Peculiarities of 7-hydroxyflavone oxidation in relation to our model for the estimation of the first oxidation potential of flavonoids

Dear Editor,

in 2023 in your journal I published models for the estimation of the first oxidation potential, E_p1, of flavonoids connected with the three different mechanisms of electrochemical oxidation – SETPT, SPLET, and HAT (1). The models were made on a calibration set of 35 flavonoids for which the oxidation potentials were all measured in our laboratory under the same conditions. The best model was based on the mean values of parameters ${\displaystyle \sum _{\text{s}\left(\text{C}\right)}\Delta {\text{NAC}}_{\text{Cat-Neut}}}$ , ${\displaystyle \sum _{\text{s}\left(\text{C}\right)}\Delta {\text{NAC}}_{\text{Rad-Anion}}}$ and ${\displaystyle \sum _{\text{s}\left(\text{C}\right)}\Delta {\text{NAC}}_{\text{Rad-Neut}}}$ as the variable connected with the three different mechanisms of electrochemical oxidation, which should mean that each mechanism contributes to the electrochemical oxidation of every flavonoid pretty much equally.

Recently, I made the same calculations on 7-hydroxyflavone (7-HF, Scheme 1) and obtained 0.623 for ${\displaystyle \sum _{\text{s}\left(\text{C}\right)}\Delta {\text{NAC}}_{\text{Cat-Neut}}}$ , 0.435 for ${\displaystyle \sum _{\text{s}\left(\text{C}\right)}\Delta {\text{NAC}}_{\text{Rad-Anion}}}$ and 0.321 for ${\displaystyle \sum _{\text{s}\left(\text{C}\right)}\Delta {\text{NAC}}_{\text{Rad-Neut}}}$ . For 7-HF, we did not measure E_p1, but since it has only one hydroxyl group placed at the A ring and according to literature (2, 3) its E_p1 value is similar to that of chrysin and 5-hydroxyflavone. Therefore, I took the mean value of their E_p1 at pH=3 (1.162 and 1.164, respectively). Placing the data for 7-HF into regressions and figures of dependences of experimental E_p1 (pH=3) on the differences in the net atomic charges for 35 flavonoids (Models and Figures 2, 3, and 4 in Ref. 1) it is obvious that 7-HF was the only flavonoid that did not fit into any of the three regressions (Figure 1). That would mean that its oxidation mechanism does not correspond to any of the three proposed mechanism of oxidation. However, the mean value of three parameters for 7-HF perfectly fit the best model we obtained for the estimation of the oxidation potential of flavonoids based on the mean values of parameters ${\displaystyle \sum _{\text{s}\left(\text{C}\right)}\Delta {\text{NAC}}_{\text{Cat-Neut}}}$ , ${\displaystyle \sum _{\text{s}\left(\text{C}\right)}\Delta {\text{NAC}}_{\text{Rad-Anion}}}$ and ${\displaystyle \sum _{\text{s}\left(\text{C}\right)}\Delta {\text{NAC}}_{\text{Rad-Neut}}}$ (Model and Figure 5 in Ref. 1). Since the model implies that all three mechanisms (SET-PT, SPLET, and HAT) equally contribute to the electrochemical oxidation of all flavonoids, the case of 7-hydroxyflavone would be yet another supporting such a conclusion.

Scheme 1.

The structure of 7-hydroxyflavone

Figure 2.

The dependence of experimental E_p1 (pH=3) on the parameters ${\displaystyle \sum _{\text{s}\left(\text{C}\right)}\Delta {\text{NAC}}_{\text{Cat-Neut}}}$ , ${\displaystyle \sum _{\text{s}\left(\text{C}\right)}\Delta {\text{NAC}}_{\text{Rad-Anion}}}$ and ${\displaystyle \sum _{\text{s}\left(\text{C}\right)}\Delta {\text{NAC}}_{\text{Rad-Neut}}}$ (variables 1, 2, and 3) for the set of 35 flavonoids published previously (1), with the addition of 7-HF

Figure 2.

The dependence of experimental E_p1 (pH=3) on the mean of ${\displaystyle \sum _{\text{s}\left(\text{C}\right)}\Delta {\text{NAC}}_{\text{Cat-Neut}}}$ , ${\displaystyle \sum _{\text{s}\left(\text{C}\right)}\Delta {\text{NAC}}_{\text{Rad-Anion}}}$ and ${\displaystyle \sum _{\text{s}\left(\text{C}\right)}\Delta {\text{NAC}}_{\text{Rad-Neut}}}$ (variables 1, 2, and 3) for the set of 35 flavonoids published previously (1), with the addition of 7-HF

It is also worth mentioning that the hydroxyl group at position 7 (in the A ring of a flavonoid) is generally the most acidic hydroxyl group in flavonoids, i.e. it deprotonates more easily than the OH group at any other position in a flavonoid (4). It also has the largest value of O-H bond dissociation enthalpy (BDE) and ionisation potential (IP). Thus, it was assumed that the 7-HF mechanism of oxidation corresponds to the sequential proton loss electron transfer (SPLET).