Investigation of Lepechinia graveolens for its antioxidant activity and phenolic composition

A bioguided separation of Lepechinia graveolens (Reg.) Epling. (Lamiaceae) for antioxidant activity was carried out. The ethyl acetate fraction (EAF) from the methanolic extract was chromatographed on Sephadex LH-20 affording five active fractions. The radical scavenging activity of each fraction, as well as that of the isolated compounds, was tested using three different methods. The major isolated antioxidant compounds were identified as luteolin-7-O-glucuronide, rosmarinic acid, and rosmarinic acid methyl ester by means of 1H and 13C NMR and mass spectrometry. Twenty other minor phenolic compounds were determined by liquid chromatography-negative electrospray ionization tandem mass spectrometry. They included hydroxycinnamic acid derivatives, flavonoids and phenolic diterpenes. The major phenolic compound was found to be rosmarinic acid, which was quantitatively determined by high-performance liquid chromatography (HPLC)-diode array detector (DAD). The content of rosmarinic acid was 2.8% (referred to dry weight), this amount being around 30% of the total phenolic content (TPH) of the crude extract. It can be concluded that the antioxidant activity of Lepechinia graveolens was mainly due to rosmarinic acid, which allows this plant species to be considered as a potential new source of this well known natural antioxidant.