Chrysin and (±)-Taxifolin Electrochemical Oxidation Mechanisms

Abstract

The electrochemical behaviour of the flavone chrysin and the dihydroflavonol (AE)-taxifolin was investigated by different electrochemical techniques at different pH values. Dihydroflavonol (AE)-taxifolin presented two oxidation peaks. The first electrooxidation reaction of (AE)-taxifolin is reversible over the whole pH range. The dependence of peak current on pH showed a maximum around neutral pH values with a decrease in acidic and alkaline media. The influence of the catechol group deprotonation on ring B is related to the electron/proton donating capacity in (AE)-taxifolin and to its radical scavenging antioxidant activity. In chrysin, where the ring B has an unsubstituted phenyl, the antioxidant active moiety is in ring A. Chrysin presents an irreversible oxidation peak at higher oxidation potentials than those usually obtained for the catechol moiety ion in ring B and a pH dependent behaviour. The ÀOH groups on ring A are affected by the 4-keto group as shown by the lower pK a value of the 7-OH, indicating that the 7-OH group is the more acidic. The 5-OH group is less acid and may be explained by the acidic weakening effect of an intramolecular H-bond between the 5-OH and 4-keto group of taxifolin and chrysin.

Synthesis and electrochemical study of new 3-(hydroxyphenyl)benzo[f]coumarins

“…The electrochemistry of this selected series of synthesized coumarinnaphthalene-stilbene hybrids (6)(7)(8)(9)(10) showed that the difference in the position of the hydroxyl substituents have only a slightly effect upon the electroactivity of the selected coumarins.…”

“…The phenol group is irreversibly oxidised in compounds 6, 7, 8, and 10, and the catechol group is reversibly oxidised in compound 9. The oxidation products of the coumarin-naphthalene-stilbene hybrids (6)(7)(8)(9)(10) The monohydroxylated compounds 6-8, irreversible pH-dependent oxidation process, occurs with one electron and one proton transfer, following the phenol oxidation mechanism [43]. Compound 6, with the para position to the hydroxyl group occupied, formed a single oxidation product corresponding to a catechol moiety, Scheme 3.…”

“…To a solution of the corresponding 3-(methoxyphenyl)benzo[f]coumarin (1)(2)(3)(4)(5) (0.50 mmol) in acetic acid (5 mL) and acetic anhydride (5 mL) at 0 ºC, hydriodic acid 57% (10 mL) was added dropwise. The mixture was stirred, under reflux, for 3 h. The solvent was evaporated under vacuum and the dry residue was purified by CH 3 CN crystallization to give the corresponding 3-(hydroxyphenyl)benzo[f]coumarins (6)(7)(8)(9)(10) in yields between 60-67% in this last reaction, Scheme 2. …”

“…The dependence of the phenol derivatives oxidation potential on solution pH has been studied thoroughly for different classes of polyphenols [5][6][7][8][9][10][11], for the evaluation of the antioxidant capacity of several polyphenols and their mixtures [12], and for the understanding of their reaction mechanisms.…”

“…Their electrochemical oxidation behaviour was investigated for a wide range of solution conditions, using a glassy carbon electrode and cyclic, differential and square wave voltammetry, at different pH, and the results will play a crucial role in understanding coumarin derivatives antioxidant activity. (6)(7)(8)(9)(10) The aim was to study and compare one derivative with a catechol group (3',4'-OH derivative) and another one presenting no contiguous OH groups (3',5'-OH derivative). Both precursors (2',5'-dimethoxy and 3',5'-dimethoxy derivatives) being commercially available on Aldrich.…”