Advanced mass spectrometry techniques for identification, characterisation and structural elucidation of natural products

Abstract

Mass-based structural identification of natural products frequently employs combinations of different mass spectrometers, including triple quadrupole and ion trap instruments. These methods often provide adequate information for tentatively identifying structures of analogues of previously characterized compounds. For completely unknown compound, tentatively identified structures are often further examined by high-resolution MS, from which elemental compositions can be obtained. Here it is required to measure the mass-to-charge ratios with the smallest mass uncertainties possible. Ideally, routines with sufficient resolution and mass accuracy yield single empirical formulae for every compound analysed. In the absence of a unique empirical formula, analysis can be highly ambiguous. In most studies, quadrupole TOF and Fourier-transform MS (orbitrap, ICR) instruments are used. This presentation illustrates state-of-the-art techniques for screening and identifying natural products. (1) Characterization of hyperforin in extracts of St. John's wort (Hypericum perforatum) is illustrated. The strategy used was a combination of MS2 and MS3 experiments, performed on QqLIT and high-resolution FTICR instruments. This combination allowed fragmentation schemes and chemical structures to be proposed. (2) Analysis of Boswellia from the Boswellia serrata tree is demonstrated. Specific active anti-inflammatory ingredients in this herb have been identified, boswellic acids. In animal studies, these acids have been shown to significantly reduce inflammation. We are interested in the metabolism, as the clinical efficiency appears high despite the low observed plasma levels, possibly involving unknown metabolites. (3) Characterization of active species in marine toxin extracts is shown. Analysis of phytoplankton allowed for detection of several previously characterized toxins as well as new unexpected compounds. Purification of crude extracts into several fractions using mass-triggered preparative chromatography permitted enrichment of different species, resulting in high quality mass spectra for all known and unknown species