Influence of Chemical Structure of Some Flavonols on Their Electrochemical Behaviour

The electrochemical behaviour of three structurally related flavonols, quercetin, morin, and rutin was studied by cyclic, differential pulse, and square-wave voltammetry methods. The study reveals that their electrochemical behaviour strongly depends on their chemical structure and electronic properties, particularly on the presence of electron-donating -OH groups, i.e. their numbers and position on rings A, B, and C in the structure of these flavonols. The important factors of the electrochemical oxidation behaviour of flavonols are as follows. (i) The presence of two electron-donating -OH groups on the B ring in the ortho-position. (ii). The 2,3-double bond in conjugation with a 4-oxo group on the C ring. (iii) The electron donating 3-OH group on the C ring, and (iv) the electron-donating 5-OH and 7-OH groups on A ring. Quercetin satisfies all of the requirements mentioned above and therefore has the best electron-donating properties of all investigated flavonols. The first oxidation peak of quercetin (peak A1) corresponds to the reversible oxidation of 3',4'-OH groups (catechol moiety) at the B ring to the ortho-quinone structure by two-electron-two-proton (2e--2H+) process. This electrochemically active and unstable ortho-quinone species then undergoes chemical rearrangements or addition reactions, indicates an electrochemical-chemical (EC) reaction mechanism. At higher anodic potential the -OH group at position 3 of ring C was oxidized (peak A2), by the reversible one-electron-one-proton reaction. In this study, it was for the first time observed that the second reduction peak of quercetin (peak C2) corresponds to the 3-OH group on ring C. The hydroxyl groups at position 5 and 7 at ring A have significantly smaller electron-donating effect than -OH groups at ring B, and therefore were oxidized at higher anodic potentials (peak A3). This oxidation is an irreversible process. Morin with meta-2',4’-dihydroxyl groups (resorcinol moiety) shows the higher value of oxidation potential of peak A1 than quercetin, indicating that oxidation of 2',4'-OH groups on ring B of morin to quinone structure is more difficult in comparison to that of quercetin. This fact clearly shows the importance of the presence of two hydroxyl groups in the ortho-diphenolic arrangement on the ring B of flavonols. The first oxidation process of morin is an one-electron-one-proton reversible reaction, which proceeds in an EC mechanism. The oxidation peak of morin A3 should be associated