A Method for Measuring Heteronuclear (1H−13C) Distances in High Speed MAS NMR

Magic angle spinning (MAS) NMR structure determination is rapidly developing. We demonstrate a method to determine 1H−13C distances r CH with high precision from Lee−Goldburg cross-polarization (LG-CP) with fast MAS and continuous LG decoupling on uniformly 13C-enriched tyrosine·HCl. The sequence is γ-encoded, and 1H−13C spin-pair interactions are predominantly responsible for the polarization transfer while proton spin diffusion is prevented. 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, reflecting coherent heteronuclear transfer. Its Fourier transform yields an effective 13C frequency response that is very sensitive to the surrounding protons. This 13C spectrum can be reproduced in detail with MAS Floquet simulations of the spin cluster, based on the positions of the nuclei from the neutron diffraction structure. It is symmetric around ω = 0 and yields two well-resolved maxima. Measurement of CH distances is straightforward, since the separation Δω /2π between the maxima for a single 1H−13C pair is related to the internuclear distance according to r CH = a(Δ ω/2π)-1/3, with a = 25.86 ± 0.01 Å Hz1/3. For the 1H directly bonded to a 13C, the magnetization is transferred in a short time of ∼100 μs. After this initial rapid transfer period, the COOH, OH, or NH3 that are not directly bonded to a 13C transfer magnetization over long distances. This offers an attractive route for collecting long-range distance constraints and for the characterization of intermolecular hydrogen bonding.