Metastable rippled gel phase in saturated phosphatidylcholines: calorimetric and densitometric characterization

Abstract

A long-living metastable rippled phase P β′ mst has been earlier reported to form in aqueous dispersions of dipalmitoylphosphatidylcholine (DPPC) upon cooling from the lamellar liquid crystalline phase L α (Tenchov et al. (1989) Biophys. J. 56, 757–768). Here we demonstrate that similar metastable phases form also in distearoylphosphatidylcholine (DSPC) and dihexadecylphosphatidylcholine (DHPC) but not in dimyristoylphosphatidylcholine (DMPC). The thermodynamic parameters of P β′ mst in DPPC, DSPC and DHPC have been characterized in detail by means of differential scanning calorimetry and scanning densitometry. It is shown that the P β′ mst phase in these three lipids has higher specific heat capacity by 0.1–0.4 kcal K −1 mol −1 and higher specific volume by 1–2 μl/g than the equilibrium P β′ phase formed upon heating from the L β′ phase. The P β′ mst → L α transition in these three lipids takes place at 0.06–0.14°C lower temperature than the P β′ → L α transition. Its enthalpy is lower by 5–11% and the apparent maximum specific heat C p max is lower by 11–14%. The P β′ mst phase is a long-living phase — it does not relax into the equilibrium P β′ for at least several hours. The replacement of the ester glycerol-hydrocarbon chain linkages in DPPC with ether bonds in DHPC does not influence the formation of P β′ mst.