Third spin-assisted recoupling in SSNMR: theoretical insights and practicable application to biomolecular structure determination
Résumé
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