Enthalpy and volume changes in lipid membranes. I. The proportionality of heat and volume changes in the lipid melting transition and its implicationfor the elastic constants

Differential scanning calorimetry, pressure calorimetry, and densitometry h ave been employed to study the relation between volume and enthalpy changes in the melting regime of lipid membranes. We demonstrate a rigid proportio nal relation between volume expansion coefficient and heat capacity. This r esult is first shown in densitometric experiments. It implies that calorime tric profiles obey a simple scaling law for the temperature axes in experim ents with applied hydrostatic pressure. In a theoretical paper (Heimburg, T . Biochim. Biophys. Acta 1998, 1415, 147-162), we have argued that this rel ation has far-reaching consequences for the predictiblity of elastic consta nts from the heat capacity. The proportionality constant between volume and enthalpy changes is found to be independent of the lipid, which has intere sting consequences for the calculation of the elastic constants of biologic al lipid mixtures with unknown composition. We demonstrate this for lung su rfactant, which displays a similar relation between volume and enthalpy cha nges.