Sensitivity of NMR-based metabolomics in drug discovery from medicinal plants

One of the main limitations of NMR based metabolomic analysis is its low sensitivity. The main objective of this study was to determine the sensitivity of the standard NMR based metabolomics protocol (as published in Nature Protocols) for the analysis of plant samples. To test this limitation, we prepared two sample sets from a well-known medicinal plant. Sample set one was prepared from one plant specimen, subdivided into 24 chemically equivalent samples, spiked with different concentrations of rutin and analysed on 300, 400 and 500 MHz NMR spectrometers. Sample set two was prepared from four different plant specimens of the same species reflecting the natural variation between medicinal plants. To two of these samples different concentrations of artemisinin were added, acting as our hypothetical active samples, whereas the other two samples acted as our inactive samples. Our results indicated that there was no difference in the sensitivity between the three NMR spectrometers and that the standard protocol can differentiate between samples at a spiking level of 0.2 mg/mL of rutin (328 µm). The second sample set gave differentiation at 0.05 mg/mL (177 µm) but a significant movement in the chemical shifts of artemisinin was observed. Our study demonstrated that the sensitivity of the current NMR based metabolomics protocol is not due to instrumental limitations but rather due to methodological limitations. In the same way that binning of spectra negates the better resolution of higher field magnets the same appears to be true by employing PCA analysis to spectra which effectively negates the higher sensitivity of higher field magnets. Our study also highlights that compounds can display significant movement in chemical shifts depending on its chemical environment, which can complicate identification by database comparisons. We would like to invite the NMR metabolomics community to repeat this analysis in order to confirm this finding so that the current limitations of the NMR based metabolomics protocol can be defined. This needs to be done in order to develop improved NMR metabolomics protocols.