## 3 Results and Discussion
Trying to explore if the above discussed patterns of PYR protection can be extended to other amino acids and peptides, experiments using free tyrosine (Tyr), as well as peptides containing a single Tyr residue, were developed. The oxidation of Tyr by ROO\({}^{\bullet}\) involves the abstraction of an electron (as hydrogen atom at pH 7.4) to produce tyrosyl radicals (Tyr\({}^{\bullet}\)) [27]. Subsequently, Tyr\({}^{\bullet}\) react with O\({}_{2}\) (this is a slow reaction with k = 1 \(\times\) 10\({}^{3}\) M\({}^{-1}\)s\({}^{-1}\), [27]) to form a Tyr-OO\({}^{\bullet}\), or self-react producing di-tyrosine dimers (di-Tyr), amongst other reaction pathways [27]. As presented in Figure 5, Panel A, the consumption of 5 \(\upmu\)M PYR was inhibited by 1 mM free Tyr and peptides following the order of R\({}_{0}\):  
Tyr (0.25 \(\pm\) 0.03) \(\sim\) Met-Tyr-Pro-Gly-Ile-Ala (0.24 \(\pm\) 0.03) \(\sim\) Gln-Arg-Tyr-Arg-Val-Leu-Asn-Ala-Ser (0.24 \(\pm\) 0.09) \(\sim\) Tyr-Tyr-Tyr (0.20 \(\pm\) 0.04) \(>\) Gly-Tyr (0.12 \(\pm\) 0.07)  
Figure 3: Kinetics of 1 \(\upmu\)M PYR consumption triggered by ROO\({}^{\bullet}\) in the presence of peptides. Solutions of PYR were incubated in the absence (black circles) or presence of 1 mM free Trp and peptides. Panel (**A**): kinetic profiles in the presence of 1 mM Trp (red), Gly-Trp-Gly (olive), Glu-Val-Trp-Lys-Ser-Ala-Glu (cyan), Ser-Val-Trp-Ile-Gly-Gly-Ser-Ile (blue), and Asn-Trp-Asp-Asp-Met (magenta). Panel (**B**): Protection of 1 (\(\circ\)) and 5 (\(\upDelta\)) \(\upmu\)M PYR elicited by 1 mM Ser-Val-Trp-Ile-Gly-Spr-Ile. \(\mathrm{F/F_{0}}\) = normalized fluorescence intensity. Similarly to the peptides containing Trp, the chemical structure of peptides including a single Tyr residue also affected the protection of PYR. Interestingly, the tripeptide Tyr-Tyr showed a higher protection of PYR than free Tyr; however, this effect was not the expected to confer 3-fold-times higher protection, implying that each Tyr residue of Tyr-Tyr-Tyr did not react independently. Several reasons can be postulated to explain such an effect; however, it seems plausible to propose that the protective effect of Tyr-Tyr-Tyr is not easily explained by competitive reactions. The latter is in line with the pattern of the kinetic profiles obtained for the consumption of 1 \(\upmu\)M PYR in the presence of the Tyr residues. In the case of free Tyr and Gln-Arg-Tyr-Arg-Val-Leu-Asn-Ala-Ser, R\({}_{0}\) values of 0.20 \(\pm\) 0.06 and 0.23 \(\pm\) 0.04 \(\upmu\)M/min were determined, respectively. These values are similar to those assessed at 5 \(\upmu\)M PYR, suggesting some influence of PYR repair in the mechanism. Nonetheless, at 1 \(\upmu\)M PYR, Tyr-Tyr-Tyr showed a R\({}_{0}\) value significantly lower (0.09 \(\pm\) 0.02) than the value determined at 5 \(\upmu\)M PYR, indicating the relevance of competitive reactions for ROO\({}^{\bullet}\). In the case of Gly-Tyr, important changes were induced by changing the PYR concentration. At 5 \(\upmu\)M PYR, kinetics did not show lag times; however, a clear lag time was detected when 1 \(\upmu\)M PYR was employed (Figure 4, Panel B), demonstrating that even when lag times are present in the protection of PYR, competition by ROO\({}^{\bullet}\) is an important process. Figure 4: Panel **(A)**: Consumption of 5 \(\upmu\)M PYR triggered by AAPH-derived ROO\({}^{\bullet}\) in the absence (black circles), and presence of 2 \(\upmu\)M (green), 3 \(\upmu\)M (blue), 4 \(\upmu\)M (cyan), 5 \(\upmu\)M (magenta), and 10 \(\upmu\)M (olive) of Trolox. Panel **(B)**: Consumption of 5 \(\upmu\)M (black) and 1 \(\upmu\)M (red) PYR mediated by AAPH-derived ROO\({}^{\bullet}\) in the presence of 5 \(\upmu\)M Trolox. F/F\({}_{0}\) = normalized fluorescence intensity. Figure 5: Consumption of 5, Panel **(A)**, and 1 \(\upmu\)M, Panel **(B)**, PYR, mediated by AAPH-derived ROO\({}^{\bullet}\) in the absence (black circles) and presence of 1 mM free Tyr and peptides; free Tyr (red), Gly-Tyr (green), Tyr-Tyr-Tyr (olive), and Gln-Arg-Tyr-Arg-Val-Leu-Asn-Ala-Ser (magenta).