STRUCTURE AND THERMOTROPIC PROPERTIES OF 1-STEAROYL-2-ACETYL-PHOSPHATIDYLCHOLINE BILAYER-MEMBRANES

The structural and thermotropic properties of 1-stearoyl-2-acetyl-phos phatidylcholine (C(18):C(2)-PC) were studied as a function of hydratio n. A combination of differential scanning calorimetry and x-ray diffra ction techniques have been used to investigate the phase behavior of C (18):C(2)-PC, At low hydration (e.g., 20% H2O), the differential scann ing calorimetry heating curve shows a single reversible endothermic tr ansition at 44.6 degrees C with transition enthalpy Delta H = 6.4 kcal /mol. The x-ray diffraction pattern at -8 degrees C shows a lamellar s tructure with a small bilayer periodicity d = 46.3 Angstrom A and two wide angle reflections at 4.3 and 3.95 Angstrom A, characteristic of a tilted chain, L beta' bilayer gel structure. Above the main transitio n temperature, a liquid crystalline Let phase is observed with d = 53. 3 Angstrom A. Electron density profiles at 20% hydration suggest that C(18):C(2)-PC forms a fully interdigitated bilayer at -8 degrees C and a noninterdigitated, liquid crystalline phase above its transition te mperature (T > T-m). Between 30 and 50% hydration, on heating C(18):C( 2)-PC converts from a highly ordered, fully interdigitated gel phase ( L beta') to a less ordered, interdigitated gel phase (L beta), which o n further heating converts to a noninterdigitated liquid crystalline L alpha phase. However, the fully hydrated (>60% H2O) C(18):C(2)-PC, af ter incubation at 0 degrees C, displays three endothermic transitions at 8.9 degrees C (transition I, Delta H = 1.6 kcal/mol), 18.0 degrees C (transition II), and 20.1 degrees C (transition III, Delta H-II+III = 4.8 kcal/mol). X-ray diffraction at -8 degrees C again showed a lame llar gel phase (L beta') with a small periodicity d = 52.3 Angstrom A. At 14 degrees C a less ordered, lamellar gel phase (LP) is observed w ith d = 60.5 Angstrom A. However, above the transition III, a broad, d iffuse reflection is observed at approximate to 39 A, consistent with the presence of a micellar phase. The following scheme is proposed for structural changes of fully hydrated C(18):C(2)-PC, occurring with te mperature: L beta' (interdigitated) --> L beta (interdigitated) --> L alpha (noninterdigitated) --> Micelles. Thus, at low temperature C(18) :C(2)-PC forms a bilayer gel phase (L beta') at all hydrations, wherea s above the main transition temperature it forms a bilayer liquid crys talline phase L alpha at low hydrations and a micellar phase at high h ydrations (>60 wt% water).