Chemical potential measurements constrain models of cholesterol-phosphatidylcholine interactions

Bilayer membranes composed of cholesterol and phospholipids exhibit diverse forms of nonideal mixing. In partic-ular, many previous studies document macroscopic liquid-liquid phase separation as well as nanometer-scale heterogeneity in membranes of phosphatidylcholine (PC) lipids and cholesterol. Here, we present experimental measurements of cholesterol chemical potential (mu ,) in binary membranes containing dioleoyl PC (DOPC), 1-palmitoyl-2-oleoyl PC (POPC), or dipalmitoyl PC (DPPC), and in ternary membranes of DOPC and DPPC, referenced to crystalline cholesterol. mu , is the thermodynamic quan-tity that dictates the availability of cholesterol to bind other factors, and notably must be equal between coexisting phases of a phase separated mixture. It is simply related to concentration under conditions of ideal mixing, but is far from ideal for the majority of lipid mixtures investigated here. Measurements of mu , can vary with phospholipid composition by 1.5 kBT at constant cholesterol mole fraction implying a more than fivefold change in its availability for binding receptors and other reactions. Experimental mea-surements are fit to thermodynamic models including cholesterol-DPPC complexes or pairwise interactions between lipid species to provide intuition about the magnitude of interactions. These findings reinforce that mc depends on membrane composition over-all, suggesting avenues for cells to alter the availability of cholesterol without varying cholesterol concentration.SIGNIFICANCE The chemical potential of cholesterol (mu,) can vary with phospholipid composition even at fixed cholesterol concentration. In this report, we connect measurements of mu, in bilayer membranes to the phase behavior of those membranes, focusing on mixtures composed of cholesterol with one or two phospholipids. mu, reflects cholesterol's availability to interact with other molecules raising the possibility that mu, links perturbations of phase behavior in cell membranes to some of their functional consequences.

Automated summary

Bilayer membranes composed of cholesterol and phospholipids exhibit nonideal mixing and the chemical potential of cholesterol can vary with phospholipid composition, affecting its availability for binding and other reactions. These findings suggest that cells can regulate cholesterol availability by altering membrane composition.