## 3 Results
### Concentration Effect of Syr and Van for Regeneration of Trp from the Trp\({}^{\bullet+}\)  
Syr and Van are commonly used, naturally occurring reducing agents. The kinetics analyses at their characteristic wavelengths demonstrate their ability to regenerate the Trp from its radical cation (Trp\({}^{\bullet+}\)), as shown in Figure 3A,B. Moreover, the reduction rate of Trp\({}^{\bullet+}\) is dependent on the concentration of Syr and Van, as illustrated in Figure 3C,D. The observed pseudo-first-order rate constant was found to depend on the excess concentration of plant phenols according to the Stern-Volmer equation [25]:  
\[\frac{\tau_{0}}{\tau}=1+k_{\text{q}}\tau_{0}[Q] \tag{4}\]  
where \(\tau_{0}\) and \(\tau\) are the Trp\({}^{\bullet+}\) lifetimes in absence and presence of plant phenols, respectively, and [\(Q\)] is the concentration of plant phenols. The quenching rate \(k_{\text{q}}\) thus determined. From the kinetics monitored at 560 nm, the second-order reaction rate constants were determined from the pseudo-first-order rate constants for various Syr and Van concentrations within the concentration of 50 uM to 150 uM. The obtained results for Syr and Van are qualitatively  
Figure 2: _ns_-TA spectra at the representative delay time (\(\Delta t\)) and kinetic traces under aerobic conditions upon photoexcitation riboflavin (Rib) at 445 nm. (**A**) _ns_-TA spectra of the triplet state of riboflavin (\({}^{3}\)Rib\({}^{\bullet}\)). (**B**) _ns_-TA spectra of Rib with the presence of tryptophan (Trp). A long lifetime (>150 us) peak at 500–600 nm indicates the formation of Trp\({}^{\bullet+}\). (**C**) Kinetic traces of Rib. (**D**) Kinetic traces of Rib and Trp. Thicker solid lines in panels (**C**,**D**) are derived from the fitting curve. in agreement with the reported results for the reaction of Trp with \(\beta\)-Car\({}^{\bullet+}\)[13]. The rate constants display linear dependence on the concentration for \(\mathrm{Syr}\) (Figure 3C) and Van (Figure 3D), which suggests that the reaction between the Trp\({}^{\bullet+}\) and plant phenols follows second-order kinetic characteristics. The Stern-Volmer equation yields a quenching rate constant on Trp\({}^{\bullet+}\) at pH 7.2, for \(\mathrm{Syr}\) of \(4.6\times 10^{7}\) M\({}^{-1}\cdot\mathrm{s}^{-1}\), and for Van of \(1.4\times 10^{7}\) M\({}^{-1}\cdot\mathrm{s}^{-1}\) (Table 1). Higher pH Enhanced \(\mathrm{Syr}\) and Van for Regeneration of Trp from the Trp\({}^{\bullet+}\)  
In the decay of Trp\({}^{\bullet+}\) and the regeneration of Trp by \(\mathrm{Syr}\) and Van, an intriguing behavior was observed, with the reaction rate constants showing a pronounced dependence on pH. The pH dependence of the rate constant for \(\mathrm{Syr}\) and Van in the pH range from 2 to 10 could be described by a single acid-base equilibrium corresponding to the following pH dependence for the observed rate constant in Equation (5) [9], \(k_{\mathrm{obs}}\):  
\[k_{\mathrm{obs}}=k_{\mathrm{acid}}\left[10^{-\mathrm{pH}}/\left(10^{-\mathrm{ pH}}+10^{-pK_{\mathrm{s}}}\right)\right]+k_{\mathrm{base}}\left[10^{-\mathrm{ pH}}/\left(10^{-\mathrm{pH}}+10^{-pK_{\mathrm{s}}}\right)\right] \tag{5}\]  
The results obtained for the \(\mathrm{Syr}\) and Van showed that the kinetics of deactivation by plant phenols were similar to what was observed for the free amino acids (Figure 4A,B).