Simultaneous UHPLC/DAD/(+/-)HESI-MS/MS analysis of phenolic acids and nepetalactones in methanol extracts of Nepeta species: a possible application in chemotaxonomic studies.

Abstract

Nepeta species contain a variety of secondary metabolites, including iridoid monoterpenes - nepetalactones and phenolic acids - that are considered the main bioactive constituents. This work represents the first attempt to comparatively explore variations in these two major groups of secondary metabolites within the genus. To develop an efficient analytical methodology for simultaneous analysis of nepetalactones and phenolic acids in methanol extracts of selected Nepeta species, and to evaluate its potential application in chemotaxonomic studies. A UHPLC combined with linear-trap quadrupole (LTQ) orbitrap MS method was used to characterise chemical diversity and complexity of phenolics among 12 selected Nepeta species. A targeted metabolomic approach using UHPLC coupled to a diode array detector (DAD) and combined with (+/-) heated electrospray ionisation (HESI) MS/MS was developed and validated for quantitative analysis of six hydroxycinnamic acid derivatives and four nepetalactones. Phenolic profiling provided a valuable database of bioactive compounds in the plant group studied, including phenolic acids (hydroxybenzoic and hydroxycinnamic acids) and flavonoids (flavones, flavonols and flavanones). Principal component analysis and cluster analysis suggested the applicability of 10 targeted compounds as chemomarkers for chemotaxonomic studies. Pearson's correlation analysis revealed significant positive correlations between metabolites involved in different biosynthetic pathways (phenylpropanoid or monoterpenoid). The described targeted metabolomic approach proved to be highly beneficial in designing a phytochemical overview of the genus Nepeta, and might have applications in further clarification of phylogenetic relations. Furthermore, it has the potential to be implemented in a routine quality control of plant material and herbal preparations.