Sphingomyelin is a major constituent of most eukaryotic cell plasma membranes. During apoptosis, sphingomyelin is converted into ceramide. It is hypothesized that this conversion induces a structural change in membranes that leads to downstream signaling. Deuterium nuclear magnetic resonance spectroscopy is used to create a partial phase diagram of multilamellar aqueous dispersions of palmitoyl sphingomyelin and ceramide in excess water to characterize the structural changes associated with increased ceramide content (0--40 mol ceramide) and varying temperature (25--80degD). The two lipids are fully miscible at high temperatures and at 40 mol ceramide. A variety of solid-liquid coexistence phase behavior is observed at lower concentrations. A gel phase is observed at progessively higher temperatures in the sphingomyelin:ceramide membranes as ceramide content increase. This implies that at physiological temperatures, ceramide may increase the gel phase propensity of cell membranes
