## 1 Introduction  
The biological activity of free amino acids and peptides is a matter of particular interest for the food industry and scientists [1, 2]. Significant efforts have been made by researchers to find and characterize natural peptides that have displayed different bioactivities [1, 2]. Amongst the biological activities reported, the capacity of peptides to inhibit the reactions initiated by radicals involved in the deterioration of foods or biological matrices has been extensively explored [3]. In this context, classical methodologies originally employed for quantifying the antioxidant capacity (AC) of enriched polyphenable samples have been applied to free amino acids, peptides, and proteins [2, 4, 5, 6]. Reported data have shown the bleaching of stable radicals, such as 2,2-Diphenyl-I-picrylhydrazyl (DPPH) and 2,2'-azino-bis(3-ethylbenzothazine-6-sulfonic acid) radical anions (ABTS), elicited by free amino acids, peptides, and their complex mixtures [3, 7, 8]. The reducing activity of these samples has also been studied with regard to ferric ions (ferric reducing antioxidant power, FRAP) [3, 9]. Particular importance has been given to the methodologies based on competitive reactions, where a target molecule (probe) competes with additives (antioxidants, XH) for peroxyl radicals(ROO\({}^{\bullet}\)), with the latter species commonly generated by the thermolysis of 2,2\({}^{\prime}\)-azo-bis(2-amidinopropane) dihydrochloride (AAPH), according to Reactions (1) to (4) [10; 11]. \[\text{AAPH}\to 2\text{R}^{\bullet}+\text{N}_{2} \tag{1}\]  
\[\text{R}^{\bullet}+\text{O}_{2}\rightarrow\text{ROO}^{\bullet} \tag{2}\]  
\[\text{Probe}+\text{ROO}^{\bullet}\rightarrow\text{bleaching} \tag{3}\]  
\[\text{XH}+\text{ROO}^{\bullet}\rightarrow\text{non radical products} \tag{4}\]  
In these assays, the AC of a particular sample is frequently assessed by determinations of the area under the curve (AUC) or the initial consumption rate of the kinetic profiles of the consumption of the probe (usually followed by UV-visible or fluorescence spectroscopy). These data are commonly normalized to the response given by 6-hydroxy-2,5,8-tetramethyl chroman-2-carboxylic acid (Trolox), a hydrosoluble vitamin E analog. While the determination of the AUC is employed in the oxygen radical absorbance (ORAC) assay, analysis of the initial consumption rates has been lesser used to asses AC. Fluorescein (FL) is the most accepted probe in the ORAC assay (ORAC-FL); however, other molecules, such as pyrogallol red (PGR) and pyranine (PYR), have also been employed as target molecules in this methodology [12; 13; 14]. The reactivity of FL, and the low concentration employed in the assay (usually 70 nM), explain why this probe is easily protected by XH [12]. The latter is inferred from the presence of lag times in the kinetic profiles which suggest that the ORAC-FL index is mainly governed by the stoichiometry of the reactions. In addition to this, the use of FL as probe would imply that alkoxyl radicals (RO\({}^{\bullet}\)) generated from self-reactions of ROO\({}^{\bullet}\) (Reactions (5) and (6)) are the principal oxidants of the system [15]. \[\text{2 ROO}^{\bullet}\rightarrow\text{ROO-OOR} \tag{5}\]  
\[\text{ROO-OOR}\rightarrow\text{2 RO}^{\bullet}+\text{O}_{2} \tag{6}\]  
In contrast to FL, the high reactivity of PGR, and its use at a higher concentration (5 uM), would explain why this probe is usually protected by XH without generating lag times in the kinetics [12]. As the consumption of PGR follows a zero-order kinetic limit, all ROO\({}^{\bullet}\) generated by the thermolysis of AAPH are neutralized, minimizing the formation of RO\({}^{\bullet}\)[12]. Thus, the ORAC assay using PGR as a probe (ORAC-PGR) gives values mainly associated with the reactivity of the sample towards ROO\({}^{\bullet}\)[12]. Similarly to PGR, PYR is highly reactive towards ROO\({}^{\bullet}\) with its consumption, in the micromolar concentration range, also following a zero-order kinetic limit [13].