Metastable Rippled Gel Phase in Saturated Phosphatidylcholines: Calorimetric and Densitometric Characterization

A long-living metastable rippled phase P beta' mst has been earlier reported to form in aqueous dispersions of dipalmitoylphosphatidylcholine (DPPC) upon cooling from the lamellar liquid crystalline phase L alpha (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 beta' 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 beta' 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 microliters/g than the equilibrium P beta' phase formed upon heating from the L beta' phase. The P beta' mst-->L alpha transition in these three lipids takes place at 0.06-0.14 degree C lower temperature than the P beta'-->L alpha transition. Its enthalpy is lower by 5-11% and the apparent maximum specific heat Cp max is lower by 11-14%. The P beta' mst phase is a long-living phase-it does not relax into the equilibrium P beta' 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 beta' mst.