NMR Science and Development Division, RIKEN SPring-8 Center, and Nano-Crystallography Unit, RIKEN-JEOL Collaboration Center, Yokohama, Kanagawa 230-0045, Japan.
Department of Chemistry and NIS Centre, University of Torino, V.P. Giuria 7, 10125, Italy.
J Magn Reson. 2019 Nov;308:106559. doi: 10.1016/j.jmr.2019.07.046. Epub 2019 Jul 13.
The combination of a phase-modulated (PM) saturation pulse and symmetry-based dipolar recoupling into a rotational-echo saturation-pulse double-resonance (RESPDOR) sequence has been employed to measure H-N distances. Such a measurement is challenging owing to the quadrupolar interaction of N nucleus and the intense H-H homonuclear dipolar interactions. Thanks to the recent advances in probe technology, the homonuclear dipolar interaction can be sufficiently suppressed at a fast MAS frequency (ν ≥ 60 kHz). PM pulse is robust to large variations of parameters on quadrupolar spins, but it has not been demonstrated under very fast MAS conditions. On the other hand, the RESPDOR sequence is applicable to such condition when it employs symmetry-based pulses during the recoupling period, but a prior knowledge on the system is required. In this article, we demonstrated the PM-RESPDOR combination for providing accurate H-N distances at a very fast MAS frequency of 70 kHz on two samples, namely L-tyrosine⋅HCl and N-acetyl-L-alanine. This sequence, supported by simulations and experiments, has shown its feasibility at ν = 70 kHz as well as the robustness to the N quadrupolar interaction. It is applicable to a wide range of H-N dipolar coupling constants when a radio frequency field on the N channel is approximately 80 kHz or more, while the PM pulse length lasts 10 rotor periods. For the first time, multiple H-N heteronuclear dipolar couplings, thus multiple quantitative distances, are simultaneously and reliably extracted by fitting the experimental fraction curves with the analytical expression. The size of the H-N dipolar interaction is solely used as a fitting parameter. These determined distances are in excellent agreement with those derived from diffraction techniques.
采用相位调制（PM）饱和脉冲和基于对称的偶极子重聚组合成旋转回波饱和脉冲双共振（RESPDOR）序列，用于测量 H-N 距离。由于 N 核的四极相互作用和强烈的 H-H 同核偶极相互作用，这种测量具有挑战性。由于探头技术的最新进展，可以在快速 MAS 频率（ν≥60 kHz）下充分抑制同核偶极相互作用。PM 脉冲对四极自旋的参数变化具有很强的鲁棒性，但在非常快速的 MAS 条件下尚未得到证明。另一方面，当 RESPDOR 序列在重聚期间采用基于对称的脉冲时，它适用于这种条件，但需要对系统有先验知识。在本文中，我们在两个样品（即 L-酪氨酸·HCl 和 N-乙酰-L-丙氨酸）上证明了 PM-RESPDOR 组合在非常快的 MAS 频率 70 kHz 下提供准确的 H-N 距离的能力。该序列通过模拟和实验得到支持，证明了在 ν=70 kHz 下的可行性以及对 N 四极相互作用的鲁棒性。当 N 通道上的射频场约为 80 kHz 或更大，并且 PM 脉冲长度持续 10 个转子周期时，它适用于广泛的 H-N 偶极耦合常数。首次通过将实验分数曲线与解析表达式拟合，同时可靠地提取多个 H-N 异核偶极耦合，从而提取多个定量距离。H-N 偶极相互作用的大小仅用作拟合参数。这些确定的距离与来自衍射技术的距离非常吻合。
