Analysis of the lithiated leucosceptroids from Leucosceptrum canum to facilitate their identification and differentiation by electrospray ionization tandem mass spectrometry.

Abstract

Leucosceptroids, isolated from Leucosceptrum canum (L.canum), are sesterterpenoids with novel molecular scaffolds which possess potent antifeedant activities. Their molecular scaffolds comprise a 5,6,5-framework with a great diversity due to different hydroxylation and substituent positions. The biological activities of leucosceptroids are affected by the subtle structural differences. The structural characterization and differentiation of the leucosceptroid isomers are of great importance. Firstly, different kinds of cation adducts of leucosceptroids were examined by adding alkali metal ions to the solution, and the lithiated adducts of leucosceptroid were found to be readily formed and yielded characteristic fragment ions under collision-induced dissociation (CID). Then, a systematic mass spectrometric investigation of the [M+Li](+) ions was carried out to clarify their characteristic fragment pathways by electrospray ionization quadrupole time-of-flight-type tandem mass spectrometry (ESI-QTOF-MS/MS). The proposed fragmentation pathways were confirmed through ion trap ESI-MS(n) (n≥3) spectra. Finally, the proposed MS/MS method was applied to investigate the extracts of L.canum. A specific fragmentation pathway of the lithiated adduct, which leads to the production of diagnostic ions of leucosceptroids, was observed. This fragmentation is initiated by cleavage of the C-ring and leads to formation of two types of ions by further dissociation. Both pathways could yield characteristic fragment ions, which could be used to define the substituents at the skeletal structure and at the C-ring. The representative characteristic fragmentations of [M+Li](+) ions and the proposed fragmentation pathways were successfully adopted to investigate the L.canum extracts, and a total of eleven leucosceptroids were identified or tentatively characterized. The characteristic fragmentation pathways of [M+Li](+) species of leucosceptroid isomers were proposed. Three types of leucosceptroid isomers were successfully differentiated. Eleven leucosceptroids were characterized from L.canum extracts. The fragmentation knowledge will facilitate the analysis of leucosceptroids and other sesterterpenoids in future research. Copyright © 2016 John Wiley & Sons, Ltd.