Spectroscopic Models for Paramagnetic Active Sites in Photosystem II

Photosynthesis in green plants and some bacteria is based on the efficient function of most sophisticated molecular machines. Like in a factory assembly line, sunlight is captured, stabilized and converted into chemical energy - which then is used for chemical reactions and to oxidize water. Particularly the last step is very difficult and the most demanding task in chemical terms. The whole process of photosynthesis is based on charge separation at specific redox centers of the photosystem and careful charge conduction and control. To avoid immediate loss of the energy by 'electrical short circuit' (and thus charge recombination) the redox potentials of the redox-active centers and the spatial arrangement of the elements in the charge transfer path were carefully tuned by evolution. We are particularly interested in the interaction and mutual control of specific redox-active groups in the so-called "reaction center" (which is the site of water oxidation) and how one could measure their interaction by physical means. The final goal of such a research program is understanding the natural systems in full detail and depth so that blueprints for technical designs and applications can be deduced. In the project presented here a series of isostructural dimeric manganese complexes with tethered organic radicals was synthesized. For the first time these compounds could mimic the paramagnetic interaction of the manganese cluster and a tyrosin Y_z^• radical in the S₂Y_z^• state of photosystem PS II. The state S₂Y_z^• is an artificially stabilized intermediate in the chain of oxidation events in the reaction center of PS II which may control the flow of electrons during catalysis. Since both sites, the manganese cluster and the radicals in S₂Y_z^• as well as in the models, carry spin and magnetic moment, their magnetic dipole interaction could be detected by using electron paramagnetic resonance techniques (EPR spectroscopy) and susceptibility measurements. For the first time the conditions of long-range interactions between mixed-valence manganese clusters and organic radicals like in PS II could be systematically explored as a function of known molecular structures. The EPR results are most relevant for the understanding of real systems and EPR distance measurements in biochemical molecules.