Identification of Oxygenated Fatty Acid as a Side Chain of Lipo-Alkaloids in Aconitum carmichaelii by UHPLC-Q-TOF-MS and a Database

Lipo-alkaloid is a kind of C19-norditerpenoid alkaloid usually found in Aconitum species. Structurally, they contain an aconitane skeleton and one or two fatty acid moieties of 3-25 carbon chains with 1-6 unsaturated degrees. Analysis of the lipo-alkaloids in roots of Aconitum carmichaelii resulted in the isolation of six known pure lipo-alkaloids (A1-A6) and a lipo-alkaloid mixture (A7). The mixture shared the same aconitane skeleton of 14-benzoylmesaconine, but their side chains were determined to be 9-hydroxy-octadecadienoic acid, 13-hydroxy-octadecadienoic acid and 10-hydroxy-octadecadienoic acid, respectively, by MS/MS analysis after alkaline hydrolysis. To our knowledge, this is the first time of the reporting of the oxygenated fatty acids as the side chains in naturally-occurring lipo-alkaloids. In order to identify more lipo-alkaloids, a compound database was established based on various combinations between the aconitane skeleton and the fatty acid chain, and then, the identification of lipo-alkaloids was conducted using the database, UHPLC-Q-TOF-MS and MS/MS. Finally, 148 lipo-alkaloids were identified from A. carmichaelii after intensive MS/MS analysis, including 93 potential new compounds and 38 compounds with oxygenated fatty acid moieties.