Dereplication of norlichexanthones in lichen extracts: LC/MS vs NMR

Xanthones are secondary metabolites derivating from the 9H-xanthen-9-one scaffold produced by a wide range of plant, bacterial and fungal species. In lichens, they occur mostly as members of two classes: lichexanthones and norlichexanthones, differing by the methylation of positions 3 and 5 for lichexanthones. Chlorination can occur on positions 2, 4, 5 and 7, resulting in a total of sixteen different norlichexanthones for only five different exact masses. This high number of isomers makes them tedious to identify. High resolution tandem mass spectrometry is of no help without HLPC separation combined to standard compounds availability, and even NMR on pure compounds could be tricky, due to their high proton- deficiency.

However, they are of chemotaxonomic interest for lichenologists to identify crustaceous lichens with very similar morphology, such as some species of Lecanora.

We therefore constituted a library of the sixteen norlichexanthones, which were obtained by a combined strategy of synthesis and isolation. An HPLC/MS method was set up to separate them efficiently by their m/z and retention times ([Fig. 1a]). In parallel, we acquired NMR data to perform the dereplication of these compounds in lichen extracts, based on the HSQC correlations observed for the non-chlorinated positions ([Fig. 1b]).

Fig. 1 Norlichexanthones in lichens: two complementary dereplication strategies.

While LC/MS is sensitive and easy to interpret, NMR is robust and highly reproducible. A combination of both methods provides a good overview of the xanthone content in lichens.

 

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Artikel online veröffentlicht:
16. November 2023