Chapter 5:Application of LC-NMR to the Study of Natural Products

NMR spectroscopy has been applied for many years to perform structure elucidation on pure compounds. Therefore, prior to NMR analysis, it was necessary to separate mixtures by means of extraction and preparative chromatography. Such procedures required large amounts of material and a chromatographic separation good enough to produce a more or less pure compound, a situation that often needed multiple chromatographic steps. In addition, NMR sensitivity required milligram amounts in order to be able to run 2D heteronuclear experiments, the cornerstone of structure elucidation. Over the years, NMR sensitivity has been enhanced by improving probehead technology and increasing magnetic field strength. With the introduction of cryogenic probes, a major enhancement in signal-to-noise ratio was achieved, commonly a factor of 4 in most solvents used. It is now possible to run the relevant experiments for structure elucidation by NMR in the low microgram range. This chapter shows how LC-NMR can be integrated into the structure verification and identification of natural product mixtures. It describes various tools that allow the increase of NMR sensitivity in such a way that <1 μg components in the active volume can be accessed by NMR. Additionally, this chapter provides examples where these tools can be applied to natural product-related samples.