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

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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 - 2004 Oct 21
Externally publishedYes

ASJC Scopus subject areas

  • Physics and Astronomy(all)
  • Physical and Theoretical Chemistry

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    Sjödin, M., Ghanem, R., Polivka, T., Pan, J., Styring, S., Sun, L., Sundström, V., & Hammarström, L. (2004). Tuning proton coupled electron transfer from tyrosine: A competition between concerted and step-wise mechanisms. Physical Chemistry Chemical Physics, 6(20), 4851-4858. https://doi.org/10.1039/b407383e