Biophysics and Department of Chemistry, University of Michigan , Ann Arbor, Michigan 48109-1055, United States.
RIKEN CLST-JEOL Collaboration Center, RIKEN , Yokohama, Kanagawa 230-0045, Japan.
J Phys Chem B. 2017 Jun 22;121(24):5944-5952. doi: 10.1021/acs.jpcb.7b03480. Epub 2017 Jun 9.
Solid-state H NMR spectroscopy has attracted much attention in the recent years due to the remarkable spectral resolution improvement by ultrafast magic-angle-spinning (MAS) as well as due to the sensitivity enhancement rendered by proton detection. Although these developments have enabled the investigation of a variety of challenging chemical and biological solids, the proton spectral resolution is still poor for many rigid solid systems owing to the presence of conformational heterogeneity and the unsuppressed residual proton-proton dipolar couplings even with the use of the highest currently feasible sample spinning speed of ∼130 kHz. Although a further increase in the spinning speed of the sample could be beneficial to some extent, there is a need for alternate approaches to enhance the spectral resolution. Herein, by fully utilizing the benefits of double-quantum (DQ) coherences, we propose a single radio frequency channel proton-based 3D pulse sequence that correlates double-quantum (DQ), DQ, and single-quantum (SQ) chemical shifts of protons. In addition to the two-spin homonuclear proximity information, the proposed 3D DQ/DQ/SQ experiment also enables the extraction of three-spin and four-spin proximities, which could be beneficial for revealing the dipolar coupled proton network in the solid state. Besides, the 2D DQ/DQ spectrum sliced at different isotropic SQ chemical shift values of the 3D DQ/DQ/SQ spectrum will also facilitate the identification of DQ correlation peaks and improve the spectral resolution, as it only provides the local homonuclear correlation information associated with the specific protons selected by the SQ chemical shift frequency. The 3D pulse sequence and its efficiency are demonstrated experimentally on small molecular compounds in the solid state. We expect that this approach would create avenues for further developments by suitably combining the benefits of partial deuteration of samples, selective excitation/decoupling pulses, heteronuclear spins for spectral editing, and nonuniform sampling.
近年来，由于超快魔角旋转（MAS）带来的显著光谱分辨率提高，以及质子检测带来的灵敏度提高，固态 H NMR 光谱学受到了广泛关注。尽管这些发展使得对各种具有挑战性的化学和生物固体的研究成为可能，但由于构象异质性的存在以及即使使用目前最高可达 130 kHz 的样品旋转速度，残留质子-质子偶极耦合仍未被抑制，许多刚性固体系统的质子光谱分辨率仍然很差。虽然进一步提高样品的旋转速度在某种程度上可能是有益的，但需要采用替代方法来提高光谱分辨率。在此，我们通过充分利用双量子（DQ）相干的优势，提出了一种基于质子的单射频通道 3D 脉冲序列，该序列关联质子的双量子（DQ）、DQ 和单量子（SQ）化学位移。除了两核同核近距离信息外，所提出的 3D DQ/DQ/SQ 实验还能够提取三核和四核近距离，这有助于揭示固态中偶极耦合质子网络。此外，在 3D DQ/DQ/SQ 光谱的不同各向同性 SQ 化学位移值上截取的 2D DQ/DQ 光谱也将有助于识别 DQ 相关峰并提高光谱分辨率，因为它仅提供与 SQ 化学位移频率选择的特定质子相关的局部同核相关信息。在固态小分子化合物上进行了实验验证了 3D 脉冲序列及其效率。我们期望通过适当结合样品部分氘化、选择性激发/去耦脉冲、用于光谱编辑的异核自旋以及非均匀采样的优势，为进一步发展创造途径。
