Efficient 2D double-quantum solid-state NMR spectroscopy with large spectral widths
2D double-quantum single-quantum correlation spectra with arbitrary spectral widths can be recorded with SR26 and related supercycled recoupling sequences when applying Supercycle-Timing-Compensation (STiC) phase shifts. This concept widely extends the applicability of supercycled sequences, most importantly for obtaining long-range distance constraints for structure determination with solid-state NMR.
Third Spin-Assisted Recoupling in SSNMR: Theoretical Insights and Practicable Application to Biomolecular Structure Determination
In solid-state nuclear magnetic resonance (SSNMR) under magic-angle spinning (MAS), the dipolar interactions (that contains distance information), averaged out by sample spinning, can be reintroduced by the application of carefully designed radiofrequency pulses called dipolar recoupling sequences. In this review, we will essentially focus on a recently introduced recoupling mechanism namely third spin-assisted recoupling (TSAR) which allows exchanging magnetization between two spins (say A and B) without relying on the reintroduction of the AB dipolar coupling. Instead, it uses a second-order effect driven by a cross-term between two dipolar terms (AH and BH) where H stands for an assisting spin (typically a proton). After a theory section delineating the principles of TSAR and of its related sequences (PAR and PAIN-CP), we will discuss some practical aspects of its experimental implementation (at moderate and ultra-high spinning frequencies) and its ability to provide structurally relevant C–C, N–C, and N–N distance information. Finally, recent applications of TSAR for structure determination of biomolecules will be overviewed.
Compensated second-order recoupling: Application to third spin assisted recoupling
We consider the effect of phase shifts in the context of second-order recoupling techniques in solid-state NMR. Notably we highlight conditions leading to significant improvements for the Third Spin Assisted Recoupling (TSAR) mechanism and demonstrate the benefits of resulting techniques for detecting long-distance transfer in biomolecular systems. The modified pulse sequences of PAR and PAIN-CP, Phase-Shifted Proton Assisted Recoupling (AH-PS-PAR) and Phase-Shifted Proton-Assisted Insensitive Nuclei Cross Polarization (ABH-PS-PAIN-CP), still rely on cross terms between heteronuclear dipolar couplings involving assisting protons that mediate zero-quantum polarization transfer between low-γ nuclei ( 13C- 13C, 15N- 15N, 15N- 13C polarization transfer). Using Average Hamiltonian Theory we show that phase inversion compensates off-resonance contributions and yields improved polarization transfer as well as substantial broadening of the matching conditions. PS-TSAR greatly improves on the standard TSAR based methods because it alleviates their sensitivity to precise RF settings which significantly enhances robustness of the experiments. We demonstrate these new methods on a 19.6 kDa protein (U-[ 15N, 13C]-YajG) at high magnetic fields (up to 900 MHz 1H frequency) and fast sample spinning (up to 65 kHz MAS frequency).