SANS study of the structural phases of magnetically alignable lanthanide-doped phospholipid mixtures

The structural phases of magnetically alignable lipid mixtures were investi gated as a function of temperature and lipid concentration using small-angl e neutron scattering (SANS). Two systems were examined: (a) an aqueous mixt ure of DMPC (dimyristoyl phosphatidylcholine) and DHPC (dihexanoyl phosphat idylcholine) lipids doped with Tm3+ ions resulting in the positive alignmen t of the system with the applied magnetic field and (b) the above aqueous T m3+ doped lipid mixture containing a negatively charged lipid, DMPG (dimyri stoylphosphatidylglycerol). For both systems, three different scattering pa tterns were observed corresponding to distinct structural phases at specifi c temperatures and lipid concentrations. At 45 degreesC and a lipid concent ration of >0.05 g/mL, the high-viscosity liquid crystalline phase was found to be a perforated and possibly undulating lamellar phase consistent with NMR results. Upon dilution ( < 0.05 g/mL) at the same temperature (45 degre esC), the perforated lamellar phase transformed into a unilamellar vesicula r phase, in which the bilayers may also be perforated. Below about 25 degre esC, the viscosity decreases considerably and the scattering data suggest t hat the lamellae present at higher temperatures break up into smaller entit ies characterized by the bicellar morphology proposed previously for the no ndoped system. The structural dimensions of the vesicular and bicellar phas es have been determined as a function of lipid concentrations from the SANS data. In the lamellar phase, the influence of Tm3+ ions and DMPG on bilaye r structure (e.g., lamellar repeat spacing, bilayer rigidity, and magnetic alignment) were also investigated.