## 2 Results and Discussion
### Antioxidant Activity
#### 2.3.4 Discussion on Structure-Activity Relationship of APs  
APs derived from dietary foods have given rise to more attention because they show enormous potential to be applied for both food and medical purposes due to their capability to promote human health by reducing oxidative stress [2; 4]. Clarifying the structure--function relationship will contribute to predicting the bioactivities of peptides and screening the high-quality protein materials for the preparation of APs [7; 22]. However, the antioxidant mechanism of food-derived peptides have not been fully worked out. Yet, investigations by [PERSON] and [PERSON] [2] and [PERSON] et al. [5] have indicated that the biological functions of peptides are distinctly affected by their amino acid composition, amino acid sequence, and molecular size. In addition, spatial conformation is thought to play a key role in their biological activities [31; 32]. APs can act as lipid peroxyl radical trap and hydrogen donor to play their functions [1]. Short peptides containing 2 to 10 amino acids usually show better activity than their parent native proteins do because they can easily react with active radicals, which allows them to display their potent actions in reaction system [2; 33]. In the experiment, F7-P3, F7-P5 and F7-P8 showed high antioxidant effect in the assays of radical scavenging, reducing power, and lipid peroxidation inhibition. These findings demonstrated that F7-P3 with five amino acid residues, F7-P5 with six amino acid residues, and F7-P8 with four amino acid residues could more easily and effectively react with free radicals and inhibit the propagating systems of lipid peroxidation in the tested model. Nevertheless, the antioxidant activity of F7-P1 (tripeptide) with the smallest molecular size among eight isolated peptides, was lower than those of F7-P3, F7-P5, and F7-P8. In addition, the reducing power of F7-P8 was weaker than those of F7-P3 and F7-P5. These findings imply that the bioactivities of APs were not only independent of molecular size, but also relied on other structural properties, such as their amino acid composition and amino acid sequences. It has been proven that the amino acid composition and sequence distinctively influence the bioactivities of APs. Previous investigations showed that Glycine (Gly) residue could supply high backbone flexibility of peptides and neutralize active radical species through donating the single hydrogen atom of its side chain [1; 34]. Gly is the richest amino acid in the sequences of collagens, and all members belonging to the collagen family are manifested by repetition domains of the tripeptides (Gly-X-Y) and drawn into the structure of the triple helix [18]. As showed in Table 1, eight isolated collagen peptides (F7-P1 to F7-P8) from hydrolysate of Spanish mackerel skins had the number of Gly residues ranging from one to three, which could be contributed to their antioxidant activities. [PERSON] et al. [1] and [PERSON] et al. [8] reported that polar amino acids, such as acidic (Asp and Glu) and basic (Lys, Arg, and His) amino acid residues in peptide sequences have a critical function in the  
Figure 5: Lipid peroxidation inhibition ability of eight antioxidant peptides (F7-P1 to F7-P8) from collagen hydrolysate fraction (F7) of Spanish mackerel (_S. nipbonius_) skin. All data are presented as the mean \(\pm\) SD of triplicate results. HO\(\bullet\) scavenging activities through metal ion chelating, which is positively related to the amino and carboxyl groups of their side chains. Arg and His residues in FPYLRH, and Glu residue in GIEWA show a significant effect on their oxidation resistance [1]. Glu, Asp, and Arg residues in LDEPDPLI and NTDGSTDYGILQINSR showed significant impact on their radical scavenging capacity [35]. Therefore, Asp residues may have a positive influence on the antioxidant capacity of F7-P3.