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Kinetic Effects of Hydrogen-bonds on Proton-Coupled Electron Transfer from Phenols
Stockholm University, Faculty of Science, Department of Organic Chemistry.
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2006 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 128, no 40, 13076-13083 p.Article in journal (Refereed) Published
Abstract [en]

The kinetics and mechanism of proton-coupled electron transfer (PCET) from a series of phenols to a laser flash generated [Ru(bpy)3]3+ oxidant in aqueous solution was investigated. The reaction followed a concerted electron−proton transfer mechanism (CEP), both for the substituted phenols with an intramolecular hydrogen bond to a carboxylate group and for those where the proton was directly transferred to water. Without internal hydrogen bonds the concerted mechanism gave a characteristic pH-dependent rate for the phenol form that followed a Marcus free energy dependence, first reported for an intramolecular PCET in Sjödin, M. et al. J. Am. Chem. Soc. 2000, 122, 3932−3962 and now demonstrated also for a bimolecular oxidation of unsubstituted phenol. With internal hydrogen bonds instead, the rate was no longer pH-dependent, because the proton was transferred to the carboxylate base. The results suggest that while a concerted reaction has a relatively high reorganization energy (λ), this may be significantly reduced by the hydrogen bonds, allowing for a lower barrier reaction path. It is further suggested that this is a general mechanism by which proton-coupled electron transfer in radical enzymes and model complexes may be promoted by hydrogen bonding. This is different from, and possibly in addition to, the generally suggested effect of hydrogen bonds on PCET in enhancing the proton vibrational wave function overlap between the reactant and donor states. In addition we demonstrate how the mechanism for phenol oxidation changes from a stepwise electron transfer−proton transfer with a stronger oxidant to a CEP with a weaker oxidant, for the same series of phenols. The hydrogen bonded CEP reaction may thus allow for a low energy barrier path that can operate efficiently at low driving forces, which is ideal for PCET reactions in biological systems.

Place, publisher, year, edition, pages
American Chemical Society , 2006. Vol. 128, no 40, 13076-13083 p.
National Category
Organic Chemistry
URN: urn:nbn:se:su:diva-22663DOI: 10.1021/ja063264fOAI: diva2:189233
Part of urn:nbn:se:su:diva-1024Available from: 2006-05-04 Created: 2006-05-04 Last updated: 2010-11-02Bibliographically approved
In thesis
1. Hydrogen Bonded Phenols as Models for Redox-Active Tyrosines in Enzymes
Open this publication in new window or tab >>Hydrogen Bonded Phenols as Models for Redox-Active Tyrosines in Enzymes
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis deals with the impact of hydrogen bonding on the properties of phenols. The possibility for tyrosine to form hydrogen bonds to other amino acids has been found to be important for its function as an electron transfer mediator in a number of important redox enzymes. This study has focused on modeling the function of tyrosine in Photosystem II, a crucial enzyme in the photosynthetic pathway of green plants.

Hydrogen bonds between phenol and amines in both inter- and intramolecular systems have been studied with quantum chemical calculations and also in some solid-state structures involving phenol and imidazole.

Different phenols linked to amines have been synthesized and their possibilities of forming intra- and intermolecular hydrogen bonds have been studied as well as the thermodynamics and kinetics of the generation of phenoxyl radicals via oxidation reactions.

Since carboxylates may in principle act as hydrogen bond acceptors in a manner similar to imidazole, proton coupled electron transfer has also been studied for a few phenols intramolecularly hydrogen bonded to carboxylates with the aim to elucidate the mechanism for oxidation. Electron transfer in a new linked phenol—ruthenium(II)trisbipyridine complex was studied as well.

The knowledge is important for the ultimate goal of the project, which is to transform solar energy into a fuel by an artificial mimic of the natural photosynthetic apparatus

Place, publisher, year, edition, pages
Stockholm: Institutionen för organisk kemi, 2006. 60 p.
electron transfer, photosystem II, radicals, imidazole, hydrogen bonding
National Category
Organic Chemistry
urn:nbn:se:su:diva-1024 (URN)91-7155-278-2 (ISBN)
Public defence
2006-05-31, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 12 A, Stockholm, 14:00
Available from: 2006-05-04 Created: 2006-05-04 Last updated: 2015-03-19Bibliographically approved

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Åkermark, Björn
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