Liquid chromatography with multistage mass spectrometry as a method for chemical differentiation of ten Nepeta species (fam. Lamiaceae)

Abstract

Nepeta species (fam. Lamiace, subfam. Nepetoideae, tribe Nepetae) are well known for their pharmacological properties, biopesticide potential and behavioral effects on cats. Different biological activities of Nepeta species are closely related to their secondary metabolites, among which iridoids predominate. Research objective of this study was to determine profiles of methanol-soluble iridoids in leaves of greenhouse-grown plants of 10 phylodiverse Nepeta species: N. grandiflora M. Bieb., N. laevigata (D. Don) Hand.-Mazz., N. nervosa Royle ex Benth., N. stewartiana Diels, N. nuda L., N. parnassica Heldr. & Sart., N. rtanjensis Diklić & Milojević, N. ernesti-mayeri Diklić & V. Nikolić, N. cataria L., and N. sibirica L. Untargeted metabolomics approach adopting UHPLC/LTQ Orbitrap MSn analysis in a negative ionization mode revealed the presence of totally 15 iridoids in samples, of which 13 were glucosides and two were aglycons of iridoids. Iridoid compounds were identified based on their monoisotopic masses, MSn fragmentation, and previously reported MS data. The results indicated that the iridoid profiles of the investigated species are very diverse both in terms of qualitative content of the compounds and their relative abundance in extracts. Among analysed Nepeta species, Balkan Peninsula endemics (N. rtanjensis, N. parnassica, and N. ernesti-mayeri) and N. cataria, are especially rich in isomers of 1,5,9-epideoxyloganic acid. 5-Deoxylamiol, nepetariaside, nepetanudoside B, and ajugol were detected in majority of analyzed species. The UHPLC-LTQ OrbiTrap MS analysis, in combination with chemometric data processing, has proven to be an efficient tool for chemical differentiation between Nepeta taxa, and could aid the efforts to unravel the chemical evolution within the genus.