METASTABLE RIPPLED GEL PHASE IN SATURATED PHOSPHATIDYLCHOLINES - CALORIMETRIC AND DENSITOMETRIC CHARACTERIZATION

A long-living metastable rippled phase P-beta'(mst) has been earlier r eported to form in aqueous dispersions of dipalmitoylphosphatidylcholi ne (DPPC) upon cooling from the lamellar liquid crystalline phase L(al pha) (Tenchov et al. (1989) Biophys. J. 56, 757-768). Here we demonstr ate that similar metastable phases form also in distearoylphosphatidyl choline (DSPC) and dihexadecylphosphatidylcholine (DHPC) but not in di myristoylphosphatidylcholine (DMPC). The thermodynamic parameters of P -beta'(mst) in DPPC, DSPC and DHPC have been characterized in detail b y means of differential scanning calorimetry and scanning densitometry . It is shown that the P-beta'(mst) phase in these three lipids has hi gher specific heat capacity by 0.1-0.4 kcal K-1 mol(-1) and higher spe cific Volume by 1-2 mu l/g than the equilibrium P-beta' phase formed u pon heating from the L(beta') phase. The P-beta'(mst)-->L(alpha) trans ition in these three lipids takes place at 0.06-0.14 degrees C lower t emperature than the P-beta'-->L(alpha) transition. Its enthalpy is low er by 5-11% and the apparent maximum specific heat C-p(max) is lower b y 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 rep lacement of the ester glycerol-hydrocarbon chain linkages in DPPC with ether bonds in DHPC does not influence the formation of P-beta'(mst).