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基于交叉偏振的超快 MAS 条件下旋转框架分离局域场 NMR 实验。

A cross-polarization based rotating-frame separated-local-field NMR experiment under ultrafast MAS conditions.

机构信息

Biophysics and Department of Chemistry, The University of Michigan, Ann Arbor, MI 48109-1055, United States.

Center for Insoluble Protein Structures (inSPIN), Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, Aarhus University, DK-8000 Aarhus C, Denmark.

出版信息

J Magn Reson. 2015 Jan;250:37-44. doi: 10.1016/j.jmr.2014.10.013. Epub 2014 Nov 15.

Abstract

Rotating-frame separated-local-field solid-state NMR experiments measure highly resolved heteronuclear dipolar couplings which, in turn, provide valuable interatomic distances for structural and dynamic studies of molecules in the solid-state. Though many different rotating-frame SLF sequences have been put forth, recent advances in ultrafast MAS technology have considerably simplified pulse sequence requirements due to the suppression of proton-proton dipolar interactions. In this study we revisit a simple two-dimensional (1)H-(13)C dipolar coupling/chemical shift correlation experiment using (13)C detected cross-polarization with a variable contact time (CPVC) and systematically study the conditions for its optimal performance at 60 kHz MAS. In addition, we demonstrate the feasibility of a proton-detected version of the CPVC experiment. The theoretical analysis of the CPVC pulse sequence under different Hartmann-Hahn matching conditions confirms that it performs optimally under the ZQ (w1H-w1C=±wr) condition for polarization transfer. The limits of the cross polarization process are explored and precisely defined as a function of offset and Hartmann-Hahn mismatch via spin dynamics simulation and experiments on a powder sample of uniformly (13)C-labeled L-isoleucine. Our results show that the performance of the CPVC sequence and subsequent determination of (1)H-(13)C dipolar couplings are insensitive to (1)H/(13)C frequency offset frequency when high RF fields are used on both RF channels. Conversely, the CPVC sequence is quite sensitive to the Hartmann-Hahn mismatch, particularly for systems with weak heteronuclear dipolar couplings. We demonstrate the use of the CPVC based SLF experiment as a tool to identify different carbon groups, and hope to motivate the exploration of more sophisticated (1)H detected avenues for ultrafast MAS.

摘要

旋转框架分离局部场固态 NMR 实验测量高分辨率的异核偶极耦合,这些偶极耦合反过来又提供了分子在固态中结构和动力学研究的宝贵原子间距离。尽管已经提出了许多不同的旋转框架 SLF 序列,但超快 MAS 技术的最新进展由于质子质子偶极相互作用的抑制,大大简化了脉冲序列的要求。在这项研究中,我们重新审视了一种简单的二维(1)H-(13)C 偶极耦合/化学位移相关实验,使用(13)C 检测交叉极化,具有可变接触时间(CPVC),并系统地研究了在 60 kHz MAS 下其最佳性能的条件。此外,我们还证明了 CPVC 实验质子检测版本的可行性。在不同的哈特曼-哈恩匹配条件下对 CPVC 脉冲序列的理论分析证实,在极化转移的 ZQ(w1H-w1C=±wr)条件下,它的性能最佳。通过对粉末样品的自旋动力学模拟和实验,精确地确定了交叉极化过程的限制,该粉末样品由均匀(13)C 标记的 L-异亮氨酸组成,其限制作为偏移和哈特曼-哈恩失配的函数。我们的结果表明,当在两个射频通道上都使用高射频场时,CPVC 序列的性能及其随后确定的(1)H-(13)C 偶极耦合对(1)H/(13)C 频偏频率不敏感。相反,CPVC 序列对哈特曼-哈恩失配非常敏感,特别是对于具有弱异核偶极耦合的系统。我们展示了基于 CPVC 的 SLF 实验作为一种工具来识别不同的碳基团,并希望激励探索更复杂的超快 MAS 中的(1)H 检测途径。

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