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

Abstract

<p>Photoionization with UV-A generates phenoxy radical <bold>ResO˙</bold> of the famous red-wine antioxidant resveratrol inside cyclodextrin cavities without the participation of auxiliary reactants. This permits the isolated exploration of the <bold>ResO˙</bold> repair by ascorbate at the water–cyclodextrin interface.</p>

Autoren

Christoph Kerzig
Martin Goez

DOI

10.1039/c6cp03742a

eISSN

1463-9084

ISSN

1463-9076

Ausgabe der Veröffentlichung

30

Zeitschrift

Physical Chemistry Chemical Physics

Sprache

en

Online publication date

2016

Paginierung

20802 - 20811

Status

Published online

Herausgeber

Royal Society of Chemistry (RSC)

Herausgeber URL

http://dx.doi.org/10.1039/c6cp03742a

Datum der Datenerfassung

2024

Titel

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

Ausgabe der Zeitschrift

18

Datenquelle: Crossref

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.

Addresses

Martin-Luther-Universität Halle-Wittenberg, Institut für Chemie, Kurt-Mothes-Str. 2, D-06120 Halle (Saale), Germany. martin.goez@chemie.uni-halle.de.

Autoren

Christoph Kerzig
Martin Goez

DOI

10.1039/c6cp03742a

eISSN

1463-9084

Externe Identifier

PubMed Identifier: 27418479

Open access

false

ISSN

1463-9076

Ausgabe der Veröffentlichung

30

Zeitschrift

Physical chemistry chemical physics : PCCP

Sprache

eng

Medium

Print

Paginierung

20802 - 20811

Datum der Veröffentlichung

2016

Status

Published

Datum der Datenerfassung

2016

Titel

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

Sub types

Journal Article

Ausgabe der Zeitschrift

18

Datenquelle: Europe PubMed Central

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.

Autoren

Christoph Kerzig
Martin Goez

Autoren-URL

https://www.ncbi.nlm.nih.gov/pubmed/27418479

DOI

10.1039/c6cp03742a

eISSN

1463-9084

Ausgabe der Veröffentlichung

30

Zeitschrift

Phys Chem Chem Phys

Sprache

eng

Country

England

Paginierung

20802 - 20811

Datum der Veröffentlichung

2016

Status

Published

Datum, an dem der Datensatz öffentlich gemacht wurde

2018

Titel

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

Sub types

Journal Article

Ausgabe der Zeitschrift

18

Datenquelle: PubMed

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

Werkzeuge