# Functional Characterization of Mung Bean Meal Protein-Derived Antioxidant Peptides
## 2 Results and Discussion
### Sequencing and Antioxidant Activities of Peptides
#### 2.3.1 DPPH Radical Scavenging Activity (DRSA)  
The peptides CGN, HC, CTN, and CSGD had the strongest DRSA as reflected in the lower EC\({}_{50}\) values when compared to the remaining peptides (Table 1). The results are consistent with a previous study, which indicated that the presence of asparagine (\(N\)), threonine (\(T\)), and aspartic acid (\(D\)) contributes strongly to the DRSA of food protein hydrolysates [27].  
Figure 6: De-convoluted annotated MS/MS fragmentation spectra of peptides in fraction 1C and the corresponding de novo sequencing: ERF (**A**), EYW (**B**), FLQL (**C**), and QFAW (**D**).  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{**Fractions**} & \multicolumn{1}{c}{**Sequences**} & \multicolumn{1}{c}{**\(m/z\)**} & \multicolumn{1}{c}{**z**} & \multicolumn{1}{c}{**Mass (Da)**} & \multicolumn{1}{c}{**DBSA (EC\({}_{\text{Bob}}\)**} & \multicolumn{1}{c}{**HBSA (EC\({}_{\text{Bob}}\)**} & \multicolumn{1}{c}{**SBSA (EC\({}_{\text{Bob}}\)**} & \multicolumn{1}{c}{**FRAP (mM Fc2\({}^{\text{a}}\)**} & \multicolumn{1}{c}{**MCA (EC\({}_{\text{Bob}}\)**}  
The presence of excess or readily donated electrons on these amino acid residues could have potentiated the DRSA of CGN, HC, CTN, and CSGD. Moreover, cysteine has been reported to enhance DRSA because the sulfur molecule and sulfhydryl group (SH group) in its molecule are stronger oxidizing agents than other amino acids without sulfur in their structure. This is more effective especially if the Cys is located in the N- and C-terminal [9; 15], which supports the strong DRSA of CGN, HC, CTN, and CSGD. The SH group can donate hydrogen atom to quench the DPPH radical, and in the process becomes S-, which reacts with and neutralizes other DPPH radical molecules to become sulfonyl (SO\({}_{2}\)) or disulfide (S-S) compounds, resulting in free radical chain termination [28]. The results obtained in this work show that all the most active peptides (CGN, HC, CTN, and CSGD) contained Cys in their N-terminal or C-terminal. The results are consistent with the role of cysteine as a major contributing amino acid to the strong antioxidant potency of GSH, a natural cellular tripeptide [9; 29]. While the peptides obtained in this study had higher EC\({}_{50}\) value of DRSA than GSH, the EC\({}_{50}\) values of CGN, HC, CTN, and CSGD are lower than that of GPP (EC\({}_{50}\) of 1.927 mg/mL), a peptide derived from Bluefin leatherjacket [30]. The lower EC\({}_{50}\) value indicates higher antioxidant potential. The results indicated that these peptides could inhibit DPPH radical better than GPP peptide. In contrast, peptide ERF had the poorest DRSA with a high EC\({}_{50}\) value of 4.87 mg/mL, which is almost 162 times lower than the activity obtained for GSH, the potent cellular antioxidant peptide. YBW, which has glutamic acid (\(E\)) at the N-terminal and QFAW also showed weaker DRSA when compared to CGN, HC, CTN, and CSGD. The presence of lysine (\(\gamma\)) has been reported to be negatively correlated with DRSA, which could be responsible for the weaker activity of YBW. It is pertinent to note that EYW, QFAW, and MMGW all have tryptophan at the C-terminal, which seem to reduce DRSA potency of the peptides. Peptide LAN had poor DPPH activities and were not able to effectively scavenge DPPH. Similarly, the three peptides with no detectable activity (LAF, QFAAD, and FLQL) all contain phenylalanine but whether this residue is responsible for the lack of DRSA remains to be investigated. However, for this set of peptides, the presence of aromatic amino acids correlated with weak DRSA. The results are consistent with previous studies that suggested that, while the low molecular weight of peptides could promote antioxidant activity, the sequence within the peptide chain and the position and type of amino acids were the main factors [11; 31; 32].