Measurement of Residual Dipolar Couplings Using Magnetically Aligned and Flipped Nanodiscs

Recent developments in lipid nanodisc technology have successfully overcome the major challenges in the structural and functional studies of membrane proteins and drug delivery. Among the different types of nanodiscs, the use of synthetic amphiphilic polymers created new directions including the applications of solution and solid-state NMR spectroscopy. The ability to magnetically align large-size (>20 nm diameter) polymer nanodiscs and flip them using paramagnetic lanthanide ions has enabled high-resolution studies on membrane proteins using solid-state NMR techniques. The use of polymer-based macro-nanodiscs (>20 nm diameter) as an alignment medium to measure residual dipolar couplings (RDCs) and residual quadrupole couplings by NMR experiments has also been demonstrated. In this study, we demonstrate the use of magnetically aligned and 90°-flipped polymer nanodiscs as alignment media for structural studies on proteins by solution NMR spectroscopy. These macro-nanodiscs, composed of negatively charged SMA-EA polymers and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipids, were used to measure residual ¹H-¹⁵N dipolar couplings (RDCs) from the water-soluble ∼21 kDa uniformly ¹⁵N-labeled flavin mononucleotide binding domain (FBD) of cytochrome-P450 reductase. The experimentally measured ¹H-¹⁵N RDC values are compared with the values calculated from the crystal structures of cytochrome-P450 reductase that lacks the transmembrane domain. The N-H RDCs measured using aligned and 90°-flipped nanodiscs show a modulation by the function (3 cos²θ - 1), where θ is the angle between the N-H bond vector and the applied magnetic field direction. This successful demonstration of the use of two orthogonally oriented alignment media should enable structural studies on a variety of systems including small molecules, DNA, and RNA.