Cu-S complexes in the heart of biological electron transfer: the homodinuclear CuA site of Cytochrome-c oxidases and N2O reductases

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The structure of cytochrome-c oxidase is fairly well known from a static (crystalline) point of view, but all attempts to assemble complexes that mimic CuA within this enzyme or to establish a simplified equivalent circuit on a complex chemical level that represents the flow of electrons from cytochrome-c to CuA failed so far. It should be noted that all previously described compounds with certain selective characteristics of CuA deviate from the biological archetype in prominent properties such as structure and redox chemistry, and thus must be improved significantly to arrive at a stage from which significant insights into dynamical aspects of the biological electron transport can be obtained.On the one hand, we will continue our work directed towards improved CuA models with bifunctional thiolate ligands and in parallel also use our novel tridentate GuaphSetS systems which have the potential to establish the biologically requested environment of the methionine-binding site of CuA by activating their NSS' donor sets a coordination which is not accessible by other ligands described in the literature.On the other hand, a novel focus will be created and directed towards photochemically excitable electron transfer cascades as equivalent circuit diagrams for the electron transfer from cytochrome-c to CuA, in which divalent ruthenium ions serve as electron sources and binuclear copper-thiolate complexes act as corresponding electron sinks.Over various stages to the end of the way, models for the CuA sites of cytochrome-c oxidases and of N2O reductases will emerge in increasing sophisticated stages which will also make decisive contributions as components of simplified equivalent circuits on a complex chemical level to a better understanding of dynamic and kinetic aspects of biological electron transport issues driven by CuA.Apart from classical methods of X-ray structure analysis and of conventional characterization by spectroscopic, (spectro)electrochemical and magneto-chemical methods, we will place particular emphasis on sophisticated spectroscopic techniques to make use of the outstanding potential of pulsed optical laser, (pulsed) free electron laser and of (pulsed) 3rd generation synchrotron sources in order to obtain time-resolved information on copper-driven biological electron transport phenomena. In this context, high-resolution X-ray absorption and emission play a central role as these techniques allow direct access to the LUMO and HOMO states of our complexes.