Maximizing efficiency of dipolar recoupling in solid-state NMR using optimal control sequences

Abstract

Dipolar recoupling is a central concept in the nuclear magnetic resonance spectroscopy of powdered solids and is used to establish correlations between different nuclei by magnetization transfer. The efficiency of conventional cross-polarization methods is low because of the inherent radio frequency (rf) field inhomogeneity present in the magic angle spinning (MAS) experiments and the large chemical shift anisotropies at high magnetic fields. Very high transfer efficiencies can be obtained using optimal control-derived experiments. These sequences had to be optimized individually for a particular MAS frequency. We show that by adjusting the length and the rf field amplitude of the shaped pulse synchronously with sample rotation, optimal control sequences can be successfully applied over a range of MAS frequencies without the need of reoptimization. This feature greatly enhances their applicability on spectrometers operating at differing external fields where the MAS frequency needs to be adjusted to avoid detrimental resonance effects.

1H-Detected Biomolecular NMR under Fast Magic-Angle Spinning

“…Recent CP RF schemes developed using optimal-control (OC) theory perform superior in recovering signal from sample in inhomogeneous B 1 field . While, in principle, a specific OC pulse is extensively optimized for a particular MAS frequency, a relatively high efficiency (≥90%) can be retained in the range of 0.7–1.3 of nominal ν R by appropriate rescaling of applied RF strength and pulse duration . Nevertheless, for largely different MAS frequency ranges (e.g., 50–70 and 80–120 kHz) individually optimized pulses must be used to account for time scale of evolution of various recoupled or suppressed spin interactions (which are not flexible with ν R ).…”

Quantum optimal control in quantum technologies. Strategic report on current status, visions and goals for research in Europe

“…Furthermore, the physical limits of the time-optimal excitation of maximum-quantum coherence was explored for spin systems consisting of up to five coupled spins [341]. Another development motivated by solid-state NMR experiments with potential applications in quantum technologies was the demonstration that it is possible to design control sequences that are robust to periodic modulations of the control amplitude with known modulation frequency but unknown amplitude and phase of the modulation [570,571]. The optimal-control-based tracking of desired spin trajectories has been used to create highly robust heteronuclear decoupling experiments [437] and more recently to tailor the detuning-dependent scaling of the spectral splitting caused by spin-spin couplings to reduce the dimensionality of heteronuclear correlation experiments [651].…”

SAA fibrils involved in AA amyloidosis are similar in bulk and by single particle reconstitution: A MAS solid-state NMR study

“…In addition, 3D CONCA and 3D CANCO experiments were performed to confirm and assign ambiguous residues ( Li et al, 2007 , Shi et al, 2009 ). In these experiments, optimal control CP (OC-CP) was used to improve sensitivity ( Tošner et al, 2018 , Tošner et al, 2021 ). Long-range interactions were obtained from long mixing time DARR experiments (100, 300, 400, and 600 ms), as well as PAR ( De Paëpe et al, 2008 ) experiments (5, 15, and 20 ms).…”