# 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
#### 3.2.2 Amino Acid Composition  
It was found that a total of 20 amino acids in the human body exist in the 42 antioxidant peptides (Table 1). To track some preferential amino acids for their antioxidant activities, the number of each component amino acid in the 42 peptides in Table 1 were analyzed and the result is illustrated in Figure 3. The differences in the percentages of amino acids indicate the unequal distribution of each amino acid in peptides. A larger number of some amino acids, such as Gly (G), Pro (P), Leu (L), account for 33.7% of total amino acids, and Ala (A), Tyr (F) and Val (V), 18.7%. Cys (S) were found as the least abundant amino acid following Met (M) and Gln (Q), which accounted for 4.9% of the number of amino acids in those antioxidant peptides. The effect of the amino acid composition of the peptides on antioxidant activity has been stressed in the literature. A high proportion of hydrophobic amino acids has been reported in peptides with high antioxidant activity, compared to other hydrophilic amino acids, which is considered as the key factor in peptide ability to scavenge radicals. For instance, the nanofiltration fraction (1-4 kDa) from tuna dark muscle by-product showed the highest superoxide radical and reducing power activities, which contained antioxidant amino acids such as Tyr, Phe, Pro, Ala, His, and Leu, which accounted for 30.3% of the total amino acids [43]. Furthermore, in three enzymolyzing systems, the highest antioxidant activity peptides from marine _Sepia breimana_ mantle was present in popsin hydrolyses, which may be due to increased exposure of internal hydrophobic amino acids during hydrolysis of the peptides in the smaller fractions compared to other hydrolyzing enzymes [21]. The antioxidant activity followed  
Figure 3: The percentage of each amino acid in the total AAs from 42 antioxidant peptides. Figure 2: The molecular weight of 42 antioxidant peptides in Table 1.  
a sequence order of Pro-Tyr-Ser-Phe-Lys > Gly-Phe-Gly-Pro-Glu-Leu > Val-Gly-Gly-Gly-Arg-Pro [12] for DPPH, OH radical and ABTS assays, while the order Trp-Pro-Pro > Gln-Pro was for hydroxyl radical scavengers, thus confirming the inference above [14]. The mechanism of action may be that the antioxidant peptides could smoothly enter into target organs through hydrophobic interactions with membrane lipid bilayers by the aid of their hydrophobicity, where they are able to exert significant capacity of scavenging radicals [30]. Hydrophobic amino acids tested frequently include His, Trp, Phe, Pro, Gly, lys, Ile and Val. His residues are credited with strong radical scavenging activity in oxidative reactions, especially for enzyme-catalyzed reactions, due to the presence of an imidazole ring as an important proton donor [3]. [PERSON] _et al._ obtained most antioxidant peptides from chickpea proteins, which commonly contains His residues [44]. Another study showed the antioxidant capacity of peptides could be enhanced notably by the presence of three aromatic amino acids (Trp, Tyr and Pre) [19]. In addition, the indole and pyrrolidine ring in Trp and Pro, respectively, could also serve as hydrogen donors via their hydroxyl groups, thus acting as hydroxyl radical scavengers. Furthermore, [PERSON] _et al._ suggested that higher radical scavenging activity was observed when peptides contained two Phe and two His residues [45]. The other amino acids, _i.e._, Gly, Lys, Ile and Val, may be responsible for forming a favorable hydrophobic micro-environment for peptide molecules. A comparison of seven peptide residues' (Leu-His-Tyr, Leu-Ala-Arg-Leu, Gly-Gly-Gly-Glu, Gly-Ala-His, Gly-Ala-Trp-Ala, Pro-His-Tyr-Leu and Gly-Ala-Leu-Ala-Ala-His) scavenging activities: showed that the first peptide (Leu-His-Tyr) displayed the highest DPPH radical-scavenging activity (63% \(\pm\) 1.57%; at 150 \(\upmu\)g/mL), which may confirm the foregoing theories [46].