The hydroperoxyl radical scavenging activity of sulfuretin: insights from theory

Sulfuretin (SFR), which is isolated from Rhus verniciflua, Toxicodendron vernicifluum, Dahlia, Bidens tripartite and Dipterx lacunifera, is one of the most important natural flavonoids. This compound is known to have numerous biological activities; among these, the antioxidant activity has not been thoroughly studied yet. In this study, the hydroperoxyl scavenging activity of SFR was examined by using density functional theory calculations. SFR is predicted to be an excellent HOO• scavenger in water at pH = 7.40 with koverall = 4.75 × 107 M−1 s−1, principally due to an increase in the activity of the anionic form following the single-electron transfer mechanism. Consistently, the activity of the neutral form is more prominent in the non-polar environment with koverall = 1.79 × 104 M−1 s−1 following the formal hydrogen transfer mechanism. Thus, it is predicted that SFR exhibits better HOO• antiradical activity than typical antioxidants such as resveratrol, ascorbic acid or Trolox in the lipid medium. The hydroperoxyl radical scavenging of SFR in the aqueous solution is approximately 530 times faster than that of Trolox and similar to ascorbic acid or resveratrol. This suggests that SFR is a promising radical scavenger in physiological environments.

Sulfuretin (SFR), which is isolated from Rhus verniciflua, Toxicodendron vernicifluum, Dahlia, Bidens tripartite and Dipterx lacunifera, is one of the most important natural flavonoids. This compound is known to have numerous biological activities; among these, the antioxidant activity has not been thoroughly studied yet. In this study, the hydroperoxyl scavenging activity of SFR was examined by using density functional theory calculations. SFR is predicted to be an excellent HOO • scavenger in water at pH = 7.40 with k overall = 4.75 × 10 7 M −1 s −1 , principally due to an increase in the activity of the anionic form following the single-electron transfer mechanism. Consistently, the activity of the neutral form is more prominent in the non-polar environment with k overall = 1.79 × 10 4 M −1 s −1 following the formal hydrogen transfer mechanism. Thus, it is predicted that SFR exhibits better HOO • antiradical activity than typical antioxidants such as resveratrol, ascorbic acid or Trolox in the lipid medium. The hydroperoxyl radical scavenging of SFR in the aqueous solution is approximately 530 times faster than that of Trolox and similar to ascorbic acid or resveratrol. This suggests that SFR is a promising radical scavenger in physiological environments.
Although the antioxidant activity of SFR is broadly examined experimentally [2,14], there are no studies on the mechanism and kinetics of its antiradical activity, particularly in physiological environments. Computer calculations offer a convenient way to predict the antioxidant activity of organic compounds in physiological media [16][17][18][19][20][21][22][23]. In this context and as a continuation of our previous studies [18,24,25], we set out in this work to evaluate the HOO • antiradical activity of SFR by a combination of thermodynamic and kinetic calculations. This study also considered the effects of solvents on the antioxidant properties of SFR in comparison with some typical antioxidants.

Results and discussion
3.1. The HOO • antiradical activity of SFR in the gas phase 3

.1.1. Thermodynamic evaluation
For SFR that contains OH and moieties, the antioxidant activity may follow either of the four main mechanisms: the formal hydrogen transfer (FHT), the sequential proton loss electron transfer (SPLET), the single-electron transfer proton transfer (SETPT) and radical adduct formation (RAF) [41,42]. The first three pathways are defined by the following thermodynamic parameters: bond dissociation enthalpy (BDE), proton affinity (PA) and ionization energy (IE), respectively. The Gibbs free energy change of the addition reaction is calculated directly for the RAF mechanism. Thus, the BDE, IE and PA values of SFR were first calculated in the gas phase, and the results are shown in table 1.
To confirm that FHT is indeed the preferred pathway of the HOO • antiradical activity of SFR, the Gibbs free energy of the SFR + HOO • reaction was calculated according to each of the four mechanisms: FHT, single-electron transfer (SET, the first step of the SETPT mechanism), sequential proton (SP, the first step of the SPLET) and RAF (table 2). It was found that the HOO • antiradical activity of SFR is only clearly spontaneous for FHT at O3 0 (O4 0 )−H bonds and RAF at the C8 position (ΔG o < 0), whereas the RAF reaction at C2 with ΔG o = 1.1 kcal mol −1 cannot be clearly excluded based on thermodynamics alone and therefore it was also included in the kinetic study. The other reactions are clearly not spontaneous with high positive ΔG o values. The ΔG o values for the reactions following the SP and SET pathways are much higher than those of the FHT mechanism. Thus, the calculated data suggest that the HOO • antiradical activity of SFR may follow either FHT or RAF mechanism (at O3 0 (4 0 )−H and C2/C8 positions, respectively), and these pathways should be investigated in the kinetic study.

Kinetic study
Based on the above results, the kinetics of the SFR + HOO • reaction in the gas phase was investigated for the thermodynamically favourable positions and mechanisms according to the QM-ORSA protocol [17], and the data are presented in table 3 and figure 2.
It is apparent that the HOO • antiradical activity of SFR occurs mostly by the H-abstraction of the O4 0 − H bond (ΔG ≠ = 11.2 kcal/mol; k Eck = 2.83 × 10 6 M −1 s −1 ; Γ = 77.0%). That is more than three times higher contribution than the hydrogen abstraction of the O3 0 −H bond (ΔG ≠ = 11.6 kcal mol −1 ; k Eck = 8.43 × 10 5 M −1 s −1 ; Γ = 23.0%). By contrast, the addition of the radical does not make any contribution (Γ = 0%) at either the C2 or C8 positions. This result is in good agreement with previous studies in phenolic compounds [46][47][48]. We can conclude that the HOO • antiradical activity of SFR is dominated by the FHT mechanism at the O3 0 (4 0 )-H bond; therefore, this is further analysed in physiological environments.

Acid-base equilibrium
Previous studies showed that the deprotonation of the OH bonds plays a key role in the HOO • antiradical activity of phenolic compounds in the aqueous solution [30,34,49]. The spontaneous dissociation of acidic moieties practically eliminates the activation energy barrier of the first step of the SPLET mechanism, simplifying it to direct electron transfer, and for this reason, this pathway can become energetically favoured in aqueous solution for the dissociated species. Thus, in this study, the    Figure 3. The acid dissociation equilibrium of SFR. Table 3. Calculated ΔH (kcal/mol), activation Gibbs free energies (ΔG ≠ , kcal/mol), tunnelling corrections (κ), k Eck (M −1 s −1 ) and branching ratios (Γ, %) for the HOO • + SFR reaction in the gas phase.

Kinetic study
Based on the results of the kinetic calculations in the gas phase, the HOO • antiradical activity in nonpolar environments was modelled by the hydrogen transfer mechanism at the O3 0 (O4 0 )−H bonds. In the aqueous environment, the SET mechanism was also investigated for the deprotonated state of SFR. The overall rate constants (k overall ) were computed following the QM-ORSA protocol [17,33], (table 4) according to equations (3.1) and (3.2).

Conclusion
The hydroperoxyl radical scavenging activity of SFR was investigated using DFT calculations. The results showed that SFR has excellent HOO • antiradical activity with k overall = 4.75 × 10 7 M −1 s −1 in water at pH = 7.40 by the SET pathway of the anion state, and good/moderate HOO • scavenging activity in lipid environment (k overall = 1.79 × 10 4 M −1 s −1 ) by the FHT mechanism via the O3 0 (O4 0 )-H bonds. The hydroperoxyl antiradical activity of SFR is better than Trolox, ascorbic acid and resveratrol in the lipid medium. This activity of SFR is approximately 530 times faster than that of Trolox and relatively similar to ascorbic acid and resveratrol in the polar environment. Thus, SFR can be a useful natural antioxidant in physiological environments.
Data accessibility. All relevant necessary data to reproduce all results in the paper are within the main text, electronic supplementary material and the Dryad Digital Repository: https://doi.org/10.5061/dryad.t4b8gtj1z [51].
The data are provided in the electronic supplementary material [52].