Volume behaviour of dipalmitoylphosphatidylcholine bilayer membrane: Pressure perturbation calorimetry and densitometry

Abstract

Pressure perturbation calorimetry (PPC) is a relatively new technique based on a commercially available calorimeter which enables us to determine the isobaric coefficient of the thermal expansion of a solute for aqueous polymer solutions and colloidal dispersions. We applied the PPC to the investigation on the volume behaviour of dipalmitoylphosphatidylcholine (DPPC) bilayer membrane and compared the obtained results of the expansivity α2 and the apparent molar volume ϕ2 of DPPC in its bilayer membrane with those from a conventional densitometry. The temperature dependence of ϕ2 from the PPC was in good agreement with that from the densitometry. Both techniques revealed that the α2 value changes with the phase state of the DPPC bilayer membrane in the order of ripple gel (Pβ') phase > liquid crystalline (Lα) phase > lamellar gel (Lβ') phase. From the dependence, also the change of ϕ2 at the main transition from the Pβ' phase to the Lα phase was estimated to be 22.7 ± 2.62 cm3 mol−1 from the PPC and at 24.0 cm3 mol-1 from the densitometry, which are comparable with 25.4 cm3mol−1 previously obtained by the calculation based on the Clapeyron equation. On the other hand, the change of ϕ2 at the pretransition from the Lβ' phase to the Pβ' phase could not be determined with sufficient reliability. This may be due to the small change in ϕ2 at the pretransition (less than 0.5% of ϕ2 at the temperature).