Sterols regulate biological processes and sustain the lateral structure of cellular membranes. The sterol cholesterol, its precursor lanosterol, the plant sterols stigmasterol and ergosterol as well as 7-dehydrocholesterol were added up to 36 mol-% to vesicles of the phospholipid 1,2-dipalmitoyl-snglycero- 3-phosphatidylcholine (DPPC). The aim of this study was to investigate the influence of the sterol side chain and ring structure on the volumetric properties of the lipid bilayer system by using pressure perturbation calorimetry (PPC), a relatively new and efficient technique, to study the thermal expansion coefficient and volumetric properties of biomolecules. The experiments were carried out in the temperature range from 10 to 85 °C, i. e., at temperatures below and above the chain-melting transition temperatures of the lipid mixtures. Additionally, corresponding differential scanning calorimetric (DSC) measurements were carried out.Whereas the conformational properties of the different sterols have a significant effect on the order parameter of the lipid acyl-chains, the thermodynamic parameters of these sterols are less influenced by the differential structural changes of the sterols. For lanosterol and stigmasterol, marked differences are found, however.
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