Highly glycosylated and acylated flavonols isolated from kale (Brassica oleracea var. sabellica) — Structure–antioxidant activity relationship

Abstract

Kale is a common Brassica vegetable rich in secondary plant metabolites especially flavonols, that comprise highly glycosylated and acylated quercetin and kaempferol derivatives. Several kale flavonols have been isolated, structurally identified and characterized by means of HPLC-DAD-ESI–MSn and NMR spectroscopy from kale for the first time. Their antioxidant activity given by a wide variety of glycosylation and acylation patterns was investigated using the Folin–Ciocalteu assay, the TEAC assay, and EPR spectroscopy. The purpose was to obtain distinct structure–antioxidant activity relationships of structures differing either in their aglycone structure, their number of glycoside substituent, and the kind of acylation. A high radical scavenging activity is mainly due to catechol like structure of e.g. quercetin derivatives. Noteworthy is the fact that flavonol derivatives esterified with methoxylated hydroxycinnamic acids possess higher antioxidant capacities than those conjugated with hydroxycinnamic acids with catechol structure. Remarkably, the influence of the acyl moiety seems to be less effective than that of the glycosylation pattern in 7-O position. In conclusion, the antioxidant behavior differs depending on the assay used and the above mentioned structural features, revealing the importance of these attributes to be responsible for a specific antioxidant capacity.