Influence of Chemical Structure of Some Flavonols on Their Electrochemical Behaviour

Abstract

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-oneproton 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 with the oxidation of 5,7-dihydroxyl moiety of ring A, which oxidation occurs at very high oxidation potentials. This oxidation is an irreversible process because no reduction peak was observed. The absence of oxidation peak A2

Photochemically Synthesized Ruthenium Nanoparticle-Decorated Carbon-Dot Nanochains: An Efficient Catalyst for Synergistic Redox Reactions

“…Quercetin has five −OH groups that can be easily oxidized, similarly to morin. However, the position of −OH groups on the B ring might affect the oxidation efficiency and the electron-donating properties of quercetin and morin. , It was reported that quercetin exhibited favorable electron-donating properties compared to those of other flavonoids, due to the conjugation of electron-donating 3-OH group on the C ring through a 2,3-double bond to the 3′,4′-catechol structure of B ring. , As a result, −OH groups on B ring of quercetin could be facilely oxidized than those of the A ring. The lower absorption peak of 263 nm and a shoulder at 322 nm are assigned to electron delocalization along with the A and C rings, whereas the longer absorption peak of 375 nm corresponds to the conjugation of B ring with C ring through the 2,3-double bond .…”

Rutin as an Electrochemical Mediator in the Determination of Captopril using a Graphite Paste Electrode

“…According to the literature , the neutral form of captopril is predominant at pH values below 4.0 and this favors accumulation of the thiol on the electrode surface. Moreover, rutin occurs principally in its the neutral form at pH 3.6 and this condition favors the kinetics of the Q/QH 2 system such that the two‐electron and two‐proton transferences are so fast that they occur as a single step . Although the electrochemical behavior of the system at pH 7.0 was similar to that observed at pH 2.0 and 3.0, application of the higher pH was considered inappropriate because of the potential deprotonation of captopril and the slower Q/QH 2 electron transfer in rutin compared with that observed at pH 4.0.…”

“…The bars represent the differences (ΔI ap ) between anodic peak currents in the absence and presence of captopril, while the insert shows peak potential as a function of pH. deprotonation of captopril [43] and the slower Q/QH 2 electron transfer in rutin [44] compared with that observed at pH 4.0.…”

Computational and in vitro validation of cardiogenic induction of quercetin on adipose‐derived mesenchymal stromal cells through the inhibition of Wnt and non‐Smad‐dependent TGF‐β pathways

“…Two benzene rings, catechol B and resorcinol A rings, connected by the heterocyclic C ring, formed the Que structure. 12 Resorcinol moiety was also presented in RSV, and catechol showed a great resemblance to RSV phenol moiety (Figure 4). Therefore, the main distinction between the two molecules was the median center of the A and B rings of the C ring.…”