Iron(III) suppresses catechin antioxidant activity in tea: Spectroscopic and electrochemical support for a coordination–passivation–oxidation mechanism

Catechin is one of the main antioxidants in tea, and its mechanisms of action have been extensively studied. However, the inhibitory effects of metal ions remain unclear. In this study, the interaction between catechin and ferric ions (Fe 3+ ) was systematically investigated to clarify the mechanism behind their reduced antioxidant activity. Results showed that Fe 3+ significantly reduced the antioxidant capacity of catechin – for example, the DPPH scavenging rate dropped from 79.8 % (in the absence of Fe 3+ ) to 60.9 % with 0.1 mM Fe 3+ . Structural and spectroscopic analyses confirmed the formation of coordination complexes, while density functional theory (DFT) calculations revealed that Fe 3+ coordination altered the electron distribution and reduced the HOMO–LUMO energy gap. Based on these findings, a coordination–passivation–oxidation mechanism was proposed, whereby Fe 3+ binding modified the electron density and oxidation potential of catechin, weakened their antioxidant function. These results offer new insight into the redox behavior of tea polyphenols and may inform the development of targeted metal–polyphenol-based antioxidant strategies in functional foods.