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 bo th lipid volume and area changes. Close to the chain melting transition of lipid bilayers volume and enthalpy fluctuations generally follow proportion al functions. This makes it possible to calculate the relationship between membrane excess heat capacity with lipid volume, area compressibility and t he membrane bending modulus, if the area fluctuations of the two monolayers are assumed to be mainly decoupled. Thus, compressibility and elasticity d isplay pronounced maxima at the chain melting transition. These maxima can also be related to pronounced minima of the sound velocity in the lipid tra nsition range, which were found in ultrasonic experiments. In the present s tudy heat capacity profiles and volume changes were obtained. The compressi bilities and the bending modulus were then deduced from the specific heat. The relevance of these findings for structural transitions and for the curv ature dependence of heat capacities is discussed. (C) 1998 Elsevier Science B.V. All rights reserved.