Photoionization access to cyclodextrin-encapsulated resveratrol phenoxy radicals and their repair by ascorbate across the phase boundary

Abstract

Repair reactions of phenoxy radicals by co-antioxidants are key parts of radical scavenging cascades in nature. Yet, kinetic and mechanistic studies of such repairs are scarce, particularly at biologically relevant interfaces. For the popular red-wine polyphenol resveratrol, we present the first example of repairing a cyclodextrin-complexed phenoxy radical by a water soluble co-antioxidant (ascorbate), a reaction of practical importance given the fact that both antioxidants and cyclodextrins are large-scale food additives. To prepare the phenoxy radical from its parent compound inside the cavities of native or hydroxypropyl-substituted α- and β-cyclodextrins, we employed laser photoionization with UV-A (355 nm), which does not rely on additional reagents, and therefore leaves the repair completely undisturbed. A global fit of the intensity dependence pinpoints the cyclodextrin influences on the biphotonic resveratrol ionization as a shift of the ground-state absorption spectrum and a longer life of the first excited state due to the suppression of the geometrical isomerization by the rigid containers, whereas the actual electron ejection from an upper excited state is almost medium-independent. The exchange of the phenoxy radical between the cyclodextrin interior and the aqueous bulk is immeasurably slow on the timescale of its repair by the ascorbate monoanion. Kinetic H/D isotope effects and activation entropies identify the repair at the cyclodextrin-water interface as a concerted proton-electron transfer with no mechanistic difference to homogeneous aqueous solution. The activation enthalpies reveal a steric repulsion between ascorbate and cyclodextrin that indicates a deeper embedding of the less hydrophilic phenoxy radical in the macrocycle compared to the parent compound, with the observed structure-rate relationships explainable on the basis of the cavity diameter and depth.