Chemists at the California Institute of Technology (Caltech) and the Lawrence Berkeley National Laboratory believe they can now explain how to build the two-atom oxygen (O2) molecule from splitting water.  The O2 forming chemical process is one of the remaining mysteries of photosynthesis that plants use to convert sunlight into usable energy and make the O2 that we need to breathe for life.

The work reveals a new way of approaching the design of catalysts that drive the water-splitting reactions of artificial photosynthesis.  The paper in Nature Chemistry titled “Redox-Inactive Metals Modulate the Reduction Potential in Heterometallic Manganese-Oxido Clusters” was published nearly a month ago.  The press release by Kimm Fesenmaier finally came out late last week.

Theodor Agapie, an assistant professor of chemistry at Caltech and principal investigator of the paper describes the point of the research, “If we want to make systems that can do artificial photosynthesis, it’s important that we understand how the system found in nature functions.”

Metal Catalyst Cluster for Photosynthesis Displayed over a Background of Photosystem II.  Click on the image for a larger view.

Metal Catalyst Cluster for Photosynthesis Displayed over a Background of Photosystem II. Click on the image for a larger view.

One key piece of biological machinery that enables photosynthesis is a conglomeration of proteins and pigments known as the Photosystem II. Within that system lies a small cluster of atoms, called the oxygen-evolving complex, where water molecules are split and the O2 molecular oxygen is made. Although this oxygen-producing process has been studied extensively, the role that various parts of the cluster play has remained unclear.

Inside the oxygen-evolving complex is a reaction that requires the transfer of electrons to take place making it an example of what is known as a “redox”, or a reduction of oxidation reaction. The cluster can be described as a “mixed-metal cluster” because in addition to oxygen, it includes two types of metals – one that is redox active, or capable of participating in the transfer of electrons (in this case, manganese), and one that is redox inactive (calcium).

“Since calcium is redox inactive, people have long wondered what role it might play in this cluster,” Agapie says.

It’s been difficult to solve the mystery mostly because the oxygen-evolving complex is just a cog in the much larger chemical reaction system that is Photosystem II.  It’s hard to study just the O2 formation in the photosystem because there is so much going on with the whole.

Here’s the innovative technology leap. To get around this, Agapie’s graduate student and the paper’s lead author Emily Tsui prepared a series of compounds that are structurally related to the oxygen-evolving complex. She built upon an organic scaffold in a stepwise fashion, first adding three manganese centers and then attaching a fourth metal. By varying that fourth metal to be calcium and then different redox-inactive metals, such as strontium, sodium, yttrium, and zinc, Tsui was able to compare the effects of the metals on the chemical properties of the compound.

Tsui explains the background, “When making mixed-metal clusters, researchers usually mix simple chemical precursors and hope the metals will self-assemble in desired structures. That makes it hard to control the product. By preparing these clusters in a much more methodical way, we’ve been able to get just the right structures.”

What the team learned is the redox-inactive metals affect the way electrons are transferred in such systems. To make molecular oxygen, the manganese atoms must activate the oxygen atoms connected to the metals in the complex. In order to do that, the manganese atoms must first transfer away several electrons. Redox-inactive metals that tug more strongly on the electrons of the oxygen atoms make it more difficult for manganese to do this. But calcium does not draw electrons strongly toward itself. Therefore, it allows the manganese atoms to transfer away electrons and activate the oxygen atoms that go on to make molecular oxygen.

The news has substantial implications.  A number of the catalysts that are currently being developed to drive artificial photosynthesis are mixed-metal oxide catalysts. Here again it has been unclear what role the redox-inactive metals in these mixed catalysts play. The new findings suggest that the redox-inactive metals affect the way the electrons are transferred.

Agapie explains the impact, “If you pick the right redox-inactive metal, you can tune the reduction potential to bring the reaction to the range where it is favorable. That means we now have a more rational way of thinking about how to design these sorts of catalysts because we know how much the redox-inactive metal affects the redox chemistry.”

The coauthors from the Lawrence Berkeley National Laboratory are Rosalie Tran and Junko Yano.

The news is quite a crack into the synthetic photosynthesis system.  Moreover, Ms Tsui’s investigative innovation could well be a process others can use to deepen their research into catalysts.

Free hydrogen is a worthy goal that remains as yet still too costly for mass application for producing synthetic fuels.  Highly efficient catalysts will help, and the stage is now set for a new crop to be found and explored.  Good work at Caltech.


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