Switching the Redox Mechanism: Models for Proton-Coupled Electron Transfer from Tyrosine and Tryptophan
- 25 February 2005
- journal article
- Published by American Chemical Society (ACS) in Journal of the American Chemical Society
- Vol. 127 (11) , 3855-3863
- https://doi.org/10.1021/ja044395o
Abstract
The coupling of electron and proton transfer is an important controlling factor in radical proteins, such as photosystem II, ribinucleotide reductase, cytochrome oxidases, and DNA photolyase. This was investigated in model complexes in which a tyrosine or tryptophan residue was oxidized by a laser-flash generated trisbipyridine−RuIII moiety in an intramolecular, proton-coupled electron transfer (PCET) reaction. The PCET was found to proceed in a competition between a stepwise reaction, in which electron transfer is followed by deprotonation of the amino acid radical (ETPT), and a concerted reaction, in which both the electron and proton are transferred in a single reaction step (CEP). Moreover, we found that we could analyze the kinetic data for PCET by Marcus' theory for electron transfer. By altering the solution pH, the strength of the RuIII oxidant, or the identity of the amino acid, we could induce a switch between the two mechanisms and obtain quantitative data for the parameters that control which one will dominate. The characteristic pH-dependence of the CEP rate (M. Sjödin et al. J. Am. Chem. Soc.2000, 122, 3932) reflects the pH-dependence of the driving force caused by proton release to the bulk. For the pH-independent ETPT on the other hand, the driving force of the rate-determining ET step is pH-independent and smaller. On the other hand, temperature-dependent data showed that the reorganization energy was higher for CEP, while the pre-exponential factors showed no significant difference between the mechanisms. Thus, the opposing effect of the differences in driving force and reorganization energy determines which of the mechanisms will dominate. Our results show that a concerted mechanism is in general quite likely and provides a low-barrier reaction pathway for weakly exoergonic reactions. In addition, the kinetic isotope effect was much higher for CEP (kH/kD > 10) than for ETPT (kH/kD = 2), consistent with significant changes along the proton reaction coordinate in the rate-determining step of CEP.Keywords
This publication has 37 references indexed in Scilit:
- De Novo Proteins as Models of Radical EnzymesBiochemistry, 1999
- PROTON-COUPLED ELECTRON TRANSFERAnnual Review of Physical Chemistry, 1998
- Mimicking Electron Transfer Reactions in Photosystem II: Synthesis and Photochemical Characterization of a Ruthenium(II) Tris(bipyridyl) Complex with a Covalently Linked TyrosineJournal of the American Chemical Society, 1997
- High-driving-force electron transfer in metalloproteins: intramolecular oxidation of ferrocytochrome c by Ru(2,2'-bpy)2(im)(his-33)3+Journal of the American Chemical Society, 1991
- The dynamic aspects of proton transfer processesBiochimica et Biophysica Acta (BBA) - Bioenergetics, 1990
- Electron transfers in chemistry and biologyBiochimica et Biophysica Acta (BBA) - Reviews on Bioenergetics, 1985
- Determination of the acidity constants of some phenol radical cations by means of electron spin resonanceJournal of the Chemical Society, Faraday Transactions 2: Molecular and Chemical Physics, 1976
- CHEMILUMINESCENCE FROM THE REDUCTION OF AROMATIC AMINE CATIONS AND RUTHENIUM(III) CHELATES*,‡Photochemistry and Photobiology, 1971
- The Chemistry of Stable Phenoxy RadicalsChemical Reviews, 1967
- Proton Transfer, Acid‐Base Catalysis, and Enzymatic Hydrolysis. Part I: ELEMENTARY PROCESSESAngewandte Chemie International Edition in English, 1964