Antioxidant activity and redox properties of flavonoids from Limonium narbonense

Limonium spp are perennial plants that are mainly distributed near coasts and in salt marshes, and also on saline and alkaline soils in continental interiors. Some Limonium species are used in folk medicine for the treatment of rheumatism and menstrual disorders. L. narbonense Mill. grows wildly along seashore areas subject to periodical floods such as the Venice lagoon. In the course of our studies we found that the methanolic extracts from roots and aerial parts of L. narbonense Mill., collected near Venice, showed antioxidant activity as has been observed by DPPH· assay. The EC₅₀ determined by the DPPH· assay were 11.8 and 23.1µg/mL for aerial parts and roots, respectively. Phytochemical analysis carried out on methanol extracts led to the identification of several polyphenols including: myricetin glycosides, kaempferol and gallic acid. These and other chemical constituents are reported for the first time for this species. Furthermore, the antioxidant activity of the main isolated flavonoids was examined using the DPPH· test. The knowledge of their redox properties can help to understand the antioxidant activity of the flavonoids isolated from L. narbonense. To this aim, we investigated by cyclic voltammetry the electrochemical properties of myricetin and myricetrin, which were present in higher concentration and, for comparison, also quercetin, rutin and kaempferol. The first oxidation process resulted a quasi-reversible electron transfer, allowing the estimation of its standard potential E°. A good correlation between EC₅₀ and E° has been found, which leads to a possible criteria for predicting the antioxidant ability of a such class of compounds.