## 3 Discussion  
At present, hundreds of antioxidant peptides have been isolated from different resources. However, there is still insufficient evidence to elucidate the structure-activity relationship of antioxidant peptides. Generally, molecular size, hydrophobicity, and amino acid composition and sequence are deemed to play key roles in their antioxidant capacities [7; 50].  
Bioactivities of antioxidant peptides are highly dependent on their molecular size because small antioxidants have the higher possibility to interact with free radicals to prevent the lipid peroxidation [3; 33]. Furthermore, smaller peptides are more likely to pass through the blood-brain barrier to perform their physiological functions in the body and have the high possibility to develop into new drugs [30; 51]. Therefore, shorter size peptides especially peptides with 2-10 amino acid residues are deemed to obtain stronger radical scavenging and lipid peroxidation inhibition activities than long-chain peptides [6; 7]. In the study, eight isolated peptides (TJP1 to TJP8) from protein hydrolysate of hairtail muscle are dipeptides (TJP2, TJP3, TJP6, and TJP7), tripeptides (TJP4, TJP5, and TJP8), or pentapeptide (TJP1), which help them to contact the target more easily to exert their bioactive properties.  
More importantly, amino acids play a critical role in the antioxidant of peptides. [PERSON] and [PERSON] reported that hydrophobic amino acids, such as Leu, Ile, Ala, Val, and Met, had high reactivity to hydrophobic PUFAs and exert their significant effects on radical scavenging in lipid-rich foods [7; 33]. [PERSON] et al. presented that the Ile and Ala residues contribute to the lipid peroxidation inhibitory and radical-scavenging and activities of GIEWA [6]. Therefore, Ala in the sequence of TJP4 and Ile residues in the sequence of TJP8 should positively influence its antioxidant activity. [PERSON] et al. [52] and [PERSON] et al. [53] confirmed that Met residue showed the highest antioxidant activity among all the amino acids because its large hydrophobic group can help peptides to facilitate the contacts with hydrophobic radical species. [PERSON] et al. further reported that Met residue in PMRGGGGYHY might work as a reactive site, where the peptide could scavenge oxidants through the formation of a sulfoxide structure after oxidation to stop free-radical chain reactions [54]. Moreover, aromatic residues of aromatic amino acids--including Phe, Trp, and Ty--can keep radical stable during the scavenging process through contributing protons to electron deficient radicals [38]. Tyr residues could turn free radicals into more stable phenoxy radicals to stop the peroxididizing chain reaction [55]. Therefore, Met residues in the sequences of TJP3 and Tyr residues in the sequences TJP8 should be the important contributors for the antioxidant activity of TJP3 and TJP8.  
Polar amino acids is reported to play a critical role in HO\(\bullet\) scavenging and metal ion chelating activities because of their carboxyl and amino groups in the side chains [56; 57]. [PERSON] et al. reported that peptides consisted of Lys, Glu, and Asp were identified to have strong abilities to chelate metal ions as well as scavenge HO\(\bullet\)[57]. [PERSON] et al. have reported that basic peptides had greater capacity to scavenge HO\(\bullet\) than acidic or neutral peptides [58]. [PERSON] et al. reported that the presence of basic amino acid of Lys was one of the main reasons for the antioxidant activity of NWDMEKIWD [10]. Gly residue is found to contribute significantly to antioxidant activity since the single hydrogen atom in its side chain can provide a high flexibility to the peptide backbone, serving as proton-donors and neutralizing active free radical species [59; 60]. Therefore, polar amino acids including Lys and Gly residues could play a critical role in the radical scavenging activities of TJP3, TJP4, and TJP8.