Small-angle scattering studies of the fully hydrated phospholipid DPPC

Small-angle neutron and x-ray scattering studies have been carried out on f ully hydrated dipalmitoylphosphatidylcholine (DPPC) multilamellar vesicles. This system is known to exhibit two distinct ripple (Pp,) phases, which de pend on sample history, at temperatures intermediate to its high-temperatur e liquid crystalline (L-alpha), phase, and its low-temperature gel (L-beta' ), phase. On cooling from the L-alpha phase, the P-beta' phase displays a c omplex multipeak diffraction pattern that differs significantly from the di ffraction pattern seen in the P-beta' phase obtained on warming from the L- beta' phase. Examining the P-beta' phase on cooling using small-angle neutr on scattering and x-ray diffraction techniques leads to the conclusion that this phase is characterized by a long wavelength ripple (lambda(r)similar to 330 Angstrom) and a highly monoclinic unit cell (gamma similar to 125 de grees). As the P-beta' phase is traversed in temperature, the ripple wavele ngth changes significantly while the monoclinicity remains unchanged. Rippl es from the P-beta' phase are seen to persist into the Lp, phase on cooling , leading to increased small-angle scattering characteristic of a disordere d stacking of the lamellae. [S1063-651X(99)11101-2].