A Floquet description of phase alternated sequences for efficient homonuclear recoupling in solid perdeuterated systems

Site-specific protein backbone deuterium 2Hα quadrupolar patterns by proton-detected quadruple-resonance 3D 2HαcαNH MAS NMR spectroscopy

A novel deuterium-excited and proton-detected quadruple-resonance three-dimensional (3D) ²H_αc_αNH MAS nuclear magnetic resonance (NMR) method is presented to obtain site-specific ²H_α deuterium quadrupolar couplings from protein backbone, as an extension to the 2D version of the experiment reported earlier. Proton-detection results in high sensitivity compared to the heteronuclei detection methods. Utilizing four independent radiofrequency (RF) channels (quadruple-resonance), we managed to excite the ²H_α, then transfer deuterium polarization to its attached C_α, followed by polarization transfers to the neighboring backbone nitrogen and then to the amide proton for detection. This experiment results in an easy to interpret HSQC-like 2D ¹H-¹⁵N fingerprint NMR spectrum, which contains site-specific deuterium quadrupolar patterns in the indirect third dimension. Provided that four-channel NMR probe technology is available, the setup of the ²H_αc_αNH experiment is relatively straightforward, by using low power deuterium excitation and polarization transfer schemes we have been developing. To our knowledge, this is the first demonstration of a quadruple-resonance MAS NMR experiment to link ²H_α quadrupolar couplings to proton-detection, extending our previous triple-resonance demonstrations. Distortion-free excitation and polarization transfer of ∼160-170 kHz ²H_α quadrupolar coupling were presented by using a deuterium RF strength of ∼20 kHz. From these ²H_α patterns, an average backbone order parameter of S = 0.92 was determined on a deuterated SH3 sample, with an average η = 0.22. These indicate that SH3 backbone represents sizable dynamics in the microsecond timescale where the ²H_α lineshape is sensitive. Moreover, site-specific ²H_α T₁ relaxation times were obtained for a proof of concept. This 3D ²H_αc_αNH NMR experiment has the potential to determine structure and dynamics of perdeuterated proteins by utilizing deuterium as a novel reporter.

Site-specific protein methyl deuterium quadrupolar patterns by proton-detected 3D 2H-13C-1H MAS NMR spectroscopy

Determination of protein structure and dynamics is key to understand the mechanism of protein action. Perdeuterated proteins have been used to obtain high resolution/sensitivty NMR experiments via proton-detection. These methods utilizes ¹H, ¹³C and ¹⁵N nuclei for chemical shift dispersion or relaxation probes, despite the existing abundant deuterons. However, a high-sensitivity NMR method to utilize deuterons and e.g. determine site-specific deuterium quadrupolar pattern information has been lacking due to technical difficulties associated with deuterium's large quadrupolar couplings. Here, we present a novel deuterium-excited and proton-detected three-dimensional ²H-¹³C-¹H MAS NMR experiment to utilize deuterons and to obtain site-specific methyl ²H quadrupolar patterns on detuterated proteins for the first time. A high-resolution fingerprint ¹H-¹⁵N HSQC-spectrum is correlated with the anisotropic deuterium quadrupolar tensor in the third dimension. Results from a model perdeuterated protein has been shown.

Dynamics of uniformly labelled solid proteins between 100 and 300 K: A 2D 2H-13C MAS NMR approach

We describe a ²H based MAS nuclear magnetic resonance (NMR) method to obtain site-specific molecular dynamics of biomolecules. The method utilizes the use of deuterium nucleus as a spin label that is proven to be very useful in dynamics studies of solid biological and functional materials. The aim is to understand overall characteristics of protein backbone and side-chain motions for CD₃, CD₂ and CD groups, in terms of timescale, type and activation energy of the underlying processes. Variable temperature two-dimensional (2D) ²H-¹³C correlation MAS NMR spectra were recorded for the uniformly ²H,¹³C,¹⁵N labelled Alanine and microcrystalline SH3 at a broad temperature range, from 320 K down to 100 K. First, the deuterium quadrupolar-coupling constant from specific D-C sites is obtained with the 2D experiment by utilizing carbon chemical shifts. Second, the static quadrupolar patterns are obtained at 100 K. Third, variable temperature approach enabled the observation of quadrupolar pattern over different motional regimes; slow, intermediate and fast. And finally, the apparent activation energies for C-D sites are determined and compared, by evaluating the temperature induced signal intensities. This information led to the determination of the dynamic processes for different D-C sites at a broad range of temperature and motional timescales. This is a first representation of 2D ²H-¹³C MAS NMR approach applied to fully isotope labelled deuterated protein covering 220 K temperature range.

Recent progress in dipolar recoupling techniques under fast MAS in solid-state NMR spectroscopy

With the recent advances in NMR hardware and probe design technology, magic-angle spinning (MAS) rates over 100 ​kHz are accessible now, even on commercial solid NMR probes. Under such fast MAS conditions, excellent spectral resolution has been achieved by efficient suppression of anisotropic interactions, which also opens an avenue to the proton-detected NMR experiments in solids. Numerous methods have been developed to take full advantage of fast MAS during the last decades. Among them, dipolar recoupling techniques under fast MAS play vital roles in the determination of the molecular structure and dynamics, and are also key elements in multi-dimensional correlation NMR experiments. Herein, we review the dipolar recoupling techniques, especially those developed in the past two decades for fast-to-ultrafast MAS conditions. A major focus for our discussion is the ratio of RF field strength (in frequency) to MAS frequency, ν₁/ν_r, in different pulse sequences, which determines whether these dipolar recoupling techniques are suitable for NMR experiments under fast MAS conditions. Systematic comparisons are made among both heteronuclear and homonuclear dipolar recoupling schemes. In addition, the schemes developed specially for proton-detection NMR experiments under ultrafast MAS conditions are highlighted as well.