Chromato-mass spectrometric identification of products of quercetin oxidation by atmospheric oxygen in aqueous solutions

Abstract

The information about structures and formation mechanisms of oxidation products of such natural flavonoid as quercetin allows explaining the nature of its antioxidant activity remaining unexplored up to present. The aims of this paper imply the oxidation of quercetin by air oxygen in water alkaline solutions (pH = 8÷9) in presence of ethanol (to increase its solubility), mass-spectrometric analysis (HPLC-ES-MS-MS) of the reaction mixture and systematization of literature data on the oxidation products for comparing with our data. The retention indices of the oxidation products of quercetin have been determined in reversed-phase HPLC for the first time. It is confirmed that many of oxidation products of quercetin are stable only in solution and cannot be isolated after drying. In order to clarify the structures of some products, special experiments have been carried out. Thus, the replacement of ethanol with methanol in the composition of the reaction mixture can in some cases eliminate ethyl esters from further consideration and identify such products as 2-(3,4-dihydroxyphenyl)-2-oxoacetic acid and 2-oxo-2-(2,4,6-trihydroxyphenyl)acetic acid. To confirm the quinone structure of one of principal products of quercetin oxidation, the UV spectrum of the reaction mixture has been recorded and its reaction with hydroxylamine has been carried out. A large number of isomers have been revealed among the quercetin oxidation products. A method to distinguish them from the dimers formed in the mass spectrometer during the electrospray ionization process has been suggested. The two main oxidation mechanisms of quercetin discussed in the literature involve intermediate formation of compounds with the trivial names “depside” and “alfitonine”. Their joint presence in the product composition has been established, but their stability and lifetimes in solution are different. Refs 17. Tables 1.