Radical Scavenging Activity of Simple Marine Bromophenols: Mechanistic and Kinetic Insights into Oxygenated Radical...

Bromophenols (BPs) are halogenated phenolic compounds widely found in marine organisms and recognized for diverse biological activities, particularly antioxidant effects. However, the intrinsic factors governing their radical scavenging behavior remain insufficiently understood. In this study, the antioxidant properties of four naturally abundant BPs, 4-bromophenol (1), 2,4-dibromophenol (2), 2,6-dibromophenol (3), and 2,4,6-tribromophenol (4), were systematically investigated. Their HO^• and HOO^• scavenging activities were evaluated using combined kinetic experiments (modified CUPRAC and PABA assays) and quantum chemical calculations in both polar and nonpolar environments. All compounds exhibited excellent HO^• scavenging activity, with rate constants around 10⁹ M^-1 s^-1 in aqueous solution and comparable reactivity in lipid-like media, indicating an environment-independent process dominated by a hydrogen atom transfer (HAT) mechanism. In contrast, HOO^• scavenging showed strong solvent dependence. In water, rate constants were on the order of 10⁴ M^-1 s^-1, while activity significantly decreased in nonpolar media. Mechanistic analysis indicates that, under polar conditions, the sequential proton loss-electron transfer (SPLET) pathway predominates, whereas electron transfer mechanisms are disfavored in non polar environments. Overall, BPs are effective radical scavengers, particularly in polar media, highlighting their potential as simple antioxidant motifs.