Researchers from the US Department of Energy’s (DOE) Argonne National Laboratory have combined two membrane-bound protein complexes to perform a complete conversion of water molecules to hydrogen and oxygen. An open-access paper describing their work is published in the journal Chemical Science.
Sunlight-driven production of hydrogen from water provides a sustainable approach to achieve a clean, renewable alternative fuel to fossil fuels. Herein, we demonstrate unique systems that link PSII water oxidation to the reductive proton-coupled chemistry of self-assembled PSI-catalyst constructs in photosynthetic membranes. Both Pt-nanoparticles and synthetic molecular catalysts readily self-assemble with thylakoids via electrostatic or hydrophobic interactions, generating viable complexes that use light to rapidly produce hydrogen directly from water.
We show that it is feasible to bind synthetic molecule catalysts to thylakoid membranes and make a functional, inexpensive solar fuel producing system, addressing a key challenge of scalability for making solar fuels a viable energy source. This work provides the basis for future studies that use synthetic catalysts, tuned through known chemical modifications, for in vivo delivery systems that target PSI. Interfacing abiotic catalysts with photosynthetic membranes provides a method to utilize Nature’s optimized light-driven Z-scheme chemistry and points to a possible means to enhance photosynthetic efficiency toward solar fuel production by creating an alternative electron transfer pathway during downregulation of photosynthesis under high light intensities.
These benchmark studies are a positive step toward the implementation of in vivo approaches to generate living photosynthetic systems as a sustainable energy solution.—Utschig et al.
The work builds on an earlier study that examined one of these protein complexes, called Photosystem I, a membrane protein that can use energy from light to feed electrons to an inorganic catalyst that makes hydrogen. This part of the reaction, however, represents only half of the overall process needed for hydrogen generation.
By using a second protein complex that uses energy from light to split water and take electrons from it, called Photosystem II, Argonne chemist Lisa Utschig and her colleagues were able to take electrons from water and feed them to Photosystem I.
The beauty of this design is in its simplicity — you can self-assemble the catalyst with the natural membrane to do the chemistry you want.—Lisa Utschig, Argonne chemist
In an earlier experiment, the researchers provided Photosystem I with electrons from a sacrificial electron donor. “The trick was how to get two electrons to the catalyst in fast succession,” Utschig said.
The two protein complexes are embedded in thylakoid membranes, like those found inside the oxygen-creating chloroplasts in higher plants.
The membrane, which we have taken directly from nature, is essential for pairing the two photosystems. It structurally supports both of them simultaneously and provides a direct pathway for inter-protein electron transfer, but doesn’t impede catalyst binding to Photosystem I.—Lisa Utschig
According to Utschig, the Z-scheme—the technical name for the light-triggered electron transport chain of natural photosynthesis that occurs in the thylakoid membrane—and the synthetic catalyst come together quite elegantly.
One additional improvement involved the substitution of cobalt or nickel-containing catalysts for the expensive platinum catalyst that had been used in the earlier study. The new cobalt or nickel catalysts could significantly reduce potential costs.
The next step for the research, according to Utschig, involves incorporating the membrane-bound Z-scheme into a living system.
Once we have an in vivo system—one in which the process is happening in a living organism—we will really be able to see the rubber hitting the road in terms of hydrogen production.—Lisa Utschig
The research was funded by the DOE Office of Science, Basic Energy Sciences Program.
Lisa M. Utschig, Sarah R. Soltau, Karen L. Mulfort, Jens Niklas and Oleg G. Poluektov (2018) “Z-scheme solar water splitting via self-assembly of photosystem I-catalyst hybrids in thylakoid membranes” Chem. Sci. 9, 8504-8512 doi: 10.1039/C8SC02841A