Antioxidant properties and five new phenylpropanoid esters of apigenin from Sideritis syriaca L

Sideritis species have been used in Greece to prepare herbal medicines and traditional teas [1] and have been reported to possess antioxidant activity. The aim of this study was to find bioactivities in Sideritis syriaca ssp. syriaca, relating to ethnopharmacological uses as antioxidant and to isolate the responsible compounds.

Four extracts from Sideritis syriaca ssp. syriaca (Lamiaceae) were screened for their possible in vitro antioxidant activities by two test systems, namely DPPH free radical scavenging and Co(II)/EDTA induced luminol chemiluminescence. In both model systems was found that the most effective extracts were the ethylacetate [2] and the ethylether. Phytochemical analysis of the second extract showed the presence of five new phenyl propanoid esters of apigenin: 1) 3-(4-Hydroxy-phenyl)-acrylic acid 5-hydroxy-2-{4-[3-(4-hydroxy-phenyl)-acryloyloxy]-phenyl}-4-oxo-4H-chromen-7-yl ester. 2) 3-(4-Hydroxy-phenyl)-acrylic acid 2-{4-[3-(3,4-dihydroxy-phenyl)-acryloyloxy]-phenyl}-5-hydroxy-4-oxo-4H-chromen-7-yl ester. 3) 3-[4-(4-{7-[3-(3,4-Dihydroxy-phenyl)-acryloyloxy]-5-hydroxy-4-oxo-4H-chromen-2-yl}-phenoxy)-phenyl]-acrylic acid. 4) 3-[3-Hydroxy-4-(4-{5-hydroxy-7-[3-(4-hydroxy-phenyl)-acryloyloxy]-4-oxo-4H-chromen-2-yl}-phenoxy)-phenyl]-acrylic acid. 5) 3-(3,4-Dihydroxy-phenyl)-acrylic acid 5-hydroxy-2-{4-[3-(4-hydroxy-phenyl)-acryloyloxy]-phenyl}-4-oxo-4H-chromen-7-yl ester. The above compounds were identified by spectroscopic (LC-DAD-MS (ESI+, ¹H-NMR) [3,4] and chemical methods. The antioxidant activity of ethyl ether extract was attributed to the presence of phenylpropanoid esters of apigenin and that can be an answer to the continual demand for new natural antioxidants. Furthermore our findings of antioxidant activity of aerial parts of Sideritis syriaca, validated the use of the plant for medical purpose.

Acknowledgements: Mediterranean Agronomic Institute of Chania, Dr. P. Kefalas, (for providing facilities for the LC-DAD-MS)

References: 1. Aligiannis, N. et. al. (2001)J. Agric. Food. Chem. 49: 811–815. 2. Armata, A. et al (2008) Food Chem. (in press) 3. Gabrieli, C. et al (1990) Phytochemistry 29: 681–683. 4. Bachir, B. et al (1992) Phytochemistry 31:2483–2486