Biophysics and Department of Chemistry, Biomedical Engineering, Macromolecular Science and Engineering, University of Michigan, Ann Arbor, MI 48109-1055, USA.
Nano-Crystallography Unit, RIKEN-JEOL Collaboration Center, Tsurumi, Yokohama, Kanagawa 230-0045, Japan; Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Institute for Advanced Study, and AIST-Kyoto University Chemical Energy Material Open Innovation Laboratory (ChEM-OIL), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan.
J Magn Reson. 2020 Apr;313:106717. doi: 10.1016/j.jmr.2020.106717. Epub 2020 Mar 17.
The advent of ultrahigh magnetic field and fast magic-angle-spinning (MAS) probe technology has led to dramatically enhanced spectral resolution and sensitivity in solid-state NMR spectroscopy. In particular, proton-based multidimensional solid-state NMR techniques have become feasible to investigate the structure and dynamics at atomic resolution, due to the increased chemical shift span and spectral resolution. Herein, the benefits of faster MAS and higher magnetic field are demonstrated on a self-assembled diphenylalanine (Phe-Phe) nanomaterial. Proton-detected 2D H/H single-quantum/single-quantum (SQ/SQ) correlation, double-quantum/single-quantum (DQ/SQ) correlation, and H chemical shift anisotropy/chemical shift (CSA/CS) correlation spectra obtained at two different spinning speeds (60 and 100 kHz) and two different magnetic fields (600 and 900 MHz) are reported. The dramatic enhancement of proton spectral resolution achieved with the use of a 900 MHz magnetic field and 100 kHz MAS is remarkable and enabled the measurement of proton CSA tensors, which will be useful to better understand the self-assembled structures of Phe-Phe nanotubes. We also show through numerical simulations that the unaveraged proton-proton dipolar couplings can result in broadening of CSA lines, leading to inaccurate determination of CSA tensors of protons. Thus, our results clearly show the insufficiency of a 600 MHz magnetic field to resolve H spectra lines and the inability of a moderate spinning speed of 60 kHz to completely suppress H-H dipolar couplings, which further justify the pursuit of ultrahigh magnetic field beyond 1 GHz and ultrafast MAS beyond 100 kHz.
超快磁场和快速魔角旋转（MAS）探头技术的出现，使得固态 NMR 光谱学中的谱分辨率和灵敏度得到了显著提高。特别是，基于质子的多维固态 NMR 技术已经变得可行，可以在原子分辨率下研究结构和动力学，这要归功于化学位移范围和谱分辨率的增加。在此，通过更快的 MAS 和更高的磁场，对自组装二苯丙氨酸（Phe-Phe）纳米材料进行了演示。报道了在两个不同的旋转速度（60 和 100 kHz）和两个不同的磁场（600 和 900 MHz）下获得的质子探测二维 H/H 单量子/单量子（SQ/SQ）相关、双量子/单量子（DQ/SQ）相关和 H 化学位移各向异性/化学位移（CSA/CS）相关谱。使用 900 MHz 磁场和 100 kHz MAS 实现的质子谱分辨率的显著提高是显著的，并能够测量质子 CSA 张量，这将有助于更好地理解 Phe-Phe 纳米管的自组装结构。我们还通过数值模拟表明，未平均的质子-质子偶极耦合会导致 CSA 线变宽，从而导致质子 CSA 张量的不准确确定。因此，我们的结果清楚地表明，600 MHz 磁场不足以分辨 H 谱线，60 kHz 的中等旋转速度不足以完全抑制 H-H 偶极耦合，这进一步证明了追求超过 1 GHz 的超高磁场和超过 100 kHz 的超快 MAS 的必要性。
