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 exh ibits a single endothermic transition at 81.3 degrees C (Delta H = 3.6 kcal/mol). At low hydration (12.1 wt % H2O), three different endother mic transitions are observed: low-temperature transition (T-1) at 39.4 degrees C (transition enthalpy (Delta H-1) = 2.8 kcal/mol), intermedi ate-temperature transition (T-2) al 45.5 degrees C, and high-temperatu re transition (T-3) at 51.3 degrees C (combined transition enthalpy (D elta H-2+3) = 5.03 kcal/mol). On increasing hydration, all three trans ition temperatures of C24:0-SM decrease slightly to reach limiting val ues of 36.7 degrees C (T-1), 44.4 degrees C (T-2), and 48.4 degrees C (T-3) at similar to 20 wt % H2O, At 22 degrees C (below T-1), x-ray di ffraction of C24:0-SM at different hydration levels shows two wide-ang le reflections, a sharp one at 1/4.2 Angstrom(-1) and a more diffuse o ne at 1/4.0 Angstrom(-1) together with lamellar reflections correspond ing to bilayer periodicities increasing from d = 65.4 Angstrom to a li miting value of 71.1 Angstrom. Electron density profiles show a consta nt bilayer thickness d(p-p) similar to 50 Angstrom. In contrast, at 40 degrees C (between T-1 and T-2) a single sharp wide-angle reflection at similar to 1/4.2 Angstrom(-1) is observed. The lamellar reflections correspond to a larger bilayer periodicity (increasing from d = 69.3- 80.2 Angstrom) and there is some increase in d(p-p) (52-56 Angstrom) w ith hydration. These structural parameters, together with calculated l ipid thickness and molecular area considerations, suggest that the low temperature endotherm (T-1) of hydrated C24:0-SM corresponds to a tra nsition from a tilted, gel state (Gel I) with partially interdigitated chains to an untilted, or less tilted, gel state (Gel II). At 60 degr ees C (above T-3), the usual liquid-crystalline L(alpha) bilayer struc ture (d = 59.5-66.3 Angstrom; d(p-p) similar to 46 Angstrom) is presen t at all hydrations. Comparison with the behavior of C18:0-SM indicate s that the inequivalence of length of the sphingosine (C18) and lignoc eryl (C24) chains results in a more complex gel phase polymorphism for C24:0-SM.