An ordered metastable phase in hydrated phosphatidylethanolamine: the Y-transition

By using time-resolved X-ray diffraction, differential scanning calorimetry and scanning densitometry, we observed rapid formation at low temperature of a metastable ordered phase, termed L-R1 phase, in fully hydrated dihexadecylphosphatidylethanolamine (DHPE). The L-R1 phase has the same lamellar repeat period as the gel L-beta phase but differs from the latter in its more ordered, orthorhombic hydrocarbon chain arrangement. It forms at about 12 degrees C upon cooling and manifests itself as splitting of the sharp, symmetric wide-angle X-ray peak of the DHPE gel phase into two reflections. This transition, designated the 'Y-transition', is readily reversible and proceeds with almost no hysteresis between cooling and heating scans. Calorimetrically, the L-R1 --> L-beta transition is recorded as a low-enthalpy (0.2 kcal/mol) endothermic event. The formation of the L-R1 phase from the gel phase is associated with a small, about 2 mu l/g, decrease of the lipid partial specific volume recorded by scanning densitometry, in agreement with a volume calculation based on the X-ray data. The formation of the equilibrium L-c phase was found to take place from within the L-R1 phase. This appears to be the only observable pathway for crystallisation of DHPE upon low-temperature incubation. Once formed, the L-c phase of this lipid converts directly into L-beta phase at 50 degrees C, skipping the L-R1 phase. Thus, the L-R1 phase of DHPE can only be entered by cooling of the gel L-beta phase. The data disclose certain similarities between the low-temperature polymorphism of DHPE and that of long-chain normal alkanes. (C) 1999 Elsevier Science B.V. All rights reserved.