MAGNETICALLY ALIGNED PHOSPHOLIPID-BILAYERS WITH POSITIVE ORDERING - ANEW MODEL MEMBRANE SYSTEM

A stable smectic phospholipid bilayer phase aligned with the director parallel to the magnetic field can be generated by the addition of cer tain trivalent paramagnetic lanthanide ions to a bicellar solution of dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholin e (DHPC) in water. Suitable lanthanide ions are those with positive an isotropy of their magnetic susceptibility, namely Eu3+, Er3+, Tm3+, an d Yb3+, For samples doped with Tm3+, this phase extends over a wide ra nge of Tm3+ concentrations (6-40 mM) and temperatures (35-90 degrees C ) and appears to undergo a transition from a fluid nematic discotic to a fluid, but highly ordered, smectic phase at a temperature that depe nds on the thulium concentration. As a membrane mimetic, these new, po sitively ordered phospholipid phases have high potential for structura l studies using a variety of techniques such as magnetic resonance (EM R and NMR), small-angle x-ray and neutron diffraction, as well as opti cal and infrared spectroscopy.