Hydration of DMPC and DPPC at 4°C produces a novel subgel phase with convex-concave bilayer curvatures

Hydration of dimyristoyl- and dipalmitoylphosphatidylcholines at 4 degrees C results in the formation of a characteristic subgel phase designated P-cc. Examination of the phase by freeze-fracture electron microscopy shows convex-concave deformations of the planar bilayer which are of two types. A smaller type with a radius of curvature of about 20 nm predominates in DMPC, and a larger type with about 70 nm radii of curvatures dominates in DPPC. The P-cc phase can also be formed in samples hydrated at temperatures above the main phase transition if the dispersion is frozen slowly and subsequently incubated at 4 degrees C for several days. The subgel P-cc phase was distinguished from the subgel L-c phase by the temperature of transition, packing of the acyl chains on the basis of wide-angle X-ray diffraction, and H-2-NMR spectra characteristic of a 'solid-ordered' phase. Vibrational spectra of the carbonyl and phosphate regions are consistent with a partially reduced hydration state. The origin of the convex-concave bilayer deformation is believed to result from constraints imposed by limiting hydration of the headgroup and a frustration arising from the spontaneous curvature of both monolayers. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.