Small-angle scattering studies of the fully hydrated phospholipid DPPC

Small-angle neutron and x-ray scattering studies have been carried out on fully hydrated dipalmitoylphosphatidylcholine (DPPC) multilamellar vesicles. This system is known to exhibit two distinct ripple (Pβ′) phases, which depend on sample history, at temperatures intermediate to its high-temperature liquid crystalline (Lα), phase, and its low-temperature gel (Lβ′), phase. On cooling from the Lα phase, the Pβ′ phase displays a complex multipeak diffraction pattern that differs significantly from the diffraction pattern seen in the Pβ′ phase obtained on warming from the Lβ′ phase. Examining the Pβ′ phase on cooling using small-angle neutron scattering and x-ray diffraction techniques leads to the conclusion that this phase is characterized by a long wavelength ripple (λr∼330 Å) and a highly monoclinic unit cell (γ∼125°). As the Pβ′ phase is traversed in temperature, the ripple wavelength changes significantly while the monoclinicity remains unchanged. Ripples from the Pβ′ phase are seen to persist into the Lβ′ phase on cooling, leading to increased small-angle scattering characteristic of a disordered stacking of the lamellae.