Recently, we published a review titled “Measurement of proton chemical shift anisotropy in solid-state NMR spectroscopy”. The review presents a retrospect of methodological development in extracting proton chemical shift anisotropy in solid samples.
In solid materials, atoms always sit among anisotropically interacting environment, leading anisotropic distribution of electrons of atoms. This feature acts as a crucial factor, named chemical shift anisotropy (CSA), of line-broadening effect in solid-state nuclear magnetic resonance spectroscopy. While the line-broadening effect renders poor resolution of spectra, the CSA itself indicates the electronic environment of corresponding nucleus, which can unveil not only the details of the local chemical structure, but also the features of molecular motion.
Proton exists extensively in organics, oxides and zeolites etc., the proton CSA therefore becomes an important probe for unveiling the dynamics and local environment in those materials. However, as dipolar interaction in solids always being much larger, the proton CSA is mostly overwhelmed, especially by the homonuclear dipolar interaction of proton itself, making the measurement of proton CSA a tough task.
Over the last half century, enormous efforts have been made to extract the proton CSA in solids. In the early years, the measurements were only accomplished in carefully-aligned single crystals which possess sparse 1H nuclei with relative large CSA. Later, with the multiple-pulse technique emerged which enabled the 1H-1H decoupling, and combined with the magic angle spinning (MAS) technique, the measurement of proton CSA could be applied to powder samples with denser 1H nuclei, such as maleic acid.
The last decay saw the introduction of R-symmetry pulse sequence (Levitt et al.), along with other CSA recoupling techniques such as rotary resonance recoupling (Duma et al.) and CSA amplification (Gan et al.). In virtue of these methods and with the help of fast/ultrafast MAS techniques, the proton CSA can be extracting in samples more complicate than ever. In complicate samples like proteins, the resolution of vast amount of protons becomes an issue, because protons are all crammed in about 20ppm chemical shift range. In 2013, we exploited a series of three dimensional spectra to extract the CSA of the protons in amide bonds. By using the chemical shift of amide 13C and 15N in a 15N/13C uniformly labeled protein CAP-Gly domain of dynactin, while introducing the 1H CSA or HX dipolar interaction in t1 dimension, 42 amide protons in 86 residues were identified. Moreover, the relative orientations between HX dipoles and proton CSAs were determined via triple fitting of three types of recoupling line shapes. The multiple-dimension technique was also used by other researchers through some different tactics, such as 3D proton DQ (double quantum)/CSA/SQ (chemical shift) spectra by Ramamoorthy's group, and 3D xCSA (CSA amplification)/13C/15N spectra by Gan et al.
Looking into the future, we anticipate that these recent advances on 1H CSA recoupling techniques would provide a vital key to the characterization of structures and hydrogen-bonding environments in a variety of solid materials.