Solid-state NMR spectra of amino acid enantiomers and their relative intensities

IF 1.8 3区 化学 Q4 CHEMISTRY, PHYSICAL
Audrey-Anne Lafrance, Manon Girard, David L. Bryce
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Abstract

Under normal experimental conditions in an achiral environment, NMR spectra of enantiomers have chemical shifts and J couplings which are not differentiable. In this work, the reproducibility of spectral intensities for pairs of amino acid enantiomers, as well as factors influencing these intensities, is assessed using 13C and 15N cross-polarization magic-angle spinning (CP/MAS) NMR spectroscopy. Prompted by a recent literature debate over a possible influence of the chirality-induced spin selectivity (CISS) effect on spectral intensities obtained in CP/MAS NMR experiments carried out on enantiomers, a number of control experiments were performed with recycle delays of at least 5T1. These included the analysis of proton-decoupled Bloch decay solid-state NMR spectra as well as solution NMR spectra where the cross polarization process is absent. Bloch decay and CP/MAS NMR spectra yield the same relative intensities for pairs of enantiomers while solution NMR spectra provide relative intensities closest to unity. Differences of plus-or-minus a few percent in the D/L spectral intensity ratios observed in all solid-state NMR experiments are due to sample preparation (i.e., grinding, particle size, partial amorphization) and limitations on sample purity. As previously described in the literature, more drastic intensity differences on the order of 50% are easily created by ball milling the samples. Finally, apodization is shown to invert the apparent D/L ratio in low signal-to-noise 15N CP/MAS NMR spectra of aspartic acid enantiomers. In summary, no spectral intensity differences attributable to enantiomerism are identified.

Abstract Image

Abstract Image

氨基酸对映体的固态 NMR 光谱及其相对强度
在非手性环境中的正常实验条件下,对映体的 NMR 光谱具有不可微分的化学位移和耦合。在这项研究中,我们使用 C 和 N 跨偏振魔角旋转 (CP/MAS) NMR 光谱评估了氨基酸对映体光谱强度的可重复性以及影响这些强度的因素。最近有文献讨论手性诱导的自旋选择性(CISS)效应可能会影响对映体进行 CP/MAS NMR 实验时获得的光谱强度,在这种讨论的推动下,我们进行了一系列至少循环延迟 5 次的对照实验。其中包括分析质子去耦的布洛赫衰变固态 NMR 图谱以及不存在交叉极化过程的溶液 NMR 图谱。布洛赫衰变和 CP/MAS NMR 图谱可得出成对对映体的相同相对强度,而溶液 NMR 图谱可得出最接近于统一的相对强度。在所有固态 NMR 实验中观察到的 D/L 光谱强度比的正负百分之几的差异是由于样品制备(即研磨、粒度、部分非晶化)和样品纯度的限制造成的。正如之前的文献所述,通过球磨样品很容易产生 50%数量级的更剧烈的强度差异。最后,在天冬氨酸对映体的低信噪比 N CP/MAS NMR 光谱中,远峰化可反转表观 D/L 比值。总之,没有发现可归因于对映体的光谱强度差异。
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来源期刊
CiteScore
5.30
自引率
9.40%
发文量
42
审稿时长
72 days
期刊介绍: The journal Solid State Nuclear Magnetic Resonance publishes original manuscripts of high scientific quality dealing with all experimental and theoretical aspects of solid state NMR. This includes advances in instrumentation, development of new experimental techniques and methodology, new theoretical insights, new data processing and simulation methods, and original applications of established or novel methods to scientific problems.
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