Selectively Enhanced H-H Correlations in Proton-Detected Solid-State NMR under Ultrafast MAS Conditions.

Proton-detected solid-state NMR has emerged as a powerful analytical technique in structural elucidation via H-H correlations, which are mostly established by broadband methods. We propose a new class of frequency-selective homonuclear recoupling methods to selectively enhance H-H correlations of interest under ultrafast magic-angle spinning (MAS). These methods, dubbed as selective phase-optimized recoupling (SPR), can provide a sensitivity enhancement by a factor of ∼3 over the widely used radio-frequency-driven recoupling (RFDR) to observe H-H contacts in a protonated tripeptide -formyl-Met-Leu-Phe (fMLF) under 150 kHz MAS and are successfully utilized to probe a long-range H-H contact in a pharmaceutical molecule, the hydrochloride form of pioglitazone (PIO-HCl). SPR is not only highly efficient in frequency-selective recoupling but also easy to implement, imparting to it great potential to probe H-H contacts for the structural elucidation of organic solids such as proteins and pharmaceuticals under ultrafast MAS conditions.