Hybrids of Gallic Acid@SiO2 and {Hyaluronic-Acid Counterpats}@SiO2 against Hydroxyl (●OH) Radicals Studied by EPR: A Comparative Study vs Their Antioxidant Hydrogen Atom Transfer Activity

Abstract

Hydrogen atom transfer (HAT) and single electron transfer (SET) are two fundamental pathways for antiradical/antioxidant processes; however, a systematic in-tandem operational evaluation of the same system is lacking. Herein, we present a comparative study of the HAT and SET processes applied to a library of well-characterized hybrid materials SiO2@GA, SiO2@GLA, SiO2@GLAM, and the doubly hybrid material {GLA@SiO2@GLAM}. Hydroxyl radicals (•OH), produced by a Fenton system, react via the single electron transfer (SET) pathway and hydrogen atom transfer, through oxygen- and carbon-atoms, respectively, while the stable-radical DPPH via the HAT pathway through oxygen-atoms. Electron paramagnetic resonance spectroscopy (EPR), eminently suited for in situ detection and quantification of free radicals, was used as a state-of-the-art tool to monitor •OH using the spin-trapping-EPR method. We found that the SiO2@GA hybrid exhibited the highest SET •OH-scavenging activity i.e., [2.7 mol of •OH per mol of grafted GA]. Then, SiO2@GLA, SiO2@GLAM, and GLA@SiO2@GLAM can scavenge 1.2, 1.3, and 0.57 mol of •OH per mol of anchored organic, respectively. The HAT efficiency for SiO2@GA was [2.0 mol of DPPH per mol of grafted GA], while SiO2@GLA, SiO2@GLAM, and GLA@SiO2@GLAM exhibited a HAT efficiency of 1.1 DPPH moles per mol of anchored organic. The data are analyzed based on the molecular structure of the organics and their −R-OH moieties. Accordingly, based on the present data we suggest that for hydroxyl (•OH) radicals, the mechanisms involved are SET from an oxygen atom and HAT from a carbon atom. In contrast, for DPPH radicals, the HAT mechanism is exclusively operating and involves hydrogen atom abstraction from OH groups.