Abstract
Sphingolipids are key lipid regulators of cell viability: ceramide is one of the key molecules in inducing programmed cell death (apoptosis), whereas other sphingolipids, such as ceramide 1-phosphate, are mitogenic. The phase behavior of bilayers comprising binary mistures of N-hexadecanoyl-D-erythro-ceramide (C16-ceramide) or N-hexadecanoyl-D-erythro-ceramide-1-phosphate (C16-ceramide-1-phosphate; C16-C1P) with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) were studied using differential scanning calorimetry (DSC) and deuterium nuclear magnetic resonance (2H-NMR). Partial phase diagrams (up to a sphingolipid mole fraction of X=0.40) were constructed for both mixtures. For C16-ceramide-containing bilayers DSC heating scans at Xcer=0.025 showed a complex structure of the main phase transition peak suggestive of lateral phase separation. The transition width increased significantly upon increasing Xcer, and the upper phase boundary temperature of the mixture shifted to ∼65°C at Xcer = 0.40. The temperature range over which 2H-NMR spectra of C16-ceramide/DPPC-d62 mixtures displayed coexistence of gel and liquid crystalline domains increased from ∼10° for Xcer=0.1 to ∼21° for Xcer=0.4. DSC and 2H-NMR observations of C16-C1P/DPPC mixtures at corresponding concentrations indicated that two-phase coexistence was limited to significantly narrower ranges of temperature for mixtures containing C16-C1P compared to those containing C16-ceramide. Work supported by funding from the Sigrid Juselius Foundation (JMH), the Finnish Cultural Foundation (JMH), Evald and Hilda Nissi Foundation (JMH), The Finnish Eye Foundation (JMH), the Academy of Finland (AH, IV, SW), and from the Natural Sciences and Engineering Research Council of Canada (MRM).
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