**3. Results**

## *3.1. Antioxidant Activity of Amino Acids and Dipeptides*

It has been reported that GSH [32] with cysteine residue showed excellent O2 •− scavenging ability than peptides with other residues. Our study confirmed that dipeptides with cysteine residue showed stronger activities than others due to the strong ability of Cys in scavenging O2 •− (shown in Figure 1a). It was seen that the O2 •− scavenging activities of these synthesized dipeptides ranked in the order CP(>GSH) > CD > CH > WC > YC > MC > IR > WH > AH > YH > MH > KP > MD > WD > KD > WY > MW > WP > YP > YD > MP > MY. However, the dipeptides with W, M, or Y residues seem to be inactive in scavenging O2 •−. Additionally, these synthesized dipeptides did not show any ability to chelate Fe2+ (data not shown).

Furthermore, as shown in Figure 1b, the amino acids Cys, Met, Trp, and Tyr were confirmed to have significant inhibition activity against linoleic acid oxidation after analyzing the antioxidant activities of 20 amino acids. The amino acid Cys showed stronger activity than GSH, followed by Tyr, Met, and Trp. The GE values of Cys, Met, Trp, and Tyr were 1.22 ± 0.12, 0.07 ± 0.01, 0.03 ± 0.01, and 0.07 ± 0.01 mmol/mmol, respectively. The remining amino acids did not show antioxidant activities. Moreover, 22 dipeptides were synthesized based on the inhibition activity of the 4 amino acids towards the oxidation of linoleic acid. It was observed that among these synthesized dipeptides, the peptide WY had the best antioxidant capacity with the GE value 80.29 ± 0.68 mmol/mmol, followed by peptides MY, MW, YH, MH, MC, WC(GSH), YC, MD, WD, CP, YD, CH, CD, YP, MP, WH, KD, WP and AH(0.12 ± 0.04 mmol/mmol) (IR and KP showed no inhibitory activity). Clearly, the more active amino acid residues existed, the stronger the inhibitory activity of dipeptides exhibited. Interestingly, Cys was observed to show an excellent antioxidation, but the dipeptides with Cys residue did not have more activity against linoleic acid oxidation than the dipeptides with Met, Trp, and Tyr residues. In addition, the dipeptides IR and KP did not show any activity in inhibiting linoleic acid oxidation even though they were reported to scavenge oxygen radicals [33]. Thus, data from our study indicated that the dipeptides containing Met, Trp, or Tyr residues should be selected as potential inhibitors of stopping linoleic acid oxidation.

**Figure 1.** Antioxidant activities of amino acids and peptides. (**a**) means activity of amino acids and dipeptides against O2 •−; (**b**) stands for activities of amino acids and dipeptides against linoleic acid; (**c**) shows effect of oligopeptides on the oxidation of O2 •−; (**d**) indicates effect of oligopeptides on the oxidation of linoleic acid, AHSVVYAIR, ADGF, HLHSAT were not presented for the poor capacity (NA: No activity was observed at concentration less than 2000 μg/mL.).
