Dimyristoylphosphatidylcholine/C16:0-ceramide binary liposomes studied by differential scanning calorimetry and wide- and small-angle X-ray scattering
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Ceramide has recently been established as a central messenger in the signaling cascades controlling cell behavior. Physicochemical studies have revealed a strong tendency of this lipid toward phase separation in mixtures with phosphatidylcholines. The thermal phase behavior and structure of fully hydrated binary membranes composed of dimyristoylphosphatidylcholine (DMPC) and N-palmitoyl-ceramide (C16:0-ceramide, up to a mole fraction X(cer) = 0.35) were resolved in further detail by high-sensitivity differential scanning calorimetry (DSC) and x-ray diffraction. Both methods reveal very strong hysteresis in the thermal phase behavior of ceramide-containing membranes. A partial phase diagram was constructed based on results from a combination of these two methods. DSC heating scans show that with increased X(cer) the pretransition temperature T(p) first increases, whereafter at X(cer) > 0.06 it can no longer be resolved. The main transition enthalpy ΔH remains practically unaltered while its width increases significantly, and the upper phase boundary temperature of the mixture shifts to ~63°C at X(cer) = 0.30. Upon cooling, profound phase separation is evident, and for all of the studied compositions there is an endotherm in the region close to the T(m) for DMPC. At X(cer) ≥ 0.03 a second endotherm is evident at higher temperatures, starting at 32.1°C and reaching 54.6°C at X(cer) = 0.30. X- ray small-angle reflection heating scans reveal a lamellar phase within the temperature range of 15-60°C, regardless of composition. The pretransition is observed up to X(cer) <0.18, Å Å. (from (ripple 0.06 0.35 31 56.5°c) 61 64 67 an at augments coexistence d distance fluid fluid) from gel high in increase increases increasing is l(α) l(β) lamellar observed phase region repeat t(p). temperatures the to together when with x(cer) ~61>0.18,> 0.03. With cooling from temperatures above 50°C we observe a slow increase in d as the coexistence region is entered. A sudden solidification into a metastable, modulated gel phase with high d values is observed for all compositions at ~24°C. The anomalous swelling for up to X(cer) = 0.30 in the transition region is interpreted as an indication of bilayer softening and thermally reduced bending rigidity.
|Number of pages||11|
|Publication status||Published - 1 Jan 2000|