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Issue 20, 2004
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Tuning proton coupled electron transfer from tyrosine: A competition between concerted and step-wise mechanisms

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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.

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Article information


Submitted
17 May 2004
Accepted
20 Aug 2004
First published
17 Sep 2004

Phys. Chem. Chem. Phys., 2004,6, 4851-4858
Article type
Paper

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

M. Sjödin, R. Ghanem, T. Polivka, J. Pan, S. Styring, L. Sun, V. Sundström and L. Hammarström, Phys. Chem. Chem. Phys., 2004, 6, 4851
DOI: 10.1039/B407383E

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