Materials and Methods Figs. S1 to S3 Table S1 References S1 SUPPORTING MATERIAL Materials and Methods Preparation of Re I (CO) 3 (dmp)(H 124)|(W 122)|AzCu I Mutant azurins were expressed and Re I (CO) 3 (dmp)(H 124)|(W 122)|AzCu I was prepared using previously published protocols (S1,S2). Crystal Structure of Re I (CO) 3 (dmp)(H 124)|(W 122)|AzCu II Crystals of Re(4,7-dimethyl-1,10-phenanthroline)(CO) 3 (H 124){T 124 H|K 122 W|H 83 Q}(Cu II)azurin (Re I (CO) 3 (dmp)(H 124)|(W 122)|AzCu II ; space group I222, cell dimensions 63.22 × 69.08 × 68.94 Å 3 ; α = β = γ = 90.00°, one molecule per asymmetric unit) grew from 4 μL drops made from equal volumes of 30 mg/mL Re I (CO) 3 (dmp)(H 124)|(W 122)|AzCu II in 25 mM HEPES pH 7.5 and reservoir by vapor diffusion. The drops were equilibrated against 500 μL of reservoir

No 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 k ET > 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.

Continuing our work toward a system mimicking the electron-transfer steps from manganese to P(680)(+) in photosystem II (PS II), we report a series of ruthenium(II)-manganese(II) complexes that display intramolecular electron transfer from manganese(II) to photooxidized ruthenium(III). The electron-transfer rate constant (k(ET)) values span a large range, 1 x 10(5)-2 x 10(7) s(-1), and we have investigated different factors that are responsible for the variation. The reorganization energies determined experimentally (lambda = 1.5-2.0 eV) are larger than expected for solvent reorganization in complexes of similar size in polar solvents (typically lambda approximately 1.0 eV). This result indicates that the inner reorganization energy is relatively large and, consequently, that at moderate driving force values manganese complexes are not fast donors. Both the type of manganese ligand and the link between the two metals are shown to be of great importance to the electron-transfer rate. In contrast, we show that the quenching of the excited state of the ruthenium(II) moiety by manganese(II) in this series of complexes mainly depends on the distance between the metals. However, by synthetically modifying the sensitizer so that the lowest metal-to-ligand charge transfer state was localized on the nonbridging ruthenium(II) ligands, we could reduce the quenching rate constant in one complex by a factor of 700 without changing the bridging ligand. Still, the manganese(II)-ruthenium(III) electron-transfer rate constant was not reduced. Consequently, the modification resulted in a complex with very favorable properties.

No 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.The development of energy systems built on solar energy is of immediate interest for most sectors of society. One idea is to construct systems for fuel production using reducing equivalents from water, a sustainable and environmentally safe substrate. In our research aiming for this goal, we have adopted a strategy to develop supramolecular complexes designed on principles from the natural enzyme, Photosystem II (PSII). 1-3 PSII is a large membrane-bound protein complex, for which the detailed structure and function is not yet completely known. Synthetic compounds that mimic its detailed chemistry can consequently not be accomplished. However, also supramolecular chemistry mimicking principally important parts of the light driven reactions in PSII are important for the advancement of artificial photosynthesis. In this report we describe a major step toward the realization of such a system.In all plants and algae, light absorption drives the electron transfer from water to carbon dioxide, producing atmospheric oxygen and providing the biosphere with an infinite source of reducing power. The light-driven oxidation of water is catalyzed by a redox "triad" in the reaction center of PSII ( Figure 1A). The absorption of a light quantum by the primary donor P 680 , which is constituted by a dimer or multimer of chlorophyll molecules, triggers charge separation and, after several electrontransfer steps, the transfer of an electron to a diffusible quinone on the acceptor side of PSII. The oxidized P 680 retrieves an electron by oxidizing a nearby tyrosine residue, Tyr Z , which then forms a neutral radical. 1,4 The tyrosyl radical in its turn oxidizes a tetranuclear Mn-cluster, which is bound to PSII close to the water-protein interface ( Figure 1A). After four consecuti...

The preparation of donor (D)-photosensitizer (S) arrays, consisting of a manganese complex as D and a ruthenium tris(bipyridyl) complex as S has been pursued. Two new ruthenium complexes containing coordinating sites for one (2a) and two manganese ions (3a) were prepared in order to provide models for the donor side of photosystem II in green plants. The manganese coordinating site consists of bridging and terminal phenolate as well as terminal pyridyl ligands. The corresponding ruthenium-manganese complexes, a manganese monomer 2b and dimer 3b, were obtained. For the dimer 3b, our data suggest that intramolecular electron transfer from manganese to photogenerated ruthenium(III) is fast, k(ET) > 5 x 10(7) s(-)(1).

No abstract