Resveratrol Radical Repair by Vitamin C at the Micelle-Water Interface: Unexpected Reaction Rates Explained by Ion-Dipole Interactions.

Repair reactions of lipophilic phenoxy radicals by hydrophilic co-antioxidants at model membranes are important for understanding the factors that govern the interactions between radical scavengers in biological systems. By using near-UV photoionization, we have selectively generated the phenoxy radical of the famous antioxidant resveratrol inside anionic (SDS), cationic (DTAC), or neutral (TX-100) micelles, as well as in homogeneous aqueous solution, and have compared its repairs in these media by the water-soluble co-antioxidants ascorbic acid and ascorbate monoanion. With all surfactants, these reactions are dynamic processes at the micelle-water interface. Whereas for the combinations ascorbate monoanion/ ionic micelle the repair rates can be rationalized by the Coulombic interactions, unexpected effects were observed with the neutral ascorbic acid and the charged micelles: for the anionic micelles, this repair is three times faster than in homogeneous solution, and two orders of magnitude faster than for the cationic micelles. Given that the repair by a concerted proton-electron transfer demands a coplanar arrangement of the resveratrol phenoxy centre sticking out into the Stern layer and the co-antioxidant hydroxy moiety approaching from the aqueous bulk, we explain these results by ion-dipole interactions: only at a negatively charged micellar surface does the direction of the large dipole moment of ascorbic acid lead to an orientation favourable for the repair.