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

High Resolution Image Download MS PowerPoint Slide 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 SiO 2 @GA, SiO 2 @GLA, SiO 2 @GLAM, and the doubly hybrid material {GLA@SiO 2 @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 SiO 2 @GA hybrid exhibited the highest SET • OH-scavenging activity i.e., [2.7 mol of • OH per mol of grafted GA]. Then, SiO 2 @GLA, SiO 2 @GLAM, and GLA@SiO 2 @GLAM can scavenge 1.2, 1.3, and 0.57 mol of • OH per mol of anchored organic, respectively. The HAT efficiency for SiO 2 @GA was [2.0 mol of DPPH per mol of grafted GA], while SiO 2 @GLA, SiO 2 @GLAM, and GLA@SiO 2 @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.

Topics & Concepts