## 4 Discussion  
\({}^{3}\)Rib\({}^{\bullet}\) has a strong oxidizing potential (1.77 V vs. NHE) capable of oxidizing Trp (1.015 V vs. NHE) to form Trp\({}^{\bullet+}\). The rate at which plant phenols regenerate Trp from Trp\({}^{\bullet+}\) depends on conditions such as plant phenols concentration, the pH of the environment, and the driving force of the reaction. The following aspects will be discussed in detail below. The efficiencies of Syr and Van in regenerating Trp from Trp\({}^{\bullet+}\) are concentration-dependent, as shown in Figure 3. Within a certain range, the reduction rate increases with a higher concentration of plant phenols, a phenomenon that aligns with the Stern-Volmer equation (Equation (4)). Similar trends have been observed in other studies, such as the reduction of carotenoid radical cations by plant phenols. For instance, as previously reported for the puerarin dianion reacting with carotenoid radical cations, the second-order  
Figure 5: \(\Delta E\) dependence on the regeneration of Trp from Trp\({}^{\bullet+}\) (**A**) Kinetics traces of Trp\({}^{\bullet+}\) were probed at 560 nm. Regeneration of Trp from Trp\({}^{\bullet+}\) by different polyphenols at pH 9 with the presence of \(\varphi\)-O\({}^{-}\). Thicker solid lines are derived from curve fitting. (**B**) Solid squares show the \(\Delta E\) dependence of log(k\({}_{2}\)), and the solid line is derived by fitting the data to Equation (6) (\(\Delta E=E_{\text{Trp}^{\bullet+}}^{\odot}-E_{\text{pheno}}^{\odot}\)) [20]. Sal: salicylic acid, m-hyd: m-hydroxyl-benzoic acid, Syr: syringic acid, Van: vanville acid, Caf: caffe acid, Cat: catechin. rate constants were consistent for \(\beta\)-carotene, which was found to increase linearly with plant phenols concentration [24]. However, at higher concentrations of plant phenolate, deviations from a simple pseudo-first-order reaction were observed, with indications of rate saturation at elevated phenol concentrations. The reduction potential decreases with increasing pH, as observed in the recovery of Trp by Syr and Van. The standard reduction potential for the Trp radical is 1.015 V at pH 7 in water but decreases to 0.65 V at pH 13 [10]. Concurrently, the proportion of \(\varphi\)-O\({}^{2-}\) increases with rising pH, from 0% at pH 2 to 74% at pH 10 for Syr and from 0% at pH 2 to 42% at pH 10 for Van (Table 2). This increase enhances the reducing ability of plant phenols and accelerates the reduction rate. The proportion of deprotonated phenol radicals is strongly influenced by pH due to the dissociation equilibrium of hydroxyl groups (-OH) in plant phenol molecules. The primary mechanisms are as follows [34; 35]: (i) Changes in dissociation equilibrium. Hydroxyl groups in plant phenols can deprotonate to form phenoxy radicals (\(\varphi\)-O\({}^{\bullet}\)) under alkaline conditions. At low pH (acidic conditions), the high concentration of H\({}^{+}\) favors the protonated state, resulting in fewer phenoxy radicals. In contrast, high pH facilitates deprotonation, significantly increasing the formation of phenoxy radicals. (ii) Role of p_Ka_. The acid dissociation constants (p_Ka_) of hydroxyl groups in plant phenols determine the balance between protonated and deprotonated forms. As pH approaches the pKa, the equilibrium shifts, leading to noticeable changes in the proportion of deprotonated radicals. (iii) Radicals stability. The stability of phenoxy radicals varies with pH. Under alkaline conditions, delocalization of the negative charge across the aromatic ring via conjugation stabilizes deprotonated phenoxy radicals. (iv) pH and redox reactions. pH also affects the rate and equilibrium of redox reactions involving plant phenols, modulating the formation and consumption of deprotonated radicals. The regeneration of Trp is further influenced by the driving force, as calculated from the standard reduction potentials. Optimal regeneration conditions align with the reorganization energy of the donor-acceptor complex, as described by [PERSON] theory [26; 36].