# The Structure-Activity Relationship of the Antioxidant Peptides from Natural Proteins
## 3 The Relationship of Chemical Structure and the Antioxidant Ability of Peptides
### The Relationship of Peptide Structure and Its Antioxidant Activity
#### 3.2.3 Amino Acids Sequence  
The interactions between amino acids may be an important factor for the antioxidant activity. Besides the hydrophobicity of peptides with high antioxidant activity, the amphiphilic nature of peptides also seems to enhance the radical-scavenging activities by increasing peptide solubility while facilitating interaction and proton exchanges with radical species. For example, when myofibrillar protein from path was hydrolys using papain, the FVNQPYLLYSVHMK peptide exhibited the highest antioxidant activity. In the sequence of the peptide, the presences of hydrophobic amino acids (Leu, Val and Phe), hydrophilic and basic amino acids (His, Pro and Lys), and aromatic amino acids (Phe and Tyr) in the peptide sequences are believed to contribute to its overall high antioxidant activity [50]. In addition, the antioxidant activity of two peptides, namely, Gly-Pro-Pro and Trp-Glu-Gly-Pro-Lys were investigated. The results showed that Gly-Pro-Pro exhibited the highest scavenging activity on DPPH radicals (EC\({}_{50}\) 6310 uM), hydroxyl radicals (EC\({}_{50}\) 7700 uM), and ABTS radicals (EC 8100 uM), whereas Trp-Glu-Gly-Pro-Lys could not effectively inhibit the peroxidation of linoleic acid [18]. The mechanism behind these results may stem from the inhibiting lipid peroxidation assays being conducted in the presence of lipids (e.g., FTC assay) or an emulsion, which would help the amphiphilic properties of the antioxidants enter into the lipid system so that they could react with the free radicals. [PERSON] and coworkers have investigated the interaction effects among the amino acids in the peptide sequences on the antioxidant capacity in detail [51]. Six peptides were synthetized according the structural motif of the most potent antioxidant peptides derived from barley protein, which contained pentapeptide Gln-Pro-Tyr-Pro-Gln with appended Gln and Pro residues. The contribution of vicinal residues to their antioxidant capacity was investigated by using repetitive sequences of Gln, Pro and Tyr. The results showed that the peptides displayed a significant positive role in stabilizing the peptide radicals in the free radical scavenging assays, as well as in effectively inactivating lipid hydroperoxides, intracellular reactive oxygen species, and inhibition of amyloid fibril formation in a cellular model. The carboxyl of Glu located next to Tyr could induce the release of the hydrogen atom of the phenolic hydroxyl in Tyr, enhancing the antioxidant activity. The results confirm that combined effect of the Gln-Pro and Pro-Tyr pairs and appending Gln and Pro on antioxidant capacity of peptides is efficient [51]. In addition, Trp in the sequence is another good example of influence on the antioxidant activity of neighboring amino acid residues. Trp has been determined to influence high antioxidant activity, and shows various activities in different sequences. The reason may be the special capacity of the indole ring of Trp to serve as an active hydrogen donor of the indole radical, and the stability it provides to neighboring amino acid residues with different physicochemical properties, which influence the antioxidant activity of the total peptides [33]. The sulfhydryl (SH) group in Cys is a hydrogen donor which comes from the SH group or the loss of an electron from its sulfur atom. The Trp residues with an aromatic ring structure have been reported to donate a proton to electron deficient radicals. Therefore, Cys and Trp would make a significant contribution to the antioxidant activities of certain peptides, when they are present in the same peptide chain. The characteristic structure of the amino acids linked to the metal chelating ability of peptides was investigated by [PERSON] _et al._[52]. According to their results, there is a excellent linear relationship between His-peptides and its metal chelating ability with \(R^{2}\) values amounting to 0.9 [52].