Structure Elucidation Group, Process and Analytical Research and Development, Merck Research Laboratories, Rahway, NJ, USA.
Analytical Research and Development, Pfizer Worldwide Research and Development, Groton, CT, USA.
Nat Protoc. 2019 Jan;14(1):217-247. doi: 10.1038/s41596-018-0091-9.
The use of anisotropic NMR data, such as residual dipolar couplings (RDCs) and residual chemical shift anisotropies (RCSAs), has emerged as a powerful technique for structural characterization of organic small molecules. RDCs typically report the relative orientations of different H-C bonds; RCSAs report the relative orientations of different carbon chemical shielding tensors and hence are more useful for proton-deficient molecules. This information is complementary to that obtained from conventional NMR data such as J couplings, isotropic chemical shifts, and nuclear Overhauser effects (NOEs)/rotational frame nuclear Overhauser effects (ROEs). Obtaining anisotropic NMR data requires the creation of an anisotropic sample environment through an alignment medium. Here, we focus on the use of compressed or stretched polymeric gels as two different but fundamentally equivalent methods for introducing sample anisotropy. Protocols are provided for the synthesis of the chloroform-compatible poly(methyl methacrylate) and dimethyl sulfoxide (DMSO)-compatible poly(2-hydroxyethyl methacrylate) gels and sample setup with a preparation time of 2-3 d. The bond-specific RDC data and the atom-specific RCSA data are extracted as changes in H-C couplings and C chemical shifts, respectively, between two measurements under different alignment conditions, with a total experimental time of 0.5-4 d. NMR data acquisition and important considerations are described in detail. We also provide step-by-step procedures for the density functional theory (DFT) calculations involved and data analysis using the commercial software MSpin. We use three example compounds, namely cryptospirolepine (505 Da), retrorsine (351 Da), and estrone (270 Da), to demonstrate some important aspects of the workflow, such as input data preparation, handling of structural flexibility, and RCSA data correction when necessary.
各向异性 NMR 数据(如残余偶极耦合(RDC)和残余化学位移各向异性(RCSA))的应用已成为有机小分子结构特征描述的有力技术。RDC 通常报告不同 H-C 键的相对取向;RCSA 报告不同碳化学屏蔽张量的相对取向,因此对于质子缺乏的分子更有用。这些信息与从常规 NMR 数据(如 J 耦合、各向同性化学位移和核 Overhauser 效应(NOE)/旋转框架核 Overhauser 效应(ROE))获得的信息互补。获得各向异性 NMR 数据需要通过取向介质创建各向异性样品环境。在这里,我们专注于使用压缩或拉伸聚合物凝胶作为引入样品各向异性的两种不同但基本等效的方法。提供了氯仿相容的聚(甲基丙烯酸甲酯)和二甲基亚砜(DMSO)相容的聚(2-羟乙基甲基丙烯酸酯)凝胶的合成方案以及具有 2-3 天制备时间的样品设置。键特异性 RDC 数据和原子特异性 RCSA 数据分别提取为在不同取向条件下两次测量之间 H-C 偶合和 C 化学位移的变化,总实验时间为 0.5-4 天。详细描述了 NMR 数据采集和重要注意事项。我们还提供了涉及的密度泛函理论(DFT)计算的分步程序以及使用商业软件 MSpin 进行数据分析。我们使用三个示例化合物,即 cryptospirolepine(505 Da)、retrorsine(351 Da)和雌酮(270 Da),来说明工作流程的一些重要方面,例如输入数据准备、结构灵活性处理以及必要时的 RCSA 数据校正。