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

The structural phases of magnetically alignable lipid mixtures were investigated as a function of temperature and lipid concentration using small-angle neutron scattering (SANS). Two systems were examined: (a) an aqueous mixture of DMPC (dimyristoyl phosphatidylcholine) and DHPC (dihexanoyl phosphatidylcholine) lipids doped with Tm³⁺ ions resulting in the positive alignment of the system with the applied magnetic field and (b) the above aqueous Tm³⁺ doped lipid mixture containing a negatively charged lipid, DMPG (dimyristoylphosphatidylglycerol). For both systems, three different scattering patterns were observed corresponding to distinct structural phases at specific temperatures and lipid concentrations. At 45 °C and a lipid concentration 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 °C), the perforated lamellar phase transformed into a unilamellar vesicular phase, in which the bilayers may also be perforated. Below about 25 °C, the viscosity decreases considerably and the scattering data suggest that the lamellae present at higher temperatures break up into smaller entities characterized by the bicellar morphology proposed previously for the nondoped system. The structural dimensions of the vesicular and bicellar phases 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 bilayer structure (e.g. lamellar repeat spacing, bilayer rigidity, and magnetic alignment) were also investigated.