Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK.
Department of Physics, University of Warwick, Coventry CV4 7AL, UK.
Phys Chem Chem Phys. 2022 Aug 31;24(34):20258-20273. doi: 10.1039/d2cp01041k.
The Lee-Goldburg condition for homonuclear decoupling in H magic-angle spinning (MAS) solid-state NMR sets the angle , corresponding to arctan of the ratio of the rf nutation frequency, , to the rf offset, to be the magic angle, , equal to tan(√2) = 54.7°. At 60 kHz MAS, we report enhanced decoupling compared to MAS alone in a H spectrum of N-glycine with at = 30° for a of ∼100 kHz at a H Larmor frequency, , of 500 MHz and 1 GHz, corresponding to a high chemical shift scaling factor () of 0.82. At 1 GHz, we also demonstrate enhanced decoupling compared to 60 kHz MAS alone for a lower of 51 kHz, , a case where the nutation frequency is less than the MAS frequency, with = 18°, = 0.92. The ratio of the rotor period to the decoupling cycle time, = , is in the range 0.53 to 0.61. Windowed decoupling using the optimised parameters for a of ∼100 kHz also gives good performance in a H spin-echo experiment, enabling implementation in a H-detected N-H cross polarisation (CP)-refocused INEPT heteronuclear correlation NMR experiment. Specifically, initial N transverse magnetisation as generated by H-N CP is transferred back to H using a refocused INEPT pulse sequence employing windowed H decoupling. Such an approach ensures the observation of through-bond N-H connectivities. For N-glycine, while the CP-refocused INEPT experiment has a lower sensitivity (∼50%) as compared to a double CP experiment (with a 200 μs N to H CP contact time), there is selectivity for the directly bonded NH moiety, while intensity is observed for the CHH resonances in the double CP experiment. Two-dimensional N-H correlation MAS NMR spectra are presented for the dipeptide β-AspAla and the pharmaceutical cimetidine at 60 kHz MAS, both at natural isotopic abundance. For the dipeptide β-AspAla, different build-up dependence on the first spin-echo duration is observed for the NH and NH moieties demonstrating that the experiment could be used to distinguish resonances for different NH groups.
李-戈尔登堡条件（Lee-Goldburg condition）用于同核去耦在 H 魔角旋转（MAS）固态 NMR 中，设定角度 ，对应于射频进动频率 的射频偏移 的正切，等于魔角 ，等于 tan(√2) = 54.7°。在 60 kHz MAS 下，我们报告了与单独 MAS 相比，在 H 谱中 N-甘氨酸的增强去耦，在 = 30°时，在 H 拉莫尔频率为 500 MHz 和 1 GHz 时， 为 ∼100 kHz，对应于高化学位移标度因子 () 的 0.82。在 1 GHz 下，我们还证明了与单独 60 kHz MAS 相比，在较低的 = 51 kHz 时也有增强的去耦，在这种情况下，进动频率小于 MAS 频率， = 18°， = 0.92。转子周期与去耦周期时间的比值， = ，在 0.53 到 0.61 之间。使用优化的参数进行窗口去耦，在 ∼100 kHz 时也能在 H 自旋回波实验中获得良好的性能，从而能够在 H 检测的 N-H 交叉极化（CP）重聚焦 INEPT 异核相关 NMR 实验中实现。具体来说，通过 H-N CP 产生的初始 N 横向磁化通过使用窗口化 H 去耦的重聚焦 INEPT 脉冲序列转移回 H。这种方法确保了通过键的 N-H 连接性的观测。对于 N-甘氨酸，虽然 CP 重聚焦 INEPT 实验的灵敏度（∼50%）比双 CP 实验（N 到 H CP 接触时间为 200 μs）低，但它对直接键合的 NH 部分具有选择性，而在双 CP 实验中观察到 CHH 共振的强度。在 60 kHz MAS 下，分别在天然同位素丰度下，为二肽 β-AspAla 和药物西咪替丁呈现二维 N-H 相关 MAS NMR 谱。对于二肽 β-AspAla，观察到 NH 和 NH 部分的第一个自旋回波持续时间的不同积累依赖性，证明该实验可用于区分不同 NH 基团的共振。
