Fragmentation pathways of oxygenated tetracyclic triterpenoids and their application in the qualitative analysis of Ganoderma lucidum by multistage tandem mass spectrometry

Abstract

The fragmentation pathways of oxygenated tetracyclic triterpenoids from Ganoderma lucidum were systematically studied based on interpreting the mass spectra of 44 known triterpenoids using a combination of multistage tandem mass spectrometry (MS(n)) experiments and high-resolution mass spectrometry (HRMS) analysis. In negative ion mode, the fragmentation pathways of triterpenoid acids are rather characteristic. After the prominent loss of H(2) O or CO(2), cleavages take place on the A, B, C and D rings. Interestingly, the cleavage mode is highly dependent on the positions of the carbonyl groups and hydroxyl groups in the tetracyclic skeleton. Characteristic cleavage of ring A occurs in 7-oxo-11-H or 7-oxo-11-hydroxy derivatives; characteristic cleavage of ring B occurs in the 7-oxo-11-hydroxy derivatives; characteristic cleavage of ring C occurs in the 7-hydroxy-15-oxo derivatives; while the cleavage of ring D can be observed in the majority of the compounds investigated. The odd-electron species, which disobey the 'even-electron rule', are also observed and discussed in this paper. These phenomena provide an easy way to determine the tetracyclic skeleton and distinguish the isomers of the triterpenoids from each other. What is more, the fragmentation pathways of triterpenoid alcohols were also investigated in positive ion mode. The accurate masses of the product ions were determined using quadrupole orthogonal time-of-flight (QTOF) instruments. Finally, the fragmentation rules were applied to identify the components of G. lucidum. As a result, 73 triterpenoids including 11 new ones were identified. The triterpenoids were classified into six subclasses according to their different fragmentation behaviors. The application of tandem mass spectrometry was further explored.