Cholesterol superlattice model is compatible with the calorimetric behavior of cholesterol/phosphatidylcholine bilayers

Previous studies have demonstrated that addition of cholesterol to bilayers of saturated phosphatidylcholine (PC) has dramatic effects on the gel-to-liquid phase-transition characteristics of the bilayer. In particular, the enthalpy of the original sharp transition diminishes strongly with increasing cholesterol concentration and a new, broad transition component appears. The enthalpy of the sharp component becomes essentially zero close to the cholesterol mole fraction of 0.25, while the enthalpy of the broad component achieves a maximum near this mole fraction. The broad component typically becomes undetectable when the cholesterol mole fraction is increased to 0.5. In this study we show that these previous experimental findings are fully compatible with a simple model proposing that (i) the cholesterol molecules tend to adopt a regular, superlattice-like lateral distribution within the PC matrix and (ii) the contribution of an acyl chain to the sharp and broad components is critically dependent on its distance from the closest cholesterol molecule. Furthermore, the parameters for PC chain length dependency of cholesterol interactions given by the model are intuitively feasible and in accordance with previous spectroscopic and modeling studies. These findings provide strong support for the cholesterol superlattice model, so far largely based on data obtained with potentially perturbing probes.