Structure–activity relationship of antioxidant dipeptides: Dominant role of Tyr, Trp, Cys and Met residues

• Tyr, Trp, Cys and Met residues played a dominant role in antioxidant dipeptides.
• Antioxidant dipeptides mainly acted as direct radical scavengers.
• Tyr- and Trp-containing dipeptides were the most potent radical scavengers.
• Tyr/Trp-containing antioxidant dipeptides preferred Tyr/Trp residue at N-terminus.
• Steric effect, hydrogen bond and secondary reaction products affected the activity.

Antioxidant activities of amino acids and synthetic dipeptides were studied to elucidate the structure–activity relationship of antioxidant dipeptides. Results showed that Tyr- and Trp-containing dipeptides showed the highest radical scavenging activities in both ABTS and ORAC assays with Trolox equivalent (TE) values ranging from 0.69 to 4.97 µmol TE/µmol peptide, followed by Cys- and Met-containing dipeptides. With respect to their reducing power and Fe2+ chelating abilities, only Cys-containing dipeptides showed moderate reducing power, and none of them displayed Fe2+chelating ability. It indicated that antioxidant dipeptides mainly acted as radical scavengers and the presence of Tyr, Trp, Cys or Met residue with electron/hydrogen donating ability was the driving force for dipeptides to scavenge radicals. Additionally, Tyr- and Trp-containing dipeptides with Tyr/Trp residue at the N-terminus showed stronger antioxidant activities than that at the C-terminus, and the neighboring residue also affected their activities by steric effect, hydrophobicity and hydrogen bonding among others.