Peptoid-based macrodiscs of variable lipid composition for structural studies of membrane proteins by oriented-sample solid-state NMR.

Solid-state Nuclear Magnetic Resonance (NMR) in combination with magnetically aligned discoidal lipid mimics allows for studying the conformations of membrane proteins in planar, lipid-rich bilayer environments and at the physiological temperature. We have recently demonstrated the general applicability of macrodiscs composed of DMPC lipids and peptoid belts, which yield magnetic alignment and NMR spectroscopic resolution comparable or superior to detergent-containing bicelles. Here we report on a considerable improvement in the magnetic alignment and NMR resolution of peptoid-based macrodiscs consisting of a mixture of the zwitterionic and negatively charged lipids (DMPC/DMPG at the 85% to 15% molar ratio). The resulting linewidths are about 30% sharper due to the higher orientational order parameter likely arising from the stabilizing electrostatic repulsion between the discs. Moreover, highly aligned, detergent-free macrodiscs can be formed with a longer-chain lipid, DPPC. Interestingly, the spectra of Pf1 in the two lipid mimetics are almost indistinguishable, which would mean that the overall transmembrane helix tilt might be governed not only by the hydrophobic matching but also possibly by the interactions of the flanking lysine and arginine residues at the membrane interface.