Photosynthesis:Study sheds new light

Crucial systems

Chemists at the California Institute of Technology (Caltech) believe they can now explain one of the last remaining mysteries concerning the process of photosynthesis in which light powers chemical conversion of carbon dioxide and water into sugars and oxygen. The work is based on synthetic, spectroscopic, and electrochemical studies, as well as X-ray spectroscopic work, and might one day lead to a range of novel photocatalysts that could be used to drive the water-splitting reactions in a synthetic analogue of photosynthesis.

"If we want to make systems that can do artificial photosynthesis, it's important that we understand how the system found in nature functions," explains Theodor Agapie, principal investigator on the research. Crucial to photosynthesis is the conglomeration of proteins and pigments we know simply as photosystem II, a name that belies not only its incredible chemical prowess but its complexity. Within PSII is the oxygen-evolving centre. This unit is at the hub of photosynthesis, it is where water molecules are split and molecular oxygen is made. Although it has been well studied, the role of the various components of the cluster remains obscure.

Complex redox

The oxygen-evolving complex hinges on electron transfer in a redox reaction involving a mixed-metal centre - one metal, manganese, is the redox active component, electron transfer allows it to shuttle between oxidation states. But the second metal, calcium, is redox inactive, so uncovering what role it plays in the system has proven difficult. Part of the problem in trying to explain the part played by the calcium ion in the oxygen-evolving complex is that the complex is a small, albeit essential, cog in the much larger PSII machine.

In order to isolate this cog, Agapie's graduate student Emily Tsui prepared a series of compounds that are structurally related to the oxygen-evolving complex. She did so by building on an organic scaffold in a stepwise manner, first adding three manganese centres and then attaching a fourth metal. By swapping out the fourth metal for calcium and other redox-inactive metals, such as strontium, sodium, yttrium, and zinc, Tsui was able to compare the effects of the metals on the overall chemical behaviour of the model compound.

"When making mixed-metal clusters, researchers usually mix simple chemical precursors and hope the metals will self-assemble in desired structures," Tsui explains. "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."

Inactive, but why?

This methodical approach reveals that the redox-inactive metals strongly 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 is not particularly electrophilic and so allows the manganese atoms to transfer away electrons and activate the oxygen atoms that go on to make molecular oxygen.

Agapie, Tsui and colleagues Rosalie Tran and Junko Yano of the Lawrence Berkeley National Laboratory have been working on a number of mixed-metal catalysts that could drive artificial photosynthesis. The new findings could help clarify how to optimise these catalysts since it is now known that the redox-inactive metals affect the way the electrons are transferred. "If you pick the right redox-inactive metal, you can tune the reduction potential to bring the reaction to the range where it is favourable," Agapie says. "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 next step is to make clusters that are capable of performing water oxidation and study the mechanism of the transformation in detail," Agapie told SpectroscopyNOW. "Thus far, we have addressed the effect of the redox inactive metal, but with respect to manganese, it is still not well understood how the multiple manganese centers work together to oxidize water and which centres supports the oxide ligands that lead to O-O bond formation. Long term, we are working toward making robust and inexpensive catalysts for artificial photosynthesis, by using the lessons learned in model systems such as the one reported."