Study of electrochemical oxidation of cyanidin glycosides by online combination of electrochemistry with electrospray ionization tandem mass spectrometry

Abstract  

Electrochemistry coupled with mass spectrometry (EC–MS) is a promising analytical tool for the online study of oxidation processes of anthocyanins. Two cyanidin glycosides, cyanidin-3-galactoside (ideain) and cyanidin-3,5-diglucoside (cyanin), were subjected to electrochemical oxidation and subsequent online mass spectrometric identification of the formed products. Application of relatively low working potentials (around 100 mV vs. Pd/H₂) using a porous graphite coulometric electrode yielded detectable oxidation products. As determined by hydrodynamic voltammetry, the monoglycosylated analogue undergoes anodic oxidation easier than the diglycosylated one. As a first step of the electrochemically induced oxidation, incorporation of a hydroxyl group was observed for both glycosides. Besides, an oxidative condensation of two anthocyanin molecules was observed. The proposed oxidative condensation was further confirmed by consecutive fragmentation (i.e., collection of MS², MS³, and MS⁴ spectra) in which corresponding subsequent losses of the sugar moiety were observed.