Frequency Selective Phase-Optimized Recoupling for Protons in Ultra-Fast Solid-State Magic-Angle Spinning NMR
We propose a new category of homonuclear frequency-selective recoupling methods for protons under ultra-fast MAS ranging from 40 kHz to 150 kHz. The methods, named as Selective Phase-optimized Recoupling (SPR) are simple in the form with defined phase schemes and RF amplitudes. SPR are robust to RF variations and efficient in frequency-selective recoupling. We demonstrated that SPR can provide a sensitivity gain of ~ 3 over the widely-used RFDR for selective 1HN-1HN correlations under 150 kHz MAS using a protonated tripeptide N-formyl-Met-Leu-Phe (fMLF). Moreover, SPR requires small ratios (~ 0.5) of RF power with respect to MAS frequency, making it perfect to probe long-range 1H-1H distance under ultra-fast MAS up to 150 kHz.