Recovery of Bulk Proton Magnetization and Sensitivity Enhancement in Ultrafast Magic-Angle Spinning Solid-State NMR

Abstract

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.

Enhancing NMR Sensitivity of Natural‐Abundance Low‐γ Nuclei by Ultrafast Magic‐Angle‐Spinning Solid‐State NMR Spectroscopy

“…For experiments using MCP, as shown in Figure a, a double spin‐lock with a ramp on the 13 C spin‐lock followed by a CW proton decoupling during 13 C signal acquisition is repeated N times. At the end of final 13 C signal acquisition, a 1 H 90° pulse is applied to flip the remaining proton transverse magnetization to the + z axis to accelerate the proton spin‐lattice relaxation ( T 1 ) process . Significant signal enhancement obtained using MCP at 60 kHz MAS is well demonstrated on a danazol/vanillin cocrystalline sample as shown in Figure .…”

Using Abundant 1H Polarization to Enhance the Sensitivity of Solid-State NMR Spectroscopy

“…However, even in the simple 1 H → 13 C CP experiment where all 13 C signals are enhanced, only around 1% of 1 H polarization is transferred to 13 C in natural abundant samples, while 99% of 1 H polarization is destroyed by T 1ρ relaxation during CP and later by heteronuclear decoupling. In simple circumstances, the residual 1 H polarization can be recycled for enhancing spin–lattice relaxation rate and thus shortening recycle delay, leading to sensitivity enhancement per unit time. ,− Such an idea was actually incorporated into a 2D 1 H-detected HETCOR experiment at fast MAS by preserving unused 1 H polarization during or after 13 C t 1 evolution time . Unfortunately, this method is only suitable for isotopically 13 C labeled samples .…”

Forcing the ‘lazy’ protons to work

“…This makes FB experiments good for NA samples to force 1 HL to participate in the following 1 H  13 C CPMAS process. Although the FB-CW measurements have been successfully used in a wide variety of applications at both moderate and fast MAS frequencies, 22,23 and even with DNP signal enhancement, 24 the experimental setup is not straightforward. The first difficulty in the FB experiments is the choice of 1 H decoupling, which must keep the same phase for all transverse 1 H magnetizations, independent of the considered crystallites.…”