Dereplication of Microbial Natural Products by LC-DAD-TOFMS

Dereplication, the rapid identification of known compounds
present
in a mixture, is crucial to the fast discovery of novel natural products.
Determining the elemental composition of compounds in mixtures and
tentatively identifying natural products using MS/MS and UV/vis spectra
is becoming easier with advances in analytical equipment and better
compound databases. Here we demonstrate the use of LC-UV/vis-MS-based
dereplication using data from UV/vis diode array detection and ESI+/ESI– time-of-flight MS for assignment of
719 microbial natural product and mycotoxin reference standards. ESI+ was the most versatile ionization method, detecting 93% of
the compounds, although with 12% ionizing poorly. Using ESI+ alone, 56.1% of the compounds could be unambiguously assigned based
on characteristic patterns of multiple adduct ions. Using ESI–, 36.4% of the compounds could have their molecular
mass assigned unambiguously using multiple adduct ions, while a further
41% of the compounds were detected only as [M – H]−. The most reliable interpretations of conflicting ESI+ and ESI– data on a chromatographic peak were from
the ionization polarity with the most intense ionization. Poor ionization
was most common with small molecules (<200 Da). In ESI–, these were often polar and basic, while in ESI+ they
were small aromatic acids or anthraquinones. No single ion-source
settings could be applied over a m/z 60–2000 range. However, continuous switching among three
settings (e.g., for 0.5 s each) during the chromatographic run allowed
MS of both small labile molecules and large peptides, and pseudo MS/MS
data on labile molecules since the settings for large molecules often
induce fragmentation into small molecules.