Unprecedented observation of days-long remnant orientation of phospholipid bicelles: a small-angle X-ray scattering and theoretical study

Nanometric bilayer-based self-assembled micelles commonly named as bicelles, formed with a mixture of long and short chains phosphatidylcholine lipids (PC), are known to orient spontaneously in a magnetic field. This field-induced orientational order strongly depends on the molecular structure of the phospholipids. Using small-angle X-ray scattering (SAXS), we performed detailed structural studies of bicelles and investigated the orientation/relaxation kinetics in three different systems: saturated-chain lipid bicelles made of DMPC (dimyristoyl PC)/DCPC (1,2-dicaproyl PC) with and without the added paramagnetic lanthanide ions Eu(3+), as well as bicelles of TBBPC (1-tetradecanoyl-2-(4-(4-biphenyl)butanoyl)-sn-glycero-3-PC)/DCPC. The structural study confirmed the previous NMR studies, which showed that DMPC bicelles orient with the membrane normal perpendicular (defined here as "nematic" orientation) to the magnetic field, whereas they orient parallel (defined here as "smectic" orientation) to the magnetic field in the presence of Eu(3+). The TBBPC bicelles also show smectic orientation. Surprisingly, the orientational order induced in the magnetic field remains even after the magnetic field is removed, which allowed us to investigate the orientation and relaxation kinetics of different bicelle structures. We demonstrate that this kinetics is very different for all three types of bicelles at the same lipid concentration; DMPC bicelles (~40 nm diameter) with and without Eu(3+) orient faster than TBBPC bicelles (~80 nm diameter). However, for the relaxation, DMPC bicelles (nematic) lose their macroscopic orientation only after one hour, whereas both DMPC bicelles with Eu(3+) and TBBPC bicelles (smectic) remarkably stay oriented for up to several days! These results indicate that the orientation mechanism of these nanometric disks in the magnetic field is governed by their size, with smaller bicelles orienting faster than the larger bicelles. Their relaxation mechanism outside the magnetic field, however, is governed by the degree of ordering. Indeed, the angular distribution of oriented bicelles is much narrower for the bicelles with smectic orientation, and, consequently, they keep aligned for much longer time (days) than those with nematic ordering (hours) outside the magnetic field. The understanding of the orientation/relaxation kinetics, as well as the morphologies of these "molecular goniometers" at molecular and supramolecular levels, allows controlling such an unprecedented long-range and long-lived smectic ordering of nanodisks and opens a wide field of applications for structural biology or material sciences.