Volumetric characterization of ester- and ether-linked lipid bilayers by pressure perturbation calorimetry and densitometry

We investigated the thermotropic volume behavior of dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC) and dihexadecylphosphatidylcholine (DHPC) membranes using pressure perturbation calorimetry (PPC) and densitometry. The ln ϕ2 vs temperature curves (ϕ2: apparent molar volume of phospholipid) obtained from the PPC data using an analysis method that we developed agreed with the results from the density measurements for these lipids within the relative difference of about 0.62%. From those curves, the volume changes with the main transition were estimated at 18.0 ± 0.49, 23.5 ± 2.33 and 23.0 ± 0.33 cm3 mol−1 for DMPC, DPPC and DHPC, respectively. For DPPC and DMPC, the average volume per methylene group of the hydrocarbon chains v CH 2 calculated by referring to the procedure by Nagle and Wilkinson was consistent with the previous result, which indicates that the DPPC bilayer in the gel state has denser hydrophobic bilayer core than the DMPC bilayer. For DHPC, the volume of the headgroup region vH was calculated to be 244 Å3 by assuming that v CH 2 of DHPC equals that of DPPC above 45 °C. This value was comparable to that of DPPC when the volume of the carbonyl groups was considered, which may signify that there is no significant conformational difference in the polar headgroups of both phospholipids. However, it was suggested from the consideration on vH of DHPC at 20 °C that expansion of the headgroup region should occur as the interdigitated structure is formed, which means some conformational change of the headgroup region is induced by the interdigitation.