Magic-angle-spinning NMR techniques for measuring long-range distances in biological macromolecules.

The determination of molecular structures using solid-state NMR spectroscopy requires distance measurement through nuclear-spin dipole-dipole couplings. However, most dipole-coupling techniques compete with the transverse (T2) relaxation of the nuclear spins, whose time constants are at most several tens of milliseconds, which limits the ability to measure weak dipolar couplings or long distances. In the last 10 years, we have developed a number of magic-angle-spinning (MAS) solid-state NMR techniques to measure distances of 15-20 Å. These methods take advantage of the high gyromagnetic ratios of (1)H and (19)F spins, multispin effects that speed up dipolar dephasing, and (1)H and (19)F spin diffusion that probes distances in the nanometer range. Third-spin heteronuclear detection provides a method for determining (1)H dipolar couplings to heteronuclear spins. We have used this technique to measure hydrogen-bond lengths, torsion angles, the distribution of protein conformations, and the oligomeric assembly of proteins. We developed a new pulse sequence, HARDSHIP, to determine weak long-range (1)H-heteronuclear dipolar couplings in the presence of strong short-range couplings. This experiment allows us to determine crystallite thicknesses in biological nanocomposites such as bone. The rotational-echo double-resonance (REDOR) technique allows us to detect multispin (13)C-(31)P and (13)C-(2)H dipolar couplings. Quantitative analysis of these couplings provides information about the structure of peptides bound to phospholipid bilayers and the geometry of ligand-binding sites in proteins. Finally, we also use relayed magnetization transfer, or spin diffusion, to measure long distances. z-Magnetization can diffuse over several nanometers because its long T1 relaxation times allow it to survive for hundreds of milliseconds. We developed (1)H spin diffusion to probe the depths of protein insertion into the lipid bilayer and protein-water interactions. On the other hand, (19)F spin diffusion of site-specifically fluorinated molecules allowed us to elucidate the oligomeric structures of membrane peptides.