Integrative Approach Based on Leaf Spray Mass Spectrometry, HPLC-DAD-MS/MS, and NMR for Comprehensive Characterization of Isoquinoline-Derived Alkaloids in Leaves of Onychopetalum amazonicum R. E. Fr.

Onychopetalum is an Amazonian botanical genus close-related with Bocageopsis and Unonopsis, whose species are used in traditional medicine. Although these genera are widely investigated from the phytochemical viewpoint, the knowledge about Onychopetalum remains almost unexplored. In order to perform a comprehensive characterization of isoquinoline-derived alkaloids from Onychopetalum amazonicum, an integrative approach based on the alkaloid profile analysis by leaf spray mass spectrometry (LS-MS), followed by a robust dereplication through high-performance liquid chromatography coupled with diode array detector and atmospheric pressure chemical ionization tandem mass spectrometry, (HPLC-DAD-APCI-MS/MS) and nuclear magnetic resonance (NMR) analysis was adopted. LS-MS allowed the tentative identification of reticuline, norjuziphine, nornuciferine, O-methylisopiline, nuciferine, N-methyl-O-methylisopiline, anonaine, and N-methylanonaine. HPLC-DAD-MS/MS analysis supported the tentative identification of N-methylasimilobine, asimilobine, norushinsunine, N-methylisopiline, isopiline, liriodenine, and lysicamine. Moreover, the integration of MS and NMR data allowed the additional identification of stepholidine and isocorypalmine. The applied integrative approach proposed herein enabled a comprehensive analysis of the alkaloid content from the O. amazonicum, showing to be a useful strategy for the dereplication of isoquinoline-derived alkaloids.