Phenolic compounds from Trifolium echinatum Bieb. and investigation of their tyrosinase inhibitory and antioxidant activities
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Two bischromones, 3,3--dimethoxy-2--oxychromone (1), 3,3--dihydroxy-2,2--oxychromone (2) and a biflavone, 5,7,4-,5 -,3-,4--hexahydroxy-3 --O-beta-glucosyl-3-,7 --O-biflavone (3) have been isolated from whole plant of Trifolium echinatum Bieb. together with five known fiavonoids. The structures of the compounds were elucidated by 1D and 2D NMR analysis as well as HRESIMS. The isolated compounds were investigated for their antioxidant activity and tyrosinase inhibitory activity. Highly potent inhibition was found for compounds 1 (IC50 = 0.41 mM), 5 (IC50 = 0.47 mM) and 8 (IC50 = 0.45 mM) compared to those of standard tyrosinase inhibitors kojic acid (IC50= 0.67 mM) and L-mimosine (IC50 = 0.64 mM). The antioxidative effect of the extracts was determined by using beta-carotene-linoleic acid, DPPH. scavenging, ABTS(+-) scavenging, and CUPRAC assays. The experimental findings indicated that the compounds 2 and 8 were found to be active in radical scavenging and CUPRAC assays. (C) 2013 Elsevier Ltd. All rights reserved.