Recovery of bulk proton magnetization and sensitivity enhancement in ultrafast magic-angle spinning solid-state NMR

The sensitivity of solid-state NMR experiments is limited by the proton magnetization recovery delay and by the duty cycle of the instrument. Ultrafast magic-angle spinning (MAS) can improve the duty cycle by employing experiments with low-power radio frequency (RF) irradiation which reduce RF heating. On the other hand, schemes to reduce the magnetization recovery delay have been proposed for low MAS rates, but the enhancements rely on selective transfers where the bulk of the (1)H magnetization pool does not contribute to the transfer. We demonstrate here that significant sensitivity enhancements for selective and broadband experiments are obtained at ultrafast MAS by preservation and recovery of bulk (1)H magnetization. We used [(13)C, (15)N]-labeled glutamine as a model compound, spinning in a 1.3 mm rotor at a MAS frequency of 65 kHz. Using low-power (1)H RF (13.4 kHz), we obtain efficient (1)H spin locking and (1)H-(13)C decoupling at ultrafast MAS. As a result, large amounts of (1)H magnetization, from 35% to 42% of the initial polarization, are preserved after cross-polarization and decoupling. Restoring this magnetization to the longitudinal axis using a flip-back pulse leads to an enhancement of the sensitivity, an increase ranging from 14% to 21% in the maximal achievable sensitivity regime and from 24% to 50% in the fast pulsing regime, and to a shortening of the optimal recycling delay to 68% of its original duration. The analysis of the recovery and sensitivity curves reveals that the sensitivity gains do not rely on a selective transfer where few protons contribute but rather on careful conservation of bulk (1)H magnetization. This makes our method compatible with broadband experiments and uniformly labeled materials, in contrast to the enhancement schemes proposed for low MAS. We tested seven different cross-polarization schemes and determined that recovery of bulk (1)H magnetization is a general method for sensitivity enhancement. The physical insight gained about the behavior of proton magnetization sharing under spin lock will be helpful to break further sensitivity boundaries, when even higher external magnetic fields and faster spinning rates are employed.