Untargeted identification of alkyne containing natural products using ruthenium catalyzed azide alkyne cycloaddition reactions coupled to LC-MS/MS.

Alkyne containing natural products have identified from plants, insects, algae, fungi, and bacteria. This class of natural products have been characterized as having a variety of biological activities. Polyynes are a sub-class of acetylenic natural products that contain conjugated alkynes and are underrepresented in natural product databases due to the fact that they decompose during purification. Here we report a workflow that utilizes alkyne azide cycloaddition (AAC) reactions, using ruthenium or copper catalysts, followed by LC-MS/MS analysis to identify acetylenic natural products. In this report we demonstrate that AAC reactions with p-bromobenzyl azide resulted in p-bromobenzyl substituted triazole products that fragment to a common brominated tropylium ion. We were able to identify a synthetic alkyne spiked into the extract of Anabaena sp. PCC 7120 at a concentration of 10 μg/ml after optimization of MS/MS conditions. We then successfully identified the known natural product fischerellin A in the extract of Fischerella muscicola PCC 9339. Lastly, we identified the recently identified natural products protegenin A and C from P. protegens through a combination of genome mining and RuAAC reactions. We also compare CuAAC and RuAAC reactions and find that CuAAC reactions produce less by-products compared to RuAAC, but is limited to terminal alkyne containing compounds. We believe that both CuAAC and RuAAC reactions coupled to LC-MS/MS represent a method for the untargeted identification of acetylenic natural products.