Dereplication of antifungal compounds from small-molecule natural product libraries by LC-MS and NMR

We present a dereplication strategy using a combination of LC-MS and ¹H NMR to facilitate compound identification towards antifungal natural product discovery. This analytical approach takes advantage of the simplicity of semi-purified column fractions (CFs) of plant extracts, which contain only a few structurally similar analogs representing chromatographically tractable small-molecule natural product compounds, thus making it easy to interpret the LC-MS and ¹H NMR data for structure identification. Antifungal screening of > 14,000 CFs afforded 12 samples (each derived from one plant) that were particularly active against the clinically important fungal pathogen Cryptococcus neoformans. Analysis of the LC-MS and ¹H NMR data of these CFs led to the identification of the known antifungal compounds including alkaloids, flavonoids, aromatic amides, saponins, polyacetylenes, and butanolides as well as the prediction of new antifungal diterpenoids. It was noted that ¹H NMR played an important role in complementing LC-MS to rapidly characterize the chemotype and structure of the compounds in the CFs. The dereplication analysis was confirmed by follow-up isolation and subsequent antifungal testing of individual compounds.