X-ray diffraction and calorimetric study of N-lignoceryl sphingomyelin membranes.

Differential scanning calorimetry and x-ray diffraction have been used to investigate hydrated multibilayers of N-lignoceryl sphingomyelin (C24:0-SM) in the hydration range 0-75 wt % H2O. Anhydrous C24:0-SM exhibits a single endothermic transition at 81.3 degrees C (delta H = 3.6 kcal/mol). At low hydration (12.1 wt % H2O), three different endothermic transitions are observed: low-temperature transition (T1) at 39.4 degrees C (transition enthalpy (delta H1) = 2.8 kcal/mol), intermediate-temperature transition (T2) at 45.5 degrees C, and high-temperature transition (T3) at 51.3 degrees C (combined transition enthalpy (delta H2 + 3) = 5.03 kcal/mol). On increasing hydration, all three transition temperatures of C24:0-SM decrease slightly to reach limiting values of 36.7 degrees C (T1), 44.4 degrees C (T2), and 48.4 degrees C (T3) at approximately 20 wt % H2O. At 22 degrees C (below T1), x-ray diffraction of C24:0-SM at different hydration levels shows two wide-angle reflections, a sharp one at 1/4.2 A-1 and a more diffuse one at 1/4.0 A-1 together with lamellar reflections corresponding to bilayer periodicities increasing from d = 65.4 A to a limiting value of 71.1 A. Electron density profiles show a constant bilayer thickness dp-p approximately 50 A. In contrast, at 40 degrees C (between T1 and T2) a single sharp wide-angle reflection at approximately 1/4.2 A-1 is observed. The lamellar reflections correspond to a larger bilayer periodicity (increasing from d = 69.3-80.2 A) and there is some increase in dp-p (52-56 A) with hydration.(ABSTRACT TRUNCATED AT 250 WORDS)