Phase Behavior of Binary Mixtures of SOPC and Cholesterol.

The major components of cell membranes are phospholipids. Due to their amphiphilic structure, in a solution they arrange in bilayers. The phase behavior of lipid bilayers and thus their functions, such as cellular organization, cellular transport, membrane fusion, drug delivery, and others, are susceptible to temperature changes and/or admixing with biologically active materials. A typical representative of phospholipids is SOPC (1-stearoyl-2-oleoyl--glycero-3-phosphocholine) possessing a saturated hydrocarbon acyl chain and an unsaturated one, exhibiting a single cis double bond. Cholesterol plays a primordial role in maintaining the mechanical stability of the cell membrane. We present a comprehensive review of the phase behavior of the binary mixture of SOPC with cholesterol as a function of the temperature explored via atomistic molecular dynamics and Slipids force field in the vicinity of the experimental melting point, = 279 K, corresponding to the temperature driven phase transition from the gel phase (L) to the liquid disordered phase (L). The behavior of the thermodynamic properties and structural characteristics with different concentrations of cholesterol show that the pure SOPC bilayer and its counterpart mixed with 10 mol % cholesterol exhibit phase transitions at 277 and 276 K, respectively, and that cholesterol reinforces the fluidity of the bilayer leading to the emergence of a liquid ordered phase (L). At cholesterol contents larger than 30 mol %, the bilayer exhibits a liquid ordered phase (L) at any temperature. The ensuing phase diagram is found to reproduce reasonably well its counterpart constructed experimentally.