Investigation of fragmentation pathways of norpimarane diterpenoids by mass spectrometry combined with computational chemistry.

RATIONALE: Norpimarane diterpenes possess plentiful bioactivities and are widely distributed in herbs, such as Flickingeria fimbriata. Rapid characterization of these natural products in complicated plant extracts is of great importance, and electrospray ionization tandem mass spectrometry is a powerful tool for chemical constituent profiling. However, limited researches on their fragmentation mechanisms seriously hinder identification via mass spectrometry. METHODS: Three norpimarane diterpenes isolated from F. fimbriata via multiple types of column chromatography served as reference compounds, and collision-induced dissociation experiments were performed on them with a series of variable collision energies. Plausible fragmentation pathways were proposed based on product ions. To further validate the fragmentation mechanisms, the proton affinity and product ion energy were simulated by density functional theory at the B3LYP/6-31+G(d, p) level. RESULTS: Three main cleavage reactions induced skeleton breakage and resulted in characteristic ions, methyl (CH3 -20) migration, hydrogen arrangement and Retro-Diels-Alder reaction, among which methyl migration was firstly proposed for pimarane diterpenes. A series of common diagnostic ions were identified, such as m/z 133.1012, 121.1012, 119.0805 and 107.0855. Additionally, the constructed fragmentation mechanisms were successfully applied for fragment ion rationalization of previously reported isopimarane diterpenes. CONCLUSIONS: Fragmentation mechanisms of norpimarane diterpenes have been uncovered. Carbocation located at the C ring tends to result in methyl (CH3 -20) migration which has been rarely reported before. This characteristic dissociation reaction allows multiple diagnostic ions to be rationalized and aids in rationalizing fragmentation patterns of other diterpenes. The uncovered mechanisms also shed light on rapid identification of norpimarane diterpenes.

Two New Alkaloids from Fuzi and Their Metabolites Study.

Former study suggests alkaloids from herbs of Aconitum genus plants possess excellent bioactivities, which exert great value for related deeper chemical constituent investigation. Herein, chemical isolation was performed and four alkaloids were isolated from Fuzi, of which two were new ones, and the other two were reported NMR data for the first time. Their chemical structures were identified by NMR data, high resolution MS, UV and IR analysis. Additionally, the MS fragmentation patterns were explored, formerly, that of hetisane alkaloid was rarely reported, and fragmentation mechanism of the diagnostic ion was proposed. Based on these fragment pathway, metabolites and metabolic pathways of four compounds were investigated in rat liver microsomes using UPLC-Q/TOF-MS, and dehydrogenation product was firstly found from metabolites of hetisane alkaloid.