Magnetically induced orientation of phosphatidylcholine membranes.

Lipid bilayers prepared from natural phospholipids orient in magnetic fields with the long axis of the lipid molecules perpendicular to the magnetic field. This magnetically induced orientation was studied at high (11.7 Tesla (T)), mid (9.36 T), and low (4.68 T) magnetic field strengths using lipid aggregates prepared from natural and synthetic phosphatidylcholine analogs. Phosphatidylcholine analogs containing saturated diacylated chains (12 to 16 carbons/chain) exhibited extensive orientation of the lipid when bilayer formation occurred by gentle hydration conditions. Gentle hydration involved incubating dried phosphatidylcholine C above the main phase transition (Tm); brief shaking or swirling by hand was occasionally needed to completely disperse the lipids. The method of bilayer formation significantly influenced the amount of lipid that orients in magnetic fields. Thus the supramolecular structures (and % orientation) above Tm in an 11.7 T field of dimyristoylphosphatidylcholine (DMPC) bilayers are SUV (0%), LUV (approximately 15%), SPLV (approximately 40%), vortexed-MLV (approximately 60%) and non-vortexed MLV (approximately 90%). Single layered vesicles prepared by the REV method exhibited orientation at 11.7 T similar to LUV prepared by freeze thaw cycles. Aqueous dispersions of eggPC prepared by gentle hydration exhibit approximately 40% orientation at 11.7 T which decreased to approximately 30% orientation if 30% cholesterol is added to the membrane. Magnetic orientation of bilayers thus appears to be a general phenomenon for both saturated and unsaturated natural phospholipids either with or without cholesterol in the membrane.