Estimating flavonoid oxidation potentials: mechanisms and charge-related regression models

Abstract

In this paper, I tested our quadratic regression models for the estimation of flavonoid oxidation potentials based on spin populations, the differences in the net atomic charges between a cation and a neutral flavonoid, between a radical and an anion of a flavonoid, and between a radical and a neutral flavonoid on a larger set of flavonoids (N = 35). By including six new flavonoids (5,6,7-trihydroxyflavone, 3,3’,4’,7-tetrahydroxyflavone, 3,7-dihydroxyflavone, 4’,7-dihydroxyflavone, 4’,5,7-trihydroxyisoflavone, and 6-hydroxyflavone), we created a respectable calibration set of 35 flavonoids with their oxidation potentials all measured at the same conditions by the same experimentalist. The best model was based on the mean values of the three variables using differences in the net atomic charges (R 2 = 0.970, S.E. = 0.043), which are connected with the three different mechanisms of electrochemical oxidation, SET-PT, SPLET, and HAT.

Flavonoid Oxidation Potentials and Antioxidant Activities-Theoretical Models Based on Oxidation Mechanisms and Related Changes in Electronic Structure

“…[9]). But after the calculations based on the mechanism that includes a 5-OH group (see Section 2), the Σ s(C) ∆NAC Cat-Neut was reduced to 0.508 and thus fitted the regression on E p1 on the set of 35 flavonoids [20] (Figure 3). Also, the greatest sum of differences in charges were now found on the A ring (0.444, 0.024 and 0.040 for rings A, C and B, respectively).…”

“…Figure 3. The dependence of experimental Ep1 (pH = 3) on ΔNACCat-Neut, calculated using the PM6 method, for the set of 35 flavonoids [20]. Quadratic regression yielded R 2 = 0.943, S.E.…”

“…Although having a small number of OH groups attached, these flavonoids have E p1 values much lower than would be expected. Thus, we reached for quantum chemical theoretical methods in order to take a closer look into flavonoid electronic structure and its changes during the initial step of electrochemical oxidation [9][10][11][12][19][20][21].…”

“…Combining experimental results with quantum chemical calculations, we tried to elucidate the relationship between the oxidation potentials of flavonoids and their electronic structure in order to obtain comprehensive and reliable model for the estimation of their oxidation potentials, E p1 [9][10][11][12][19][20][21]. Such a model would enable the fast and cheap prediction of E p1 for a flavonoid of interest, which may not be available at the moment or even be synthesized yet.…”

“…For the calculations of atomic charges, atomic spin densities and energies of species (Equations ( 1)-( 3)), for each flavonoid in the set, we used the following quantum chemical methods: semi-empirical parametrization method 6 (PM6) and density functional theory (DFT) [9][10][11][12][19][20][21]. All these calculated parameters gave us valuable information about the redox reactions of flavonoids and, consequently, about their antioxidant capacity.…”