Tuning proton coupled electron transfer from tyrosine

A competition between concerted and step-wise mechanisms

Martin Sjödin, Raed Ghanem, Tomas Polivka, Jie Pan, Stenbjörn Styring, Licheng Sun, Villy Sundström, Leif Hammarström

Research output: Contribution to journalArticle

64 Citations (Scopus)

Abstract

The intra-molecular, proton-coupled electron transfer from a tyrosine residue to covalently linked tris-bipyridine ruthenium(III) complexes in aqueous solution (RuIII-TyrOH → RuII-TyrO. + H+) is studied in two complexes. The RuIII-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 (λ = 1.4 eV at pH = 7) and a significant kinetic isotope effect: kH/kD = 1.5-3. We can explain the pH-dependence and the high λ by the pH-dependent ΔG°′ 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 λ from the temperature dependent rate constant, correction is made for the large entropy increase of the reaction (TΔSrxn ≈ 0.41 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 RuIII/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.

Original languageEnglish
Pages (from-to)4851-4858
Number of pages8
JournalPhysical Chemistry Chemical Physics
Volume6
Issue number20
DOIs
Publication statusPublished - Oct 21 2004

Fingerprint

tyrosine
Tyrosine
Protons
electron transfer
Tuning
tuning
protons
Electrons
Deprotonation
Isotopes
Rate constants
isotope effect
Paraquat
Kinetics
Proton transfer
Ruthenium
Photooxidation
energy
Oxidants
photooxidation

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Atomic and Molecular Physics, and Optics

Cite this

Tuning proton coupled electron transfer from tyrosine : A competition between concerted and step-wise mechanisms. / Sjödin, Martin; Ghanem, Raed; Polivka, Tomas; Pan, Jie; Styring, Stenbjörn; Sun, Licheng; Sundström, Villy; Hammarström, Leif.

In: Physical Chemistry Chemical Physics, Vol. 6, No. 20, 21.10.2004, p. 4851-4858.

Research output: Contribution to journalArticle

Sjödin, Martin ; Ghanem, Raed ; Polivka, Tomas ; Pan, Jie ; Styring, Stenbjörn ; Sun, Licheng ; Sundström, Villy ; Hammarström, Leif. / Tuning proton coupled electron transfer from tyrosine : A competition between concerted and step-wise mechanisms. In: Physical Chemistry Chemical Physics. 2004 ; Vol. 6, No. 20. pp. 4851-4858.
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AU - Sjödin, Martin

AU - Ghanem, Raed

AU - Polivka, Tomas

AU - Pan, Jie

AU - Styring, Stenbjörn

AU - Sun, Licheng

AU - Sundström, Villy

AU - Hammarström, Leif

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