Antioxidant activity and chelating capacity of tryptophan, tyrosine and phenylalanine

Abstract

Aromatic amino acids such as tyrosine (Tyr), tryptophan (Trp), and phenylalanine (Phe) are important constituents of marine dissolved organic matter (DOM), where they may influence trace metal chemistry through redox and chelating mechanisms. These amino acids are known to participate in key biogeochemical processes by reducing transition metal ions and forming stable complexes, thereby modulating metal speciation, bioavailability, and oxidative stress in marine systems. In this study, the antioxidant activity of Tyr, Trp, and Phe was evaluated through two redox-based assays, ferric reducing antioxidant power (FRAP) and cupric reducing antioxidant capacity (CUPRAC), as well as by assessing their ability to chelate Fe²⁺ and Cu²⁺. These amino acids were studied at concentrations ranging from 100 to 500 μM. The results revealed that Trp exhibited the highest ferric reducing capacity (absorbance = 0.232 at 500 μM), followed by Tyr (0.177 at the same concentration), while Phe showed minimal activity. In the CUPRAC assay, Tyr presented a strong and nonlinear reduction of Cu²⁺, reaching an absorbance of 0.321 (at 500 μM), while Trp and Phe followed with lower values. Chelation assays showed limited Fe²⁺ binding (max. 13.21% for Phe), but all three amino acids demonstrated high Cu²⁺ complexation at 500 μM: 84.60% for Trp, 82.60% for Phe, and 79.20% for Tyr. These findings highlight the influence of side-chain structure, pKa, and assay conditions on the antioxidant and chelating behavior of amino acids. The strong reactivity of Trp and Tyr, in particular, supports their potential role in regulating metal availability and oxidative balance in marine environments, contributing to the broader understanding of DOM–metal interactions.