Shen Ming, Trébosc J, Lafon O, Pourpoint F, Hu Bingwen, Chen Qun, Amoureux J-P
Physics Department & Shanghai Key Laboratory of Magnetic Resonance, East China Normal University, Shanghai 200062, China; UCCS, University Lille North of France, Villeneuve d'Ascq 59652, France.
UCCS, University Lille North of France, Villeneuve d'Ascq 59652, France.
J Magn Reson. 2014 Aug;245:38-49. doi: 10.1016/j.jmr.2014.05.006. Epub 2014 May 21.
Connectivities and proximities between protons and low-gamma nuclei can be probed in solid-state NMR spectroscopy using two-dimensional (2D) proton-detected heteronuclear correlation, through Heteronuclear Multiple Quantum Correlation (HMQC) pulse sequence. The indirect detection via protons dramatically enhances the sensitivity. However, the spectra are often broadened along the indirect F1 dimension by the decay of heteronuclear multiple-quantum coherences under the strong (1)H-(1)H dipolar couplings. This work presents a systematic comparison of the performances of various decoupling schemes during the indirect t1 evolution period of dipolar-mediated HMQC (D-HMQC) experiment. We demonstrate that (1)H-(1)H dipolar decoupling sequences during t1, such as symmetry-based schemes, phase-modulated Lee-Goldburg (PMLG) and Decoupling Using Mind-Boggling Optimization (DUMBO), provide better resolution than continuous wave (1)H irradiation. We also report that high resolution requires the preservation of (1)H isotropic chemical shifts during the decoupling sequences. When observing indirectly broad spectra presenting numerous spinning sidebands, the D-HMQC sequence must be fully rotor-synchronized owing to the rotor-synchronized indirect sampling and dipolar recoupling sequence employed. In this case, we propose a solution to reduce artefact sidebands caused by the modulation of window delays before and after the decoupling application during the t1 period. Moreover, we show that (1)H-(1)H dipolar decoupling sequence using Smooth Amplitude Modulation (SAM) minimizes the t1-noise. The performances of the various decoupling schemes are assessed via numerical simulations and compared to 2D (1)H-{(13)C} D-HMQC experiments on [U-(13)C]-L-histidine⋅HCl⋅H2O at various magnetic fields and Magic Angle spinning (MAS) frequencies. Great resolution and sensitivity enhancements resulting from decoupling during t1 period enable the detection of heteronuclear correlation between aliphatic protons and ammonium (14)N sites in L-histidine⋅HCl⋅H2O.
使用二维（2D）质子检测异核相关，通过异核多量子相关（HMQC）脉冲序列，可以在固态核磁共振光谱中探测质子与低γ核之间的连接性和邻近性。通过质子进行间接检测可显著提高灵敏度。然而，在强（1）H - （1）H偶极耦合作用下，异核多量子相干的衰减常常会使光谱在间接F1维度上变宽。这项工作对双极介导的HMQC（D - HMQC）实验间接t1演化期间各种去耦方案的性能进行了系统比较。我们证明，在t1期间的（1）H - （1）H偶极去耦序列，如基于对称性的方案、相位调制的Lee - Goldburg（PMLG）和使用令人难以置信的优化去耦（DUMBO），比连续波（1）H照射提供更好的分辨率。我们还报告说，高分辨率要求在去耦序列期间保留（1）H各向同性化学位移。当间接观察呈现大量旋转边带的宽光谱时，由于采用了转子同步间接采样和偶极重新耦合序列，D - HMQC序列必须完全转子同步。在这种情况下，我们提出了一种解决方案，以减少在t1期间去耦应用前后窗口延迟调制引起的伪边带。此外，我们表明使用平滑幅度调制（SAM）的（1）H - （1）H偶极去耦序列可将t1噪声降至最低。通过数值模拟评估了各种去耦方案的性能，并与在不同磁场和魔角旋转（MAS）频率下对[U - （13）C] - L - 组氨酸·HCl·H2O进行的二维（1）H - {（13）C} D - HMQC实验进行了比较。在t1期间去耦带来的高分辨率和灵敏度增强使得能够检测L - 组氨酸·HCl·H2O中脂肪族质子与铵（14）N位点之间的异核相关性。
