The chemical compositions and antioxidant evaluation of the essential oils (EOs) obtained by hydrodistillation from the leaf and stem of Indigofera spicata Forssk (Fabaceae) grown in Nigeria have been studied. The EOs were analyzed using gas chromatography coupled with mass spectrometry (GC-MS) and the antioxidant potential was determined using 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging ability method. The essential oils from the leaf and stem obtained in 0.007% and 0.009% yield have been found to contain 18 and 17 compounds respectively. 13 compounds identified in the stem EOs make 90.2% of it. Sesquiterpenes (49.2%) and alcohols (30.7%) are dominant classes of compounds in the leaf EOs, with the most abundant compounds as caryophyllene (38.2%), humulene (6.2%) and m-eugenol (27.5%). Whereas esters (47.2%), and monoterpenoids (20.8%) dominate the stem essential oils with major constituents as linalyl acetate (23.9%), α-terpinyl acetate (12.8%), 3,5,5-trimethyhexyl acetate (9.4%), and linalool (20.8%). Common to both EOs were linalool, (leaf, 1.9%) caryophyllene (stem 5.9%), linanyl acetate (leaf, 2.5%). Comparison of the composition pattern of the leaf and stem EOs of I. spicata revealed significant qualitative and quantitative differences. Monoterpenes, sesquiterpenoid, diterpenoid, epoxide and ether were exclusive to the leaf oil while saturated, unsaturated hydrocarbons and anhydride were found only in the stem oil. The IC50 values of antioxidant evaluations show the leaf EO (36.97 µg/mL) has more potential than the stem oil (39.89 µg/mL) and comparable to that of the controls Vitamin C and Butylhydroxyl Anisole with IC50 values 24.20 and 24.21 µg/mL respectively. Most of these identified compounds have been known for various pharmacological activities such as antioxidant, antitumor, anti-inflammatory, even as fragrances and the antioxidant potential of the oils justify the ethno-medicinal uses of I. spicata.
Key words: Indigofera spicata, linalool, m-eugenol, caryophyllene, essential oil, sesquiterpene, antioxidant, 2,2-diphenyl-1-picrylhydrazyl (DPPH).
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