Biphenyl phosphatidylcholine: a promoter of liposome deformation and bicelle collective orientation by magnetic fields.

Membrane lipids with long saturated or unsaturated acyl chains are usually not sensitive to magnetic fields. We report in this review a few exceptions with potential use in structural biology or drug delivery. Mixtures of short and long chain phospholipids called bicelles can form discs-shaped nanoobjects (40nm) that can indeed be oriented in magnetic fields. This is due to the cooperative effect of the small diamagnetic negative anisotropic susceptibility of each of the individual lipids that build up a macroscopic magnetic moment that orients in the field like a compass. Chain saturated lipids have a tendency to be oriented with their long molecular axis perpendicular to the field, thus leading to a disc plane with a parallel orientation. Newly synthesized phosphatidylcholine (PC) containing a biphenyl group in one of its acyl chains (1-tetradecanoyl-2-(4-(4-biphenyl)butanoyl)-sn-glycero-3-PC, TBBPC) shows very unusual macroscopic orienting properties due to the strong positive anisotropy of the biphenyl diamagnetic susceptibility. Mixing with short chain lipids leads to bicelles of 80nm diameter that are oriented by magnetic fields such that the disc plane is perpendicular to the field. Tuning the lipid molecular structure thus affords controlling the orientation of this "molecular goniometer". Because the magnetic alignment is remnant for tens of hours even outside the field, applications in structural biology and biotechnology, are discussed. Of great interest, micrometer-sized liposomes made from such a new lipid are strongly deformed into oblates when placed in a magnetic field greater than a few Tesla. Increasing the magnetic field leads to even greater deformations which could potentially be used in medicine for specific drug delivery purposes, under magnetic resonance imaging.