Free radical reactions with hydroxytyrosol and tyrosol in aqueous media: insight into radical scavenging and antioxidant behaviour

In the present work, pulse radiolysis studies have been employed to investigate the reactivity of HTy and Ty toward a variety of reducing and oxidizing free radicals, such as hydrated electron ( e ─ 𝑎 ⁢ 𝑞 ), hydroxyl radical ( H O · ) and azide radical ( N · 3 ). A comparative approach was adopted to establish how the additional phenolic hydroxyl group in HTy modulates the radical-induced redox pathways relative to Ty. The transient absorption behaviour, formation/decay kinetics, and dose-dependent studies may provide direct insight into the initial steps governing their antioxidant action. Complementary antioxidant assays based on ABTS •+ and DPPH radicals further support the radiation-chemical observations and reveal remarkably stronger radical scavenging by HTy than by Ty. The superior reactivity of HTy is attributed to its catechol functionality, which favours rapid hydrogen atom/electron transfer and enhanced stabilization of the resulting phenoxyl-type intermediates, whereas the monophenolic Ty exhibits substantially weaker response. Overall, this study provides a fundamental kinetic and mechanistic basis for understanding the antioxidant behaviour of these olive phenolic compounds and highlights how subtle structural variation governs their redox efficiency against biologically relevant radical species.