| 1. |
- Berglund-Baudin, Helena, et al.
(author)
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Intramolecular Electron Transfer from Manganese(II) Coordinatively Linked to a Photogenerated Ru(III)-Polypyridine Complex: A Kinetic Analysis
- 1998
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In: The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory. - : American Chemical Society (ACS). - 1520-5215. ; 102:15, s. 2512-2518
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Journal article (peer-reviewed)abstract
- For further investigations in the field of artificial photosynthesis, a model compound, 1, has been developed to mimic the electron-transfer steps from the manganese cluster to P680+ in photosystem II. In this model compound the photosensitizer ruthenium(II)-trisbipyridyl was linked to a manganese(II) ion through a bridging ligand. Photoexcitation of 1 in the presence of the electron acceptor methyl viologen (MV2+) lead to electron transfer from the Ru moiety to MV2+. Laser flash photolysis experiments at different concentrations of 1 were performed in order to follow the subsequent reduction of the photooxidized Ru(III) species. A kinetic model, taking different parallel reactions into account, could explain the experimental data. It was shown that the major part of the photooxidized Ru(III) created was reduced again by intramolecular electron transfer from the attached Mn(II), with a rate constant of 1.8 × 105 s-1. However, Mn(II) was partially dissociated from 1, giving a fraction of Ru(III) without Mn(II) attached. In these complexes electron transfer could occur only after a rate-limiting reassociation of Mn(II), with a rate constant 2.9 × 109 M-1 s-1. In the analysis of the data, the fraction of dissociated Mn(II) could be determined independently at each concentration of 1, utilizing the fact that bound Mn(II) quenched the excited state, probably by energy transfer.
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| 2. |
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| 3. |
- Hammarström, Leif, et al.
(author)
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Artificial photosynthesis: towards functional mimics of photosystem II?
- 1998
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In: Biochimica et Biophysica Acta - Bioenergetics. - 0005-2728. ; 1365:1-2, s. 193-199
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Journal article (peer-reviewed)abstract
- This paper describes the initial development of a project aiming at the construction of functional mimics of the oxygen-evolving complex of photosystem II, coupled to photoinduced charge separation. Biomimetic electron donors, manganese complexes and tyrosine, have been linked to a Ru(II)-polypyridine photosensitiser. Oxidation of the donors by intramolecular electron transfer from the photo-oxidised Ru(III) complex was demonstrated using optical flash photolysis and EPR experiments. A step-wise electron transfer Mn->tyrosine->Ru(III) was demonstrated.
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| 4. |
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| 5. |
- Hammarström, Leif, et al.
(author)
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Mimicking Photosystem II Reactions in Artificial Photosynthesis
- 1999
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In: Zeitschrift für Physikalische Chemie. - 0942-9352. ; 213:2, s. 157-163
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Journal article (peer-reviewed)abstract
- The paper describes a project aiming at the construction of functional mimics of the oxygen evolving complex of Photosystem II, coupled to photoinduced charge separation. Biomimetic electron donors - manganese complexes and tyrosine - have been linked to a Ru(II)-polypyridine photosensitizer. Oxidation of the donors by intramolecular electron transfer from the photo-oxidized Ru(III) complex was demonstrated using optical flash photolysis and EPR experiments. A step-wise electron transfer Mn → tyrosine → Ru(III) was demonstrated, in a sequence analogous to reactions of the PSII donor side.
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| 6. |
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| 7. |
- Magnuson, Ann, et al.
(author)
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A Biomimetic Model System for the Water Oxidizing Triad in Photosystem II
- 1999
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 121:1, s. 89-96
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Journal article (peer-reviewed)abstract
- In plants, solar energy is used to extract electrons from water, producing atmospheric oxygen. This is conducted by Photosystem II, where a redox ”triad” consisting of chlorophyll, a tyrosine, and a manganese cluster, governs an essential part of the process. Photooxidation of the chlorophylls produces electron transfer from the tyrosine, which forms a radical. The radical and the manganese cluster together extract electrons from water, providing the biosphere with an unlimited electron source. As a partial model for this system we constructed a ruthenium(II) complex with a covalently attached tyrosine, where the photooxidized ruthenium was rereduced by the tyrosine. In this study we show that the tyrosyl radical, which gives a transient EPR signal under illumination, can oxidize a manganese complex. The dinuclear manganese complex, which initially is in the Mn(III)/(III) state, is oxidized by the photogenerated tyrosyl radical to the Mn(III)/(IV) state. The redox potentials in our system are comparable to those in Photosystem II. Thus, our synthetic redox “triad” mimics important elements in the electron donor ”triad” in Photosystem II, significantly advancing the development of systems for artificial photosynthesis based on ruthenium−manganese complexes.
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| 8. |
- Magnuson, Ann, et al.
(author)
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Mimicking electron transfer reactions in photosystem II : synthesis and photochemical characterization of a ruthenium(II) tris(bipyridyl) complex with a covalently linked tyrosine.
- 1997
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 119:44, s. 10720-10725
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Journal article (peer-reviewed)abstract
- In the natural photosynthetic reaction center photosystem II, absorption of a photon leads to photooxidationof the primary electron donor P680, which subsequently retrieves electrons from a tyrosyl residue, functioning as aninterface to the oxygen-evolving manganese complex. In a first step toward mimicking these reactions, we havemade a Ru(II)-polypyridine complex with an attached tyrosyl moiety. The photoexcited ruthenium complex playedthe role of P680and was first oxidized by external acceptors. Combined transient absorbance and EPR studies provided evidence that the Ru(III) formed was reduced by intramolecular electron transfer from the attached tyrosine, with a rate constant of 5104s-1. Thus we show that a tyrosine radical could be formed by light-induced electrontransfer reactions, and we indicate future directions for developing a closer analogy with the photosystem II reactions.
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| 9. |
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| 10. |
- Sun, Licheng, et al.
(author)
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Hydrogen-Bond Promoted Intramolecular Electron Transfer to Photogenerated Ru(III): A Functional Mimic of TyrosineZ and Histidine 190 in Photosystem II
- 1999
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 121:29, s. 6834-6842
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Journal article (peer-reviewed)abstract
- As a model for redox components on the donor side of photosystem II (PS II) in green plants, a supramolecular complex 4 has been prepared. In this, a ruthenium(II) tris-bipyridyl complex which mimics the function of P680 in PS II, has been covalently linked to a tyrosine unit which bears two hydrogen-bonding substituents, dipicolylamine (dpa) ligands. Our aim is to mimic the interaction between tyrosineZ and a basic histidine residue, namely His190 in PSII, and also to use the dpa ligands for coordination of manganese. Two different routes for the synthesis of the compound 4 are presented. Its structure was fully characterized by 1H NMR, COSY, NOESY, 13C NMR, IR, and mass spectrometry. 1H NMR and NOESY gave evidence for the existence of intramolecular hydrogen bonding in 4. The interaction between the ruthenium and the substituted tyrosine unit was probed by steady-state and time-resolved emission measurements as well as by chemical oxidation. Flash photolysis and EPR measurements on 4 in the presence of an electron acceptor (methylviologen, MV2+, or cobalt pentaminechloride, Co3+) showed that an intermolecular electron transfer from the excited state of Ru(II) in 4 to the electron acceptor took place, forming Ru(III) and the methylviologen radical MV+ or Co2+. This was followed by intramolecular electron transfer from the substituted tyrosine moiety to the photogenerated Ru(III), regenerating Ru(II) and forming a tyrosyl radical. In water, the radical has a g value of 2.0044, indicative of a deprotonated tyrosyl radical. In acetonitrile, a radical with a g value of 2.0029 was formed, which can be assigned to the tyrosine radical cation. In both solvents the electron transfer is intramolecular with a rate constant kET > 1 × 107 s-1. This is 2 orders of magnitude greater than the one for a similar compound 3, in which no dpa arm is attached to the tyrosine unit. Therefore the hydrogen bonding between the substituted tyrosine and the dpa arms in 4 is proposed to be responsible for the fast electron transfer. This interaction mimics the proposed His190 and tyrosineZ interaction in the donor side of PS II.
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