Antioxidant Hydrogen-Atom-Transfer to DPPH Radicals by Hybrids of {Hyaluronic-Acid Components}@SiO₂

Hydrogen-atom-transfer (HAT) is among the key mechanisms of antioxidant and antiradical activity in natural systems. Hyaluronic acid (HyA) is currently used extensively in health and cosmetics applications. Herein it is shown that {HyA@SiO₂} hybrids based on hyaluronic acid (HyA) components grafted on SiO₂ nanoparticles enable significant HAT activity versus DPPH radicals, while the homogeneous HyA counterparts are practically inactive. The {HyA@SiO₂} hybrids consist of the two building blocks of HyA [d-glucuronic acid (GLA) and N-acetyl-d-glucosamine (GLAM)] covalently grafted on SiO₂ nanoparticles. Based on the kinetic-thermodynamic Arrhenius study, we show that the {SiO₂@GLA} hybrids operate spontaneously via hydrogen-atom-transfer (HAT) with a low activation energy barrier, i.e., by ΔΕ ∼ 20 kJ/mol vs the nongrafted counterparts. Moreover, a doubly grafted {GLA@SiO₂@GLAM} nanohybrid, i.e. that contains both components of HyA, shows the most significant antioxidant activity. FTIR and Raman analysis reveal that local H-bonding between the SiO₂ matrix, GLA, and GLAM in {GLA@SiO₂@GLAM} decreases the activation barrier of the HAT mechanism. Thus, {GLA@SiO₂@GLAM} nanohybrids exemplify a novel family of materials that are not the mere sum of their components. The present case is the first example of non-phenolic molecules being able to perform antiradical HAT, opening new perspectives not foreseen until today.