## 2 Results and Discussion
### Structure-Antioxidant Activity Relationship of Peptides  
The structural characteristics of peptides provide guides for the evaluation of food-derived proteins as potential precursors of antioxidant peptides and predict the possible release of bioactive peptides from various proteins using an appropriate protease [1]. Many researchers found that the antioxidant activity of peptides was highly dependent on their amino acid sequence and composition. [PERSON] et al. (2012) reported that the Gly residue may contribute significantly to antioxidant activity since the single hydrogen atom in the side chain of Gly serves as a proton-donating source and neutralizes active free radical species [44]. In addition, [PERSON] et al. (2015) reported that the single hydrogen atom of Gly (G) can provide high flexibility to the peptide backbone and positively influence the antioxidant properties [8]. Therefore, Gly residues might be important contributors to the antioxidant activity of SCPE-A, SCPE-B, and SCPE-C because there are one, two and four Gly residues in their amino acid sequences, respectively. The pyrrolidine ring of proline (P) can interact with the secondary structure of the peptide, thereby increasing the flexibility, and it is also capable of quenching singlet oxygen due to its low ionization potential [6]. [PERSON] and [PERSON] (2011) reported that the Pro residue plays an important role in the antioxidant activity of the peptide purified from _Saccharomyces cerevisiae_ protein hydrolysate [45]. Therefore, the one, one and two Pro residues in the amino acid sequences of SCPE-A, SCPE-B, and SCPE-C should enhance the radical-scavenging activities of the three peptides. Aromatic amino acids, such as Phe, Tyr, His, and Trp, and hydrophobic amino acids, including Ala, Val, and Leu, have been reported to be critical to the antioxidant activities of peptides [1]. [PERSON] et al. (2005) reported that amino acids with aromatic residues, such as Phe, Tyr and Trp, can quench free radicals by direct electron transfer [46]. The results from [PERSON] et al. (2015) indicated that hydrophobic amino acids (e.g., Val, Ala, Leu) and aromatic amino acids (Phe, His, Tyr and Trp) can enhance the radical-scavenging abilities of peptides from Chinese cherry seeds [4]. Therefore, the presence of the one Ala residue and two Phe residues in the sequences of SCPE-B and SCPE-C, respectively, should have a positive impact on their radical-scavenging and lipid peroxidation inhibitory activities. Figure 7: Lipid peroxidation inhibition assays of SCPE-A, SCPE-B and SCPE-C. All data are presented as the mean \(\pm\) SD of triplicate results. The presence of acidic and basic amino acids plays a critical role in the metal ion chelating activity, which is related to the carboxyl and amino groups in their side chains [47]. Similar results were reported by [PERSON] et al. (2012), who found that the basic (Arg) and acidic (Asp and Glu) amino acid residues in the sequences of Asn-Thr-Asp-Gly-Ser-Thr-Asp-Tyr-Gly-Ile-Leu-Gln-Ile-Asn-Ser-Arg (NTDGSTDYGILQINSR) and Leu-Asp-Glu-Pro-Asp-Pro-Leu-Ile (LDEPDPLI) were critical to their antioxidant activities [48]. [PERSON], et al. (2003) found that Glu is an effective cation chelator that forms complexes with calcium, iron and zinc and may contribute to the antioxidant activity [49]. Therefore, Glu in SCPE-A and Arg in SCPE-B might be favorable to their antioxidant activities. In addition, the antioxidant activities of peptides are dependent on their molecular size, and shorter peptides, especially peptides with 2-10 amino acid residues, have stronger radical-scavenging and lipid peroxidation inhibition activities than their parent native proteins or long-chain peptides [1; 46].