Mechanical aspects of membrane thermodynamics. Estimation of the mechanical properties of lipid membranes close to the chain melting transition from calorimetry.

Changes in the internal energy of lipids with temperature are related to both lipid volume and area changes. Close to the chain melting transition of lipid bilayers volume and enthalpy fluctuations generally follow proportional functions. This makes it possible to calculate the relationship between membrane excess heat capacity with lipid volume, area compressibility and the membrane bending modulus, if the area fluctuations of the two monolayers are assumed to be mainly decoupled. Thus, compressibility and elasticity display pronounced maxima at the chain melting transition. These maxima can also be related to pronounced minima of the sound velocity in the lipid transition range, which were found in ultrasonic experiments. In the present study heat capacity profiles and volume changes were obtained. The compressibilities and the bending modulus were then deduced from the specific heat. The relevance of these findings for structural transitions and for the curvature dependence of heat capacities is discussed.