An ordered metastable phase in hydrated phosphatidylethanolamine: the Y-transition (original) (raw)

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 L phase but differs from the latter in its more ordered, orthorhombic hydrocarbon chain arrangement. It forms at about 12³C 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 CL L 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 Wl/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 L phase at 50³C, skipping the L R1 phase. Thus, the L R1 phase of DHPE can only be entered by cooling of the gel L L phase. The data disclose certain similarities between the low-temperature polymorphism of DHPE and that of long-chain normal alkanes. ß -ray di¡raction 0005-2736 / 99 / $^see front matter ß 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 5 -2 7 3 6 ( 9 8 ) 0 0 2 5 9 -