Dilute spin-exchange assignment of solid-state NMR spectra of oriented proteins: Acetylcholine M2 in bilayers

The assignment of amide resonances in the two-dimensional PISEMA (Polarizat ion Inversion with Spin Exchange at the Magic Angle) spectrum of uniformly N-15 labeled M2 peptide corresponding to the channel-lining segment of the acetylcholine receptor in oriented phospholipid bilayers is described. The majority of the resonances were assigned through comparisons with spectra f rom selectively N-15 labeled recombinant peptides and specifically N-15 lab eled synthetic peptides. Some resonances were assigned to specific amino ac id residues by means of homonuclear N-15 spin-exchange spectroscopy. A modi fication to the conventional spin-exchange pulse sequence that significantl y shortens the length of the experiments by combining the intervals for N-1 5 spin-exchange and H-1 magnetization recovery is described.