An international research team, including the Mülheim Max Planck Institute for Chemical Energy Conversion (MPI CEC) and the Australian National University (ANU), has published new results on the process of water splitting in natural photosynthesis, a process of fundamental importance for life the earth. The results obtained are also important for the development of CO2-free solar fuels.
The oxidation of two water molecules to molecular oxygen by sunlight is one of the first steps in so-called oxygen photosynthesis, a process that occurs in all plants, algae and cyanobacteria. The reaction is similar to light conversion in photovoltaic systems. However, here instead of electricity, high-energy chemical compounds are produced, carbohydrates, which are then available to us as food and in fossil form as fuels (oil, gas, coal).
The study shows how the complex is activated and the controlled uptake of water from the environment of the protein takes place.
By releasing oxygen, photosynthesis has also created our oxygen-rich earth's atmosphere and the ozone layer in the stratosphere, which protects us from the harsh UV radiation of the sun.
The investigations, under the direction of dr. Nick Cox, former group leader in the department of Prof. Wolfgang Lubitz at the MPI CEC, focused on a photosynthetic enzyme responsible for biological water splitting. The reaction takes place in the so-called photosystem II on a metal cluster consisting of four manganese and one calcium metal ion.
This complex binds to adjacent centers two water molecules, which form an oxygen molecule after oxidation and proton release and release. The study shows how this complex is activated and how the controlled uptake of water from the environment of the protein takes place.
"Our results show that the enzyme breathes to absorb the water molecules," says Nick Cox. "Halfway through the reaction cycle, the enzyme begins to stretch like an accordion, allowing the binding of a water molecule to the manganese cluster."
It is important that this movement is not possible at other points of the reaction cycle, which prevents water absorption. This is the basis of the highly specific formation of an oxygen molecule from two water molecules.
"Without the accurate sequential binding and conversion of the two water molecules on the manganese cluster, the process would easily form aggressive oxygen intermediates (called reactive oxygen species) that react with and destroy the protein," Dr. Maria Chrysina from the MPI CEC.
If science succeeds in mimicking the water splitting in the laboratory with similar efficiency as in nature, humanity would have an inexhaustible source of sustainable energy at its disposal.
"The energy of sunlight falling on the earth's surface in one hour is enough to cover the current energy needs of all humanity for a whole year," says Professor Wolfgang Lubitz, director emeritus at the MPI CEC.
The development of an artificial water-splitting enzyme would make it possible to produce “solar fuels”, a green alternative to gasoline, diesel and natural gas. Such a solar fuel would be e.g. B. Hydrogen, which burns without releasing the greenhouse gas CO2. Hydrogen from the splitting of water could then also be used to prepare other important chemicals, e.g. B. for the synthesis of fertilizers in agriculture - essential for the food supply of the world population.
The investigations are based on a spectroscopic technique (Electron Paramagnetic Resonance, EPR), which is used for the investigation of magnetic materials as well as for metal centers in biological molecules. This technique provides information on the electronic structure of a catalytic center, such as a metallo-enzyme at the molecular level, such as in the water-splitting enzyme of photosynthesis.
The results were obtained by a collaboration of scientists from the Max Planck Institutes for Chemical Energy Conversion (MPI CEC) and Coal Research (MPI KOFO) in Mülheim / Ruhr, the Ruhr University Bochum, the University of Uppsala (Sweden) and the Australian National University (ANU) in Canberra. The work has recently been published in Proceeding of the National Academy of Sciences of the United States of America:
Chrysina, M., Heyno, E., Kutin, Y., Reus, M., Nilsson, H., Nowaczyk, MM, DeBeer, S., Neese, F., Messinger, J., Lubitz, W., Cox , N. (2019). Five-coordinate MnIV intermediate in the activation of nature's water splitting cofactor. Proceedings of the National Academy of Sciences Aug 2019, 201817526; DOI: 10.1073 / pnas.1817526116
Scientific contact:
Prof. Dr. Wolfgang Lubitz
Max Planck Institute for Chemical Energy Conversion
Mulheim / Ruhr
Phone: + 49 (0) 208 306 3508
Email ppgad@pucrs.br
Original publication:
DOI: 10.1073 / pnas.1817526116
Esther Schlamann | Max Planck Institute for Chemical Energy Conversion
