Magnetic-Alignment of Polymer Nanodiscs Probed by Solid-State NMR Spectroscopy

The ability of amphipathic polymers to self-assemble with lipids and form nanodiscs has been a boon for the field of functional reconstitution of membrane proteins. In a field dominated by detergent micelles, a unique feature of polymer nanodiscs is their much-desired ability to align in the presence of an external magnetic field. Magnetic alignment facilitates the application of solid-state NMR spectroscopy and aids in the measurement of residual dipolar couplings (RDCs) via well-established solution NMR spectroscopy. In this study, we comprehensively investigate the magnetic-alignment properties of SMA-QA polymer based nanodiscs by using 31P and 14N solid-state NMR experiments under static conditions. The results reported herein demonstrate the spontaneous magnetic-alignment of large-size (≥ 20 nm diameter) SMA-QA nanodiscs (also called as macro-nanodiscs) with the lipid-bilayer-normal perpendicular to the magnetic field direction. Consequently, the orientation of macro-nanodiscs are further shown to flip their alignment axis parallel to the magnetic field direction upon the addition of a paramagnetic lanthanide salt. These results demonstrate the use of SMA-QA polymer nanodiscs for solid-state NMR applications including structural studies on membrane proteins.