CHAPTER 6: Magnetic Liposomes and Bicelles: New Tools for Membrane-Peptide Structural Studies

The structure and topology of membrane peptides and proteins in a natural membrane environment can be approached with solid-state NMR by making use of the magnetic properties of lipids, which in certain conditions lead to magnetically oriented membrane samples. Lipids possess an intrinsic very small magnetic susceptibility anisotropy, Δχ, which leads to interesting annealing properties in very high magnetic fields of ca. 20 Tesla. Saturated chain lipids have a negative Δχ, leading to liposome (multilamellar vesicle, MLV) deformation to prolate or to lipid bicelles (40–100 nm nanodiscs) oriented with the normal to the disc surface perpendicular to the field. Biphenyl chain-containing lipids exhibit a positive Δχ that leads to MLV oblate deformation or bicelle orientation with the normal to the nanodisc parallel to the field. Using different lipids either in the form of liposomes or bicelles to tune the membrane orientation with respect to the field, a collection of NMR experiments can be performed to gain membrane peptide/protein topology and structural information. Representative examples are given of the Pf1 coat membrane protein, the antimicrobial peptide surfactin and the neurotransmitter methionine enkephalin.