Nuclear Magnetic Resonance of Small Molecules in Natural Products

Abstract

This chapter provides basic guidance to understand nuclear magnetic resonance (NMR) spectroscopy in the context of natural product analysis. It is intended to provide the reader an introduction to the field and also to the following chapters of this handbook. The most important 1D‐ and 2D‐NMR experiments are described; pitfalls of the structure elucidation process are sketched.

NMR-Based Chromatography Readouts: Indispensable Tools to “Translate” Analytical Features into Molecular Structures

“…Due to principally different acquisition modes, NMR experiments can be classified by their dimensionality. In one-dimensional NMR spectroscopy (1D-NMR), NMR signals are recorded for a specific NMR active nucleus, in natural product research, usually 1 H or 13 C, rarely 15 N or 31 P. One-dimensional NMR spectra show NMR signals with all the details outlined above, but do not allow to deduce relationships between the nuclei directly from the spectrum. In two-dimensional NMR spectroscopy (2D-NMR), spin-spin correlations can be observed and made visible via plotting spin-spin correlations in a two-dimensional contour plot.…”

“…However, for a more detailed insight into the coupling network of the protons, at least homonuclear 2D shift correlation spectra must be recorded. If one wants to gain additional knowledge about the neighboring carbon centers (C-H pairs) correlating with the protons or to establish 1 H-13 C- 13 C coupling networks, heteronuclear 2D shift correlation spectra must be recorded [15].…”

“…NMR signals are signals from individual atoms of the molecule and depend on both the observed nuclear species and the molecular environment of the observed atom (“chemical shift”). The signal intensities are directly proportional to the relative number of observed nuclei, and the neighboring nuclei in scalar networks are correlated by the effect of quantum coherence (“scalar coupling”) [ 15 ].…”

“…The purpose of this review article was to demonstrate that while it is possible to use mass spectrometry and similar methods to differentiate, group, and often assign the differentiating variables to entities that can be recognized as single molecules, the structural characterization of these putative biomarkers usually requires the use of NMR spectroscopy. This overview is based on three previous publications by the authors [ 10 , 15 , 16 ]. Based on the findings reported therein, some of which are repeated in Section 3 , Section 4 and Section 5 of this review, the readership is informed about the latest developments in this field.…”

Combining HPLC-DAD-QTOF-MS and HPLC-SPE-NMR to Monitor In Vitro Vitetrifolin D Phase I and II Metabolism

“…Therefore, the HR-MS-based labeling of chromatographic features as putative metabolites of a parent substance must be combined with a technology whose main application is the structural characterization of unknown organic molecules in solution—NMR spectroscopy is such a method with well-understood possibilities and limitations [ 19 ]. Besides its unquestioned use in plant science for structure elucidation [ 20 ], it is a major cornerstone of the dereplication of mixtures, as encountered in metabolomics, metabolism studies, or bioactivity-guided fractionation efforts [ 21 ].…”