of cholesterol agreed favorably with the predictions from the simulation. Under the right conditions, the multibody interactions can also lead to the formation of cholesterol superlattices. Also, an intrinsic thermodynamic connection between a jump in chemical potential and a regular distribution (RD) of membrane molecules was uncovered. This study shows that combining thermodynamics with computer simulation can be a productive approach for analyzing and interpreting complex experimental data, and thermodynamics can yield a predicting power in bioscience research.

Assess the nature of cholesterol-lipid interactions through the chemical potential of cholesterol in phosphatidylcholine bilayers.

Cholesterol plays a vital role in determining the physiochemical properties of cell membranes. However, the detailed nature of cholesterol-lipid interactions is a subject of ongoing debate. Existing conceptual models, including the Condensed Complex Model, the Superlattice Model, and the Umbrella Model, identify different molecular mechanisms as the key to cholesterol-lipid interactions. In this work, the compositional dependence of the chemical potential of cholesterol in cholesterol/phosphatidylcholine mixtures was systematically measured at high resolution at 37 degrees C by using an improved cholesterol oxidase (COD) activity assay. The chemical potential of cholesterol was found to be much higher in di18:1-PC bilayers than in di16:0-PC bilayers, indicating a more favorable interaction between cholesterol and saturated chains. More significantly, in 16:0,18:1-PC and di18:1-PC bilayers, the COD initial-reaction rate displays a series of distinct jumps near the cholesterol mole fractions (chi(C)) of 0.15, 0.25, 0.40, 0.50, and 0.57 and a peak at the cholesterol maximum solubility limit of 0.67. These jumps have been identified as the thermodynamic signatures of stable cholesterol regular distributions. In contrast, no such jumps were evident in di16:0-PC bilayers below chi(C) of 0.57. The observed chemical potential profile is in excellent agreement with previous Monte Carlo simulations based on the Umbrella Model but not with the predictions from the other models. The data further indicate that the cholesterol regular distribution domains (superlattices) are not the hypothesized condensed complexes. Those complexes were mainly implicated from studies on lipid monolayer that may not be relevant to the lipid bilayer in cell membranes.

Exploration of molecular interactions in cholesterol superlattices: effect of multibody interactions.

Experimental evidences have indicated that cholesterol may adapt highly regular lateral distributions (i.e., superlattices) in a phospholipid bilayer. We investigated the formations of superlattices at cholesterol mole fraction of 0.154, 0.25, 0.40, and 0.5 using Monte Carlo simulation. We found that in general, conventional pairwise-additive interactions cannot produce superlattices. Instead, a multibody (nonpairwise) interaction is required. Cholesterol superlattice formation reveals that although the overall interaction between cholesterol and phospholipids is favorable, it contains two large opposing components: an interaction favoring cholesterol-phospholipid mixing and an unfavorable acyl chain multibody interaction that increases nonlinearly with the number of cholesterol contacts. The magnitudes of interactions are in the order of kT. The physical origins of these interactions can be explained by our umbrella model. They most likely come from the requirement for polar phospholipid headgroups to cover the nonpolar cholesterol to avoid the exposure of cholesterol to water and from the sharp decreasing of acyl chain conformation entropy due to cholesterol contact. This study together with our previous work demonstrate that the driving force of cholesterol-phospholipid mixing is a hydrophobic interaction, and multibody interactions dominate others over a wide range of cholesterol concentration.