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Tuning proton coupled electron transfer from tyrosine: A competition between concerted and step-wise mechanisms

Sjodin, M; Ghanem, Raed LU ; Polivka, Tomas LU ; Pan, Jie LU ; Styring, Stenbjörn LU ; Sun, LC; Sundström, Villy LU and Hammarstrom, L (2004) In Physical Chemistry Chemical Physics 6(20). p.4851-4858
Abstract
The intra-molecular, proton-coupled electron transfer from a tyrosine residue to covalently linked tris-bipyridine ruthenium(III) complexes in aqueous solution (Ru-III-TyrOH --> Ru-II-TyrO(.) + H+) is studied in two complexes. The Ru-III-TyrOH state is generated by laser flash-induced photo-oxidation in the presence of the electron acceptor methyl viologen. The reaction is shown to follow either a concerted electron transfer-deprotonation (CEP) mechanism or a step-wise mechanism with electron transfer followed by deprotonation (ETPT). The CEP is characterised by a pH-dependent rate constant, a large reorganisation energy (lambda = 1.4 eV at pH = 7) and a significant kinetic isotope effect: k(H)/k(D) = 1.5-3. We can explain the... (More)
The intra-molecular, proton-coupled electron transfer from a tyrosine residue to covalently linked tris-bipyridine ruthenium(III) complexes in aqueous solution (Ru-III-TyrOH --> Ru-II-TyrO(.) + H+) is studied in two complexes. The Ru-III-TyrOH state is generated by laser flash-induced photo-oxidation in the presence of the electron acceptor methyl viologen. The reaction is shown to follow either a concerted electron transfer-deprotonation (CEP) mechanism or a step-wise mechanism with electron transfer followed by deprotonation (ETPT). The CEP is characterised by a pH-dependent rate constant, a large reorganisation energy (lambda = 1.4 eV at pH = 7) and a significant kinetic isotope effect: k(H)/k(D) = 1.5-3. We can explain the pH-dependence and the high lambda by the pH-dependent DeltaGdegrees' for proton release to bulk water, and by the additional reorganisation energy associated with the proton transfer coordinate (both internal and solvent), respectively. In the calculation of lambda from the temperature dependent rate constant, correction is made for the large entropy increase of the reaction (TDeltaS(rxn) approximate to0.41 eV at pH = 7 and T = 298 K). The step-wise ETPT mechanism on the other hand shows a pH-independent rate, a lower reorganisation energy and no kinetic isotope effect. We propose that our complexes can be used as models to understand proton-coupled electron transfer in radical proteins. We show that the mechanism can be switched between CEP and ETPT by tuning the reaction pH and the electrochemical potential of the Ru-III/II oxidant. With a low driving force for the overall reaction the "energy conservative" CEP mechanism may dominate, in spite of the higher reorganisation energy as compared to ETPT. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Physical Chemistry Chemical Physics
volume
6
issue
20
pages
4851 - 4858
publisher
Royal Society of Chemistry
external identifiers
  • wos:000224755500017
  • scopus:8144223109
ISSN
1463-9084
DOI
10.1039/b407383e
language
English
LU publication?
yes
id
41e659e2-dc5e-4bff-a635-0b00025b4e98 (old id 138668)
date added to LUP
2007-07-03 16:53:38
date last changed
2017-12-10 04:36:20
@article{41e659e2-dc5e-4bff-a635-0b00025b4e98,
  abstract     = {The intra-molecular, proton-coupled electron transfer from a tyrosine residue to covalently linked tris-bipyridine ruthenium(III) complexes in aqueous solution (Ru-III-TyrOH --> Ru-II-TyrO(.) + H+) is studied in two complexes. The Ru-III-TyrOH state is generated by laser flash-induced photo-oxidation in the presence of the electron acceptor methyl viologen. The reaction is shown to follow either a concerted electron transfer-deprotonation (CEP) mechanism or a step-wise mechanism with electron transfer followed by deprotonation (ETPT). The CEP is characterised by a pH-dependent rate constant, a large reorganisation energy (lambda = 1.4 eV at pH = 7) and a significant kinetic isotope effect: k(H)/k(D) = 1.5-3. We can explain the pH-dependence and the high lambda by the pH-dependent DeltaGdegrees' for proton release to bulk water, and by the additional reorganisation energy associated with the proton transfer coordinate (both internal and solvent), respectively. In the calculation of lambda from the temperature dependent rate constant, correction is made for the large entropy increase of the reaction (TDeltaS(rxn) approximate to0.41 eV at pH = 7 and T = 298 K). The step-wise ETPT mechanism on the other hand shows a pH-independent rate, a lower reorganisation energy and no kinetic isotope effect. We propose that our complexes can be used as models to understand proton-coupled electron transfer in radical proteins. We show that the mechanism can be switched between CEP and ETPT by tuning the reaction pH and the electrochemical potential of the Ru-III/II oxidant. With a low driving force for the overall reaction the "energy conservative" CEP mechanism may dominate, in spite of the higher reorganisation energy as compared to ETPT.},
  author       = {Sjodin, M and Ghanem, Raed and Polivka, Tomas and Pan, Jie and Styring, Stenbjörn and Sun, LC and Sundström, Villy and Hammarstrom, L},
  issn         = {1463-9084},
  language     = {eng},
  number       = {20},
  pages        = {4851--4858},
  publisher    = {Royal Society of Chemistry},
  series       = {Physical Chemistry Chemical Physics},
  title        = {Tuning proton coupled electron transfer from tyrosine: A competition between concerted and step-wise mechanisms},
  url          = {http://dx.doi.org/10.1039/b407383e},
  volume       = {6},
  year         = {2004},
}