## 3 Results and Discussion
### Amino Acid Composition  
The functions of the peptide are determined by the amino acid composition. The amino acid composition of peptide fractions was shown in Table 2. The hydrophilic amino acids including aspartic acid (Asp), arginine (Arg), and histidine (His) were correlated with metal-binding activities [23] and there was no significant difference on the contents of amino acids in different molecular weight peptides fractions. However, we found that Asp residue of three peptide fractions all did not participate in the reaction, particularly. Furthermore, the contents of other hydrophilic amino acids (Arg and His) in P3 had decreased after chelating with zinc.  
Compared with high molecular weight peptide fraction P3, various of amino acids in low molecular weight peptide fraction P1 with strong zinc chelating capacity participated in zinc chelation reaction except the hydrophilic amino acids. Additionally, the acidic-amino acids (His and Cys), serine (Ser), and glycine (Gly) in P1 were involved in the chelating reaction, but the contents of those amino acids had decreased in P3 after binding with zinc. This can be attribute to the low molecular weight peptide with higher surface charge, which provide more sites for chelating zinc and the occurrence of higher DH lead to the exposure of more amino acid residues whose side chains can participate in zinc coordination [10; 21]. A study of chelating peptides from oyster protein hydrolysates indicated that acidic amino acid residues including His and Cys were the major binding sites of zinc ion [15]. [PERSON] et al. [11] reported that Ser residues played an important role in the metal chelation reaction. [PERSON] et al. [25] reported that Gly can participate in the formation of complex Al(III). In addition, the imidazole ring of His and thiolate group of Cys in low molecular weight peptide fraction was able to efficiently chelate metal ions [26]. Metal ions could simultaneous bound with more bind sites including the N-terminal amino group and a distant donor atom in a macrochelate structure which caused by the presence of Pro in the internal position of the peptide sequence [26; 27], and the content of Pro with metal chelation capacity in P2 and P3 was remarkably lower than P1, but participated in the reaction. A previous study found that the combination of high His content and small peptide size was correlated with high copper chelating activity [28]. This result suggests that a variety of amino acids in low weight molecular peptide fraction with strong zinc chelation capacity involve in the chelation than in high molecular weight peptide fraction and it can provide more site for bounding zinc to forming a stable complex.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Amino Acid** & **P1** & **P1-Zn** & **P2** & **P2-Zn** & **P3** & **P3-Zn** \\ \hline Glu & 7.76 & 7.80 & 7.82 & 7.63 & 7.91 & 7.61 \\ Ser & 3.17 & 3.21 & 3.17 & 3.17 & 3.22 & 3.07 \\ His & 0.53 & 0.55 & 0.53 & 0.56 & 0.52 & 0.49 \\ Gly & 40.50 & 40.61 & 41.06 & 39.51 & 41.25 & 39.62 \\ Thr & 2.48 & 2.57 & 2.52 & 2.51 & 2.57 & 2.46 \\ Arg & 5.05 & 5.12 & 5.23 & 5.09 & 5.15 & 4.89 \\ Ala & 13.75 & 13.76 & 13.86 & 13.57 & 13.92 & 13.38 \\ Tyr & 0.23 & 0.17 & 0.15 & 0.14 & 0.17 & 0.13 \\ Cys & 1.52 & 1.64 & 1.44 & 2.31 & 1.63 & 1.57 \\ Val & 1.93 & 1.97 & 1.92 & 1.94 & 1.98 & 1.90 \\ Met & 1.00 & 0.87 & 0.75 & 0.94 & 0.80 & 0.81 \\ Phe & 1.26 & 1.26 & 1.25 & 1.24 & 1.27 & 1.22 \\ Ile & 0.96 & 0.96 & 0.95 & 0.95 & 0.96 & 0.93 \\ Leu & 2.17 & 2.17 & 2.17 & 2.16 & 2.19 & 2.11 \\ Lys & 2.62 & 2.67 & 2.71 & 2.56 & 2.73 & 2.60 \\ Pro & 10.39 & 10.05 & 9.82 & 11.13 & 9.02 & 12.67 \\ Asp & 4.66 & 4.65 & 4.66 & 4.61 & 4.71 & 4.54 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Amino acid composition (mole/%) of three molecular weight peptide fractions and their zinc complexes.