Phenolic profile and antioxidant evaluation of Mentha x piperita L. (peppermint) extracts

Seven extracts were prepared from Mentha x piperita (peppermint) leaves in sequence using a Soxhlet apparatus, viz. (40-60 degrees) light petroleum (PE), dichloromethane (CH2Cl2), acetonitrile (ACN), ethyl acetate (EtOAc), methanol (MeOH), n-butanol and water (H2O) extracts. The phenolic and flavonoid content of each extract were estimated using spectrophotometric methods whilst a qualitative-quantitative analysis was made by reverse-phase high performance liquid chromatography coupled with photodiode array detection (HPLC-PDA). Each extract was assessed in a battery of six antioxidant-related assays so as to determine their iron(III) reductive, iron(II) chelating and free radical scavenging abilities. The MeOH-soluble extract contained the greatest content of total phenols and flavonoids based upon the Folin-Ciocalteu and 2-aminoethyl diphenylborate reagent data and HPLC-PDA analysis. Based upon the chromatographic and UV-spectral data, the leaves principally contained the cinnamic acid caffeic acid, the depside rosmarinic acid and flavonoids (flavones and flavanones). Eriocitrin (383.3 +/- 2.2 mg/g extract) and rosmarinic acid (381.2 +/- 1.9 mg/g extract) were the most abundant components identified within the leaves, whilst naringenin-7-O-glucoside (0.8 +/- 0.01 mg/g extract) was the least abundant component identified being found only in the EtOAc-soluble extract. The EtOAc, ACN and H2O-soluble extracts demonstrated the most potent iron(III) reductive and 1,1'-diphenyl-2-picrylhydrayl, 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate) and hydroxyl free radical scavenging properties; however, the H2O and CH2Cl2-soluble extracts were the most potent extracts in the beta-carotene-linoleic acid bleaching inhibition assay. In terms of iron(II) chelation--an important antioxidant property--the PE, MeOH and H2O extracts demonstrated moderate iron(II) chelating activity.