title: Structure and thermotropic properties of 1-stearoyl-2-acetyl-phosphatidylcholine bilayer membranes.
abstract: The structural and thermotropic properties of 1-stearoyl-2-acetyl-phosphatidylcholine (C(18):C(2)-PC) were studied as a function of hydration. A combination of differential scanning calorimetry and x-ray diffraction techniques have been used to investigate the phase behavior of C(18):C(2)-PC. At low hydration (e.g., 20% H2O), the differential scanning calorimetry heating curve shows a single reversible endothermic transition 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 structure with a small bilayer periodicity d = 46.3 A and two wide angle reflections at 4.3 and 3.95 A, characteristic of a tilted chain, L beta' bilayer gel structure. Above the main transition temperature, a liquid crystalline L alpha phase is observed with d = 53.3 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 temperature (T > Tm). 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 on further heating converts to a noninterdigitated liquid crystalline L alpha phase. However, the fully hydrated (> 60% H2O) C(18):C(2)-PC, after 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 HII+III = 4.8 kcal/mol). X-ray diffraction at -8 degrees C again showed a lamellar gel phase (L beta') with a small periodicity d = 52.3 A. At 14 degrees C a less ordered, lamellar gel phase (L beta) is observed with d = 60.5 A. However, above the transition III, a broad, diffuse reflection is observed at approximately 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 temperature: 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, whereas above the main transition temperature it forms a bilayer liquid crystalline phase L alpha at low hydrations and a micellar phase at high hydrations (> 60 wt% water).
authors.affiliation: Department of Biophysics, Boston University School of Medicine, Housman Medical Research Center, Massachusetts 02118.
authors.forename: J
authors.initials: J
authors.lastname: Shah
authors.forename: R I
authors.initials: RI
authors.lastname: Duclos
authors.forename: G G
authors.initials: GG
authors.lastname: Shipley
mesh_terms: D055592:Biophysical Phenomena; D001703:Biophysics; D002152:Calorimetry, Differential Scanning; D066298:In Vitro Techniques; D007700:Kinetics; D008051:Lipid Bilayers / Q000737:chemistry*; D008823:Micelles; D008968:Molecular Conformation; D015394:Molecular Structure; D010713:Phosphatidylcholines / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry; D014961:X-Ray Diffraction
chemical_list: C088962:1-stearoyl-2-acetylphosphatidylcholine; D008051:Lipid Bilayers; D008823:Micelles; D010713:Phosphatidylcholines; D014867:Water