One of the annoying things that happens when you track developing science is that you keep seeing interesting results on a topic, but none of them quite reaches the significance to justify a news story. This week, another paper that fit the description came to my attention. Again, these particular results weren’t especially exciting, but I’ve decided it gives me an excuse to introduce you to an interesting and potentially significant area of chemistry.
The area of research that keeps grabbing my attention is a fusion of photovoltaic technology and biochemistry. Photovoltaics are useful because they provide a way to conveniently liberate some electrons. And a lot of enzymes work because they do interesting things with electrons they obtain from other molecules. So, in theory, it should be possible to use a photovoltaic device to supply an enzyme what it needs to catalyze useful reactions. And, in many cases, reality matches up nicely with theory.
The new paper focuses on using photovoltaic nanoparticles to drive an enzyme that uses carbon dioxide, incorporating it into a larger molecule. But the researchers behind it also discover the process doesn’t work especially efficiently, and they make some progress toward figuring out why.
All chemical reactions involve rearranging the locations of electrons; electron transfer reactions, however, specifically involve changing the charge states of molecules, oxidizing some and reducing others. While enzymes catalyze a lot of bond rearranging reactions, many of the reactions central to metabolism involve electron transfers. There are entire electron transfer chains involved in breaking down sugars and others that are central to photosynthesis.
(The electrons involved in these reactions often come from metals, which is why so many of these enzymes have iron or some other metal incorporated as a co-factor.)
Many of these enzymes do things that we would be very interested in doing on an industrial scale. They form interesting molecules, make key intermediates in drugs, turn nitrogen gas into fertilizer, or even potentially pull carbon dioxide from the air, incorporating it into useful chemicals. So, it would be great if we could pull these enzymes out of the complex maze of biochemical pathways they’re embedded in and use them separated from all the complexities of the cell.
Unfortunately, that’s far, far easier said than done. Many of these enzymes take starting materials that only exist transiently within the cell. Others rely on chemicals or co-factors that are hard to produce or expensive; without those, they don’t have any way to get the electrons they need to do anything useful.
Photovoltaics offer an alternative to all this messy biochemistry. If the starting chemicals of a reaction are easy to obtain, a little bit of light and a photovoltaic will supply the electrons that are needed. By using photovoltaic nanoparticles, it’s also possible to tune the energy of the electrons to the needs of the enzyme. And it works. Photovoltaic-driven enzymes have made hydrogen from acidic solutions, converted nitrogen gas to ammonia, and pulled one of the oxygens off carbon dioxide.
While these were valuable demonstrations, we haven’t learned a lot about what’s going on at a biochemical level when an enzyme interacts with a photovoltaic material. As a result, it’s hard to know in advance which enzymes will play nice with them. And, if the process isn’t working as well as we’d like, there’s no obvious way to improve things.