## 4 Conclusions  
The antioxidant activity towards peroxyl radicals of Trp and Tyr residues, evaluated by competitive reactions using pyranine as a probe, involves complex mechanisms where intermediates and oxidation products of Trp and Tyr would play an important role. Particularly, reactions between the secondary radical of pyranine (pyranine*) and Trp, or Tyr (free or included in peptides), to repair pyranine, could be relevant; however, these processes are not necessarily evidenced by the presence of lag times in the kinetics. Conversely, the absence of lag times in the kinetic profiles would not indicate (exclusive) competition for peroxyl radicals. The antioxidant behavior of Trp and Tyr residues contrasts with the high antioxidant activity determined for a phenolic compound (Trolox), which is mainly associated with a repairing mechanism. A comparison of the results with data obtained by using fluorescein as a probe shows that the antioxidant capacity of free and peptide Trp and Tyr residues strongly depends on the target molecule employed. Thus, the antioxidant activity of Trp and Tyr, determined by competitive assays, should be carefully examined. Despite such a limitation, it seems clear that the antioxidant activity of Trp and Tyr residues would be significantly lower than natural polyphenols, implying that the role that peptides play as antioxidants in complex systems is not totally established. Therefore, new investigations aimed at understanding the mechanisms behind the antioxidant activity of peptides in complex systems (foods, natural products, etc.) are required.  
The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/app15084241/s1](https://www.mdpi.com/article/10.3390/app15084241/s1)]([https://www.mdpi.com/article/10.3390/app15084241/s1](https://www.mdpi.com/article/10.3390/app15084241/s1)). Figure S1. Kinetic profiles of 5 uM PYR consumption mediated by ROO* in the absence (black) and presence of 1 mM (blue) and 3 mM (red) of free Gly. PYR solutions were incubated at 37 \({}^{\circ}\)C in phosphate buffer (75 mM, pH 7.4) in the presence of 10 mM AAPH. PYR consumption was followed by its emission of fluorescence at 510 nm (excitation at 460 nm). Figure S2. Circular dichroism spectra of tryptophan- and tyrosine containing peptides (4-10 mM) in phosphate buffer 75 mM, pH 7.4. A) Glu-Val-Trp-Lys-Ser-Ala-Glu (4.0 mM), B) Ser-Val-Trp-Ile-Gly-Gly-Ser-Ile (5.0 mM), C) Asn-Trp-Asp-Asp-Met (5.0 mM), D) Gln-Arg-Tyr-Arg-Val-Leu-Asn-Ala-Ser (9.0 mM), E) Glu-Tyr-Asp-Glu-Ala-Gly-Pro-Ser (10.7 mM), F) Met-Tyr-Pro-Gly-Ile-Ala (9.1 mM). Spectra were registered in a JASCO J-1700 circular Dichroism spectrophotometer. Conditions: response of 1 s, data pitch of 0.025 nm, 0.5 nm bandwidth, and 0.1 cm path length cell. Figure S3. Kinetic profiles of 5 uM PYR consumption mediated by ROO* in the absence (black) and presence of 5 mM (blue) of free Met. PYR solution was incubated at 37 \({}^{\circ}\)C in phosphate buffer (75 mM, pH 7.4) in the presence of 10 mM AAPH. PYR consumption was followed by its emission of fluorescence at 510 nm (excitation at 460 nm).  
Conceptualization, formal analysis, methodology, validation, writing--review and editing; C.L.-A., M.E.A. and A.C.F.-G.; investigation, A.C.F.-G.; resources, C.L.-A.; data curation, C.L.-A.; writing--original draft preparation, C.L.-A.; visualization, C.L.-A. and A.C.F.-G.; supervision, C.L.-A.; project administration, C.L.-A.; funding acquisition, C.L.-A. All authors have read and agreed to the published version of the manuscript.  
This research was funded by ANID Chile through Fondecyt, grant numbers 1220459 and Fondecquip EQM230048 (to C.L.-A), Fondecyt, grant number 1210751 (to M.E.A.), and Postdoctoral grant 3240525 (to A.F.-G).  
Not applicable.  
The data supporting this article have been included in the Supplementary Materials.  
The authors declare no conflicts of interest.