AUDIO-FREQUENCY NMR IN A NUTATING FRAME - APPLICATION TO THE ASSIGNMENT OF PHENYLALANINE RESIDUES IN ISOTOPICALLY ENRICHED PROTEINS

The effect of chemical shift can be removed to high order by quantizing nuclear spins along a strong radio-frequency (RF) field and applying a second audio-frequency magnetic field, orthogonal to the direction of the effective RF spin lock field. By adjusting the audio-modulation frequency of this second magnetic field to the rate at which spins nutate about the RF field, a resonance condition in the spin-locked frame is established which, to second order, is independent of the resonance offset of the homonuclear spins. The phase and amplitude of the audio-frequency field are under operator control and make it possible to conduct complex pulse sequences in the nutating frame. The approach is demonstrated for transferring in a single step magnetization from phenylalanine C-13(beta) to all aromatic ring carbons in a sample of uniformly C-13-enriched calmodulin. A three-dimensional version of the experiment yields complete resonance assignments for the structurally informative aromatic Phe signals, including all C-13(zeta) and H-1(zeta) resonances.