Quantitative H-H Distances in Protonated Solids by Frequency-Selective Recoupling at Fast Magic Angle Spinning NMR.

Nuclear magnetic resonance (NMR) spectroscopy of protons in protonated solids is challenging. Fast magic angle spinning (MAS) and homonuclear decoupling schemes, in conjunction, with high magnetic fields have improved the proton resolution. However, experiments to quantitatively measure H-H distances still remain elusive due to the dense proton-proton dipolar coupling network. A novel MAS solid-state NMR pulse sequence is proposed to selectively recouple and measure interproton distances in protonated samples. The phase-modulated sequence combined with a judicious choice of transmitter frequency is used to measure quantitative H-H distances on the order of 3 Å in l-histidine·HCl·HO, despite the presence of other strongly coupled protons. This method provides a major boost to NMR crystallography approaches for structural determination of pharmaceutical molecules by directly measuring H-H distances. The band-selective nature of the sequence also enables observation of selective H-H correlations (e.g., H-H/H-H/Η-Η) in peptides and proteins, which should serve as useful restraints in structure determination.