Calorimetric analysis of lipid-sterol systems: a comparison between structurally similar cholesterol and vitamin D3 interacting with phospholipid bilayers of different thickness

We compared the thermotropic properties (transition temperatures and associated enthalpies) of phosphatidylcholine (PC) bilayers of varying chain length containing either cholesterol or vitamin D-3. The two sterols are structurally similar but behave in rather different ways. Both transition temperature and associated enthalpy decrease on raising the amount of sterol dissolved in the lipid matrix; the decrease, however, depends on the PC's chain length and is markedly different for cholesterol and vitamin D-3. These differences have been rationalized on the basis of a simple theoretical model by employing a mean field statistical thermodynamics approach. The model has been developed in a perturbation scheme by considering cholesterol as reference molecule and vitamin D-3 as a slightly modified cholesterol. Taking together experimental and theoretical results, we infer that the different thermotropic behavior of the two sterols is mainly due to (a) weaker PC-vitamin D-3 pair interactions as compared with PC-cholesterol and (b) greater ability of cholesterol to reduce the co-operative melting of PC lipid chains.