Oxidative Degradation of Quercetin with Hydrogen Peroxide Using Continuous-Flow Kinetic Electrospray–Ion Trap–Time-of-Flight Mass Spectrometry

The time-dependent hydrogen peroxide-induced oxidative degradation of aqueous quercetin at pH 7.4 was measured using an in-house-built online continuous-flow device made of concentric capillary tubes, modified to fit a photodiode array (PDA) detector and the inlet of an electrospray ionization–ion trap–time-of-flight mass spectrometer (ESI-IT-TOF-MS). As the reaction time was increased, the deprotonated quercetin ion signal, [Q – H]−, decreased, and the formation of degradation product ions was observed. Structures for degradation product ions were proposed using higher order tandem mass spectrometry (up to MS3) and high mass accuracy. The determined degradation pathways included oxidation, hydroxylation, cyclic peroxylation, ring cleavage, and small molecule loss. The most intense degradation product observed was 2,4,6-trihydroxybenzoate, which was proposed to be the end point of the peroxylation pathway and the favored degradation pathway under these conditions. This pathway is believed to be the result of nucleophilic attack by hydrogen peroxide at the C2 position of quercetin. This was followed by a cross ring cyclic peroxylation event at C2–C4, which resulted in an intermediate depside that was defined by C-ring-opening due to loss of C3-OH and cleavage of the peroxy bond. Further cleavage of the depside resulted in the 2,4,6-trihydroxybenzoate. A sodiated pseudo adduct of the dimerized trihydroxybenzoate was believed to be induced under electrospray conditions. A computational study was performed to justify the position within the C-ring for both the attack by nucleophilic oxidants and the cyclic peroxylated intermediate structure.