Metabolomic tool to identify antioxidant compounds of Fraxinus angustifolia leaf and stem bark extracts

• Selective and comprehensive extraction from leaves and stem bark of F. angustifolia.
• Antioxidant activity guided fractionation of most active extracts.
• Phytochemical investigation of phenolics, flavonoids and tannins contents using 1H NMR spectroscopy.
• Metabolomics study by multivariate data analysis, correlations of obtained activities to 1H NMR signals.
• Assignment of signals which are correlated to activity to identify antioxidant compounds.

Fraxinus angustifolia Vahl. (Oleaceae) can be used as an industrial crop for natural dye production and textile color, as cattle food and many traditional medicinal uses. In North Algeria, F. angustifolia is used to treat inflammatory diseases like arthritis, rheumatism and gout, which are known to be mediated or promoted by oxidative stress.This study aimed to apply and compare two different ways to characterize antioxidant activity of leaves and stem bark of F. angustifolia extracts and elucidate structures of bioactive compounds. On one hand, classical way of extraction followed by bio-guided chromatographic fractionation with antioxidant in vitro assays was applied. On the other hand, 1H NMR based metabolomic study was assessed.Antioxidant potentials were evaluated using ferric reducing antioxidant power (FRAP), free radical (DPPH and ABTS+) scavenging and ferrous ion-chelating activities. Total phenolic, flavonoids and tannins contents of crude extracts were assessed by colorimetric methods. Moreover, phytochemical investigation of extracts and chromatographic fractions were also performed by 1H NMR spectroscopy.Strong antioxidant activities were shown by several extracts and fractions which are mixtures of metabolites. 1H NMR data combined with partial least square analysis (PLS scores) discriminated most active extracts and fractions and bring out major characteristic chemical shifts of related metabolites highly involved in this antioxidant activity (FRAP). Assignments of most correlated chemical shifts lead to structure elucidation of antioxidant compounds of F. angustifolia from mixtures of metabolites. Phenylethanoids were found to be major antioxidants of stem bark extracts and fractions, mainly calceolariosides (A and B) and verbascoside. The later compound was also identified in leaves fractions, moreover, secoiridoids (oleuropein and ligstroside) and at least three flavonoides glycosides; kaempferol 3-O-rutinoside, isoquercetrin (quercetin 3-O-glucoside) and rutin (quercetin 3-O-α-L-rhamnopyranosyl-1-6-glucopyranose) were elucidated and revealed to be highly involved in leaves antioxidant potential.Metabolomics approach relied much faster the antioxidant power of F. angustifolia leaves and stem bark extracts to these identified phenolic compounds comparatively to bio-guided fractionation way, even if most actives fractions were mixtures of several metabolites.