Nuclear spin diffusion under fast magic-angle spinning in solid-state NMR

Analysis of the MODIST Sequence for Selective Proton-Proton Recoupling

Theoretical and simulated analyses of selective homonuclear dipolar recoupling sequences serve as primary tools for understanding and determining the robustness of these sequences under various conditions. In this article, we investigate the recently proposed first-order dipolar recoupling sequence known as MODIST (Modest Offset Difference Internuclear Selective Transfer). We evaluate the MODIST transfer efficiency, assessing its dependence on rf-field strengths and the number of simulated spins, extending up to 10 spins. This helps to identify conditions that enhance polarization transfer among spins that are nearby in frequency, particularly among aliphatic protons. The exploration uncovers a novel effect for first-order selective recoupling sequences that we term "facilitated dipolar recoupling". This effect amplifies the recoupled dipolar interaction between distant spins due to the presence of additional strongly dipolar-coupled spins. Unlike the third spin-assisted recoupling mechanism, facilitated dipolar recoupling only requires a coupling to one of the two distant spins of interest. Experimental demonstration of MODIST, including at different rf-field strengths, was carried out with the membrane protein influenza A M2 in lipid bilayers using 55 kHz magic-angle spinning (MAS). Reducing MODIST rf-field strength by a factor of 2 unveils possibilities for detecting Hα-Hα and HMeth-HMeth correlations with a 3D (H)C(H)(H)CH experiment under fast MAS rates, all achievable without specific spin labeling.

Probing Noncovalent Interactions by Fast Magic-Angle Spinning NMR at 100 kHz and More

Noncovalent interactions are the basis for a large number of chemical and biological molecular-recognition processes, such as those occurring in supramolecular chemistry, catalysis, solid-state reactions in mechanochemistry, protein folding, protein-nucleic acid binding, and biomolecular phase separation processes. In this perspective article, some recent developments in probing noncovalent interactions by proton-detected solid-state Nuclear Magnetic Resonance (NMR) spectroscopy at Magic-Angle Spinning (MAS) frequencies of 100 kHz and more are reviewed. The development of MAS rotors with decreasing outer diameters, combined with the development of superconducting magnets operating at high static magnetic-field strengths up to 28.2 T (1200 MHz proton Larmor frequency) improves resolution and sensitivity in proton-detected solid-state NMR, which is the fundamental requirement for shedding light on noncovalent interactions in solids. The examples reported in this article range from protein-nucleic acid binding in large ATP-fueled motor proteins to a hydrogen-π interaction in a calixarene-lanthanide complex.

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

Solid-state NMR spectroscopy has been playing a significant role in elucidating the structures and dynamics of materials and proteins at the atomic level for decades. As an extremely abundant nucleus with a very high gyromagnetic ratio, protons are widely present in most organic/inorganic materials. Thus, this Perspective highlights the advantages of proton detection at fast magic-angle spinning (MAS) and presents strategies to utilize and exhaust 1H polarization to achieve signal sensitivity enhancement of solid-state NMR spectroscopy, enabling substantial time savings and extraction of more structural and dynamics information per unit time. Those strategies include developing sensitivity-enhanced single-channel 1H multidimensional NMR spectroscopy, implementing multiple polarization transfer steps in each scan to enhance low-γ nuclei signals, and making full use of 1H polarization to obtain homonuclear and heteronuclear chemical shift correlation spectra in a single experiment. Finally, outlooks and perspectives are provided regarding the challenges and future for the further development of sensitivity-enhanced proton-based solid-state NMR spectroscopy.