A method for measuring heteronuclear (H-1-C-13) distances in high speed MAS NMR

Magic angle spinning (MAS) NMR structure determination is rapidly developin g. We demonstrate a method to determine H-1-C-13 distances r(CH) with high precision from Lee-Goldburg cross-polarization (LG-CP) with fast MAS and co ntinuous LG decoupling on uniformly C-13-enriched tyrosine . HCl. The seque nce is gamma-encoded, and H-1-C-13 spin-pair interactions are predominantly responsible for the polarization transfer while proton spin diffusion is p revented. When the CP amplitudes are set to a sideband of the Hartmann-Hahn match condition, the LG-CP signal builds up in an oscillatory manner, refl ecting coherent heteronuclear transfer. Its Fourier transform yields an eff ective C-13 frequency response that is very sensitive to the surrounding pr otons. This C-13 spectrum can be reproduced in detail with MAS Floquet simu lations of the spin cluster, based on the positions of the nuclei from the neutron diffraction structure. It is symmetric around omega = 0 and yields two well-resolved maxima. Measurement of CH distances is straightforward, s ince the separation Delta omega/2 pi between the maxima for a single H-1-C- 13 pair is related to the internuclear distance according to r(CH) = a(Delt a omega/2 pi)(-1/3), with a = 25.86 +/- 0.01 Angstrom Hz(1/3). For the H-1 directly bonded to a C-13, the magnetization is transferred in a short time of similar to 100 mu s. After this initial rapid transfer period, the COOH , OH, or NH3 that are not directly bonded to a C-13 transfer magnetization over long distances. This offers an attractive route for collecting long-ra nge distance constraints and for the characterization of intermolecular hyd rogen bonding.