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- Sjodin, M., et al.
(författare)
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Proton-coupled electron transfer from tyrosine in a tyrosine-ruthenium-tris-bipyridine complex : Comparison with Tyrosine(z) oxidation in photosystem II
- 2000
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 122:16, s. 3932-3936
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Tidskriftsartikel (refereegranskat)abstract
- The pH- and the temperature dependence of the rate constant for electron transfer from tyrosine to ruthenium in Ru(II)(bpy)(2)(4-Me-4'CONH-L-tyrosine etyl ester-2,2'-bpy) 2PF(6) was investigated using flash photolysis. At a pH below the tyrosine pK(a) approximate to 10 the rate constant increased monotonically with pH. This increase was consistent with a concerted electron transfer/deprotonation mechanism. Also indicative of a concerted reaction was the unusually high reorganization energy, 2 eV, extracted from temperature-dependent measurements. Deprotonation of the tyrosine group, at pH > pK(a), resulted in a 100-fold increase in rate constant due to a decreased reorganization energy, lambda = 0.9 eV. Also, the rate constant became independent of pH, In Mn-depleted photosystem II a similar pH dependence has been found for electron transfer from tyrosine(Z) (Tyr(Z)) to the oxidized primary donor P680(+). On the basis of the kinetic similarities we propose that the mechanisms in the two systems are the same, that is, the electron transfer occurs as a concerted proton-coupled electron-transfer reaction, and at pH < 7 the Tyr(Z) proton is released directly to the bulk water.
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| 2. |
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| 3. |
- Sjodin, M., et al.
(författare)
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Tuning proton coupled electron transfer from tyrosine : A competition between concerted and step-wise mechanisms
- 2004
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Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 6:20, s. 4851-4858
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Tidskriftsartikel (refereegranskat)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.
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