Indian Institute of Technology Ropar, Nangal Road, Rupnagar, 140001, Punjab, India.
Department of Chemistry, University of Michigan, 930 N. University Ave., Ann Arbor, MI, 48109-1055, USA.
Solid State Nucl Magn Reson. 2019 Feb;97:40-45. doi: 10.1016/j.ssnmr.2018.12.002. Epub 2018 Dec 28.
The chemical shift anisotropy (CSA) interaction of a nucleus is an important indicator of the local electronic environment particularly for the contributions arising from hydrogen (H)-bonding, electrostatic and π-π interactions. CSAs of protons bonded to nitrogen atoms are of significant interest due to their common role as H-bonding partners in many chemical, pharmaceutical and biological systems. Although very fast (∼100 kHz) magic angle sample spinning (MAS) experiments have enabled the measurement of proton CSAs directly from solids, due to a narrow chemical shift (CS) distribution, overlapping NH proton resonances are common and necessitate the introduction of an additional frequency dimension to the regular 2D H CSA/H CS correlation method to achieve sufficient resolution. While this can be accomplished by using the isotropic shift frequency of N or N nuclei, the use of the naturally-abundant N nucleus avoids N isotopic labeling and therefore would be useful for a variety of solids. To this end, we propose a proton-detected 3D H CSA/N/H CS correlation method under fast MAS (90 kHz) to determine the CSA tensors of NH protons in samples without isotopic enrichment. Our experimental results demonstrate that the proposed 3D NMR experiment is capable of resolving the overlapping H resonances of amide (NH) groups through the N isotropic shift frequency dimension and enables the accurate measurement of site-specific H CSAs directly from powder samples under fast MAS conditions. In addition to the 3D H CSA/N/H CS experiment, an approach employing N-edited 2D H CSA/H CS experiment is also demonstrated as an additional means to address spectral overlap of NH resonances with aliphatic and other proton resonances in solids.
原子核的化学位移各向异性（CSA）相互作用是局部电子环境的重要指标，特别是对于氢（H）键合、静电和π-π相互作用的贡献。与氮原子键合的质子的 CSA 非常重要，因为它们在许多化学、制药和生物系统中作为 H 键合伙伴的常见作用。尽管非常快（约 100 kHz）的魔角旋转（MAS）实验能够直接从固体中测量质子 CSA，但由于 CS 分布较窄，重叠的 NH 质子共振很常见，需要在常规 2D H CSA/H CS 相关方法中引入附加的频率维度，以实现足够的分辨率。虽然可以通过使用 N 或 N 核的各向同性位移频率来实现，但使用天然丰度的 N 核可以避免 N 同位素标记，因此对于各种固体都很有用。为此，我们提出了一种在快速 MAS（90 kHz）下质子检测的 3D H CSA/N/H CS 相关方法，以确定未进行同位素富集的样品中 NH 质子的 CSA 张量。我们的实验结果表明，所提出的 3D NMR 实验能够通过 N 各向同性位移频率维度解析酰胺（NH）基团的重叠 H 共振，并能够在快速 MAS 条件下直接从粉末样品中准确测量特定位置的 H CSA。除了 3D H CSA/N/H CS 实验外，还演示了一种采用 N 编辑的 2D H CSA/H CS 实验的方法，作为解决固体中 NH 共振与脂肪族和其他质子共振重叠的另一种手段。
