Harden M. McConnell Lecture

The process of water splitting and oxygen release invented by early photosynthetic organisms led to the creation of our planet´s oxygen-rich atmosphere and the protective ozone layer in the stratosphere, both essential events for the development of higher life on earth. Photosynthesis stores the sun´s energy via CO2 reduction in form of energy-rich organic compounds. It is the only basic source of food on earth, provides us with oxygen to breath, delivers valuable biomaterials and it has generated all our fossil fuels.

The catalytic water oxidation reaction is performed by a protein-bound Mn4OxCa cluster located in the so-called photosystem II [1]. The cofactor´s reaction cycle comprised 5 intermediate redox states (Sn), in which the subscript indicates the number of stored oxidizing equivalents in the Mn cluster (n = 0 to 4) required to split two water molecules and release one O2. A redox-active tyrosine residue couples the fast light-induced single-electron charge separation to the slow catalytic four-electron water oxidation process. All these states are paramagnetic; thus electron paramagnetic resonance (EPR) is the method of choice to study the catalytic cycle and understand this important biological process. The S-states are trapped by light flash/freeze quench techniques and their electronic structure is studied by advanced pulse EPR techniques (ENDOR, ESEEM, ELDOR-detected NMR) [2]. The experimental data are corroborated and complemented by DFT calculations [3] and compared with recent structural data [4]. The results give information on the electronic structure (spin and oxidation states) of the manganese ions [3], the function of the Ca2+ [5], the effect of the amino acid surrounding as well as the binding, location and reaction dynamics of the two substrate water molecules in the cycle [6], preceding O-O bond formation and release of triplet dioxygen. A robust model for the water oxidation mechanism is derived [7]. This information is valuable for the design of bioinspired catalysts for water oxidation.

1. (a) Umena, Y., et al. (2011) Nature, 473, 55; (b) Suga, M., et al. (2015) Nature, 517, 99
2. (a) Kulik, L. V., et al. (2007) J. Am. Chem. Soc., 129, 13421; (b) Cox, N., et al. (2014) Science, 345, 804;
   (c) Lohmiller et al. (2017) J. Am. Chem. Soc., 139, 14412; (d) Chrysina, M. et al. PNAS (2019), in press
3. Krewald, V., et al. (2015) Chem. Sci., 6, 1676; Krewald, V., et al. (2016) Inorg. Chem., 55, 488
4. Kern, J. et al. (2018) Nature, 563, 421
5. (a) Lohmiller, T. et al. (2012) J. Biol. Chem., 287, 24721; (b) Cox, N. et al. (2011) J. Am. Chem. Soc., 133, 3635.
6. Rapatskiy, L., et al. (2012) J. Am. Chem. Soc., 134, 16619; Rapatskiy, L. et al. (2019) submitted
7. Lubitz, W., Chrysina, M., Cox, N. Photosyn. Res. (2019), in press

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Wolfgang Lubitz studied Chemistry at the Freie Universität (FU) Berlin (1969 – 1974), where he also received his doctoral degree (1977), and his habilitation (1982). He worked as a research scientist at UC San Diego, USA (1983/84) with George Feher (Biophysics) and subsequently as Assistant Professor and Associate Professor at the FU Berlin (1979 -1989). In 1989 he became Professor at the Physics Department of the Universität Stuttgart, before he took over a Chair of Physical Chemistry at the Technische Universität (TU) Berlin in 1991 as successor of Horst T. Witt. In 2000 he became a Scientific Member of the Max Planck Society and Director at the Max Planck Institute for Bioinorganic Chemistry (now renamed Max Planck Institute for Chemical Energy Conversion) in Mülheim/Ruhr, Germany, where he established the Department for Biophysical Chemistry. Since 2000 Professor Lubitz is Honorary Professor at the Universität Düsseldorf. Since 2017 he is Director emeritus at the Max Planck Institute in Mülheim. His scientific work is focused on energy conversion processes in natural and artificial photosynthesis, the investigation of metalloproteins (hydrogenases, water oxidase) and the development and application of molecular spectroscopy, especially magnetic resonance methods. His work has been published in about 500 scientific publications and decorated with a number of awards. He is Fellow of the Royal Society of Chemistry (UK), the International Society of Magnetic Resonance, and the International EPR Society and has received honorary doctorates (Dr. h.c.) from the Universitet Uppsala, Sweden (2008) and the Université d'Aix-Marseille, France (2014). He has been President of the International EPR Society (2005 – 2008) and is currently Vice President of the Council for the Annual Nobel Laureate Meetings in Lindau (Germany) and elected member of the board of the Society of German Natural Scientists and Physicians (GDNÄ).

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