Antioxidant activity and polyphenolic composition of water knotweed (Polygonum amphibium L.) ethanolic extracts

As a part of our ongoing phytochemical investigation of plants from Polygonum genus (Polygonaceae), we investigated the chemical composition and antioxidant acitivity of P. amphibium collected on Vlasina lake in Serbia. Plants from this genus are used in traditional medicine, diet, and as ornamental plants. Polyphenolic profile of 80% ethanolic extracts of stems, leaves, flowers, and entire herb was determined by quantitative LC-MS/MS analysis of 45 phenolics, and by spectrophotometric measuring of the total phenolic and flavonoid content. The antioxidant activity was evaluated by measuring FRAP ability of the extracts and their scavenging capacity towards DPPH, OH, NO and O₂ ^- radicals. Dominant phenols in all investigated extracts were found to be quercetin and its glycosides – hyperoside and isoquercitrin, accounting for up to 4% of leaves dry extract. Among flavan-3-ols, catechin, epicatechin and gallic acid ester – epigallocatecin gallate – were present mostly in stems (˜0.9%). Gallic acid was the most abundant phenolic acid (up to 0.5% of leaves and herb dry extract). High DPPH and O₂ ^- scavenging ability (IC₅₀ 6.8–7.4µg/mL and 5.5–8.5µg/mL, respectively), and moderate ability of scavenging OH (IC₅₀ 135–188µg/mL) and NO (IC₅₀ 82.6–102µg/mL) radicals, were observed in herb extracts, as well as high FRAP ability (461–466mg eqv. ascorbic acid/1g dw), and total phenolic content (300–307mg eqv. gallic acid/1g dw). Flavonoids content was 38.5–39.7mg eqv. quercetin/1g dw.