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 ${(P}_{{\beta }^{\prime }})$ phases, which depend on sample history, at temperatures intermediate to its high-temperature liquid crystalline ${(L}_{\alpha }),$ phase, and its low-temperature gel ${(L}_{{\beta }^{\prime }}),$ phase. On cooling from the $L_{\alpha }$ phase, the $P_{{\beta }^{\prime }}$ phase displays a complex multipeak diffraction pattern that differs significantly from the diffraction pattern seen in the $P_{{\beta }^{\prime }}$ phase obtained on warming from the $L_{{\beta }^{\prime }}$ phase. Examining the $P_{{\beta }^{\prime }}$ 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 $\left({\lambda }_r\sim 330\AA \right)$ and a highly monoclinic unit cell (γ∼125°). As the $P_{{\beta }^{\prime }}$ phase is traversed in temperature, the ripple wavelength changes significantly while the monoclinicity remains unchanged. Ripples from the $P_{{\beta }^{\prime }}$ phase are seen to persist into the $L_{{\beta }^{\prime }}$ phase on cooling, leading to increased small-angle scattering characteristic of a disordered stacking of the lamellae.