Electron Donor Systems in Natural and Artificial Photosynthesis

Sammanfattning: Photosynthesis is the process by which light energy is converted into chemical products, in photoautotrophic bacteria, algae and plants. The principal idea is the production of reducing agents by photo-induced oxidation of a sacrificial electron donor. Photosystem II in plants, algae and cyanobacteria, absorbs light and catalyzes the oxidation of water, liberating electrons that are used to form reductants. By capturing light, plants and algae provide the entire biosphere with an electron source for the buildup of living tissue. The active site for photosynthetic water oxidation contains a tetranuclear manganese cluster and a redox active tyrosyl residue. Together with the photosensitizer chlorophylls in the reaction center of Photosystem II, the manganese and the tyrosine activate and split water molecules into molecular oxygen, electrons and protons. In the first part of this thesis the nature of the manganese binding site, and the electron transfer reactions involved in the assembly of the manganese cluster were investigated. Selective chemical modification of histidyl and carboxylic acid residues was conducted on the manganese binding area of Photosystem II. Kinetic studies of manganese binding after chemical modification revealed binding sites with different affinity for manganese. Two binding sites containing histidines were found, and two or more sites of carboxylic acid residues. The participation of the redox active cofactors Tyrosine-D and cytochrome-b559 in the activation of the water oxidizing center was studied using EPR spectroscopy. Tyrosine-D and cytochrome-b559 was found to aid Photosystem II in the assembly of a functional manganese cluster, by acting as auxilliary electron donors, and in the case of cytochrome-b559 under high light intensities, also as auxilliary electron acceptor. The two cofactors thereby prevent light-induced protein damadge during activation of the water oxidizing complex. In the second part of the thesis, the properties and reactions of novel compounds, synthesized with the objective to mimic the water oxidizing complex, were studied by optical and EPR spectroscopy. A ruthenium complex, serving as photosensitizer, was covalently connetced to a tyrosine. Light-induced electron transfer from the tyrosine to the ruthenium part was generated in the complex, similar to the reactions in Photosystem II. This super-molecule was then allowed to react with a synthetic dinuclear manganese complex. Electron transfer from manganese to the photooxidized tyrosine was observed, thereby mimicking the stepwise electron transfer reactions that take place on the electron donor side of Photosystem II. This is the first functional mimic of the water oxidizing triad in Photosystem II, and a new platform for regenerative electron donor systems in artificial photosynthesis.