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Many chemical reactions require the input of energy to activate the transformation. This can often be in the form of heat, or chemical energy. One of the most efficient ways of introducing energy into a reaction is by using light. If you don't have to heat up a reaction, or add extra chemicals to it, and instead shine a light on it, you can save significant energy. However, it can be difficult to control and optimize light-driven reactions.

Research, , provides a holistic understanding of the light-driven production of hydrogen gas using a nanocrystal-enzyme complex as the catalyst, and a computational framework that can be used more generally to understand other light-driven chemical reactions in the future.

The work is a collaboration between the Dukovic Group at the University of Colorado Boulder and the King Group at the National Renewable Energy Lab.

Chemical catalysis is a special type of reaction, one that increases the speed of a transformation and often reduces the amount of waste produced by the process. Think of it like an assembly line.

The catalyst is like a station on the line, bringing together two or more components to create a new product that is then passed along. Without the catalyst, the components might, by chance, bump together and form the desired product, but it would be much slower, and much less frequent. The catalyst remains unchanged in the process and can repeat the transformation many times.

Enzymes are nature's catalysts. On the cellular level, whenever a change needs to happen, an enzyme is usually involved. The speed of an enzyme, and its selectivity, that is, its ability to only react with the desired molecules out of the soup of molecules present in a typical cell, is fantastic.

Enzymes are often superior to catalysts we can make in a lab, and as such, much research has gone into finding ways to harness such enzymes to do reactions for us in the lab. Unfortunately, it is not as easy as just grabbing some enzyme out of a cell. Enzymes often require specific environments and partners to react with.

Redox enzymes are a special, and particularly attractive, class of enzymes. They are capable of adding, or removing, an electron from a chemical reaction, a key step in the production of hydrogen gas. Redox enzymes rarely exist by themselves.

Returning to the assembly line analogy, to get a station that can add the electrons to the protons (H⁺) to make hydrogen gas, many other stations need to be added before, in a specific order. In a cell there is a chain of enzymes that pass the electrons along before the reaction can take place.

This is where the artificial component comes in. The nanocrystal, which, when exposed to light, releases an electron, replaces the long chain of enzymes and can directly transfer an electron to the enzyme. So, you reduce your assembly line down from a chain of many stations to just two.

"This work was really only possible through collaboration," explains Gordana Dukovic, the lead researcher at CU Boulder.

"The team at NREL have vast expertise in hydrogenase (the redox enzyme that creates hydrogen gas), and we have the expertise in making and tailoring the nanocrystals and studying what they do after they absorb light."

Getting the enzyme to work with the artificial electron donor took some work. The two teams first started working together in 2011 and have invested a great deal of work in understanding many aspects of this nanocrystal-enzyme hybrid.

"Working with the team at NREL has been really amazing," says Dukovic. "The opportunity to work with experts who really help you ask the important questions, and identify where our assumptions were wrong, was essential for this work."

For over more than a decade, this collaboration has interrogated the different steps of this process, such as how the nanocrystal and enzyme fit together, how the nanocrystal generates an electron when exposed to light, how the nanocrystal transfers the electron to the enzyme, and how the enzyme uses those electrons to make hydrogen.

It is only through building this comprehensive understanding of the steps that underpin this reaction that the team are in the position to provide a holistic picture of the whole transformation.

Furthermore, the framework that they have built is robust enough to be applied in improving other light-driven reactions in the future.

The work describes an improved assembly line capable of converting light energy into hydrogen gas, a clean burning fuel that provides new, more efficient ways to generate electricity.

Perhaps more excitingly, it demonstrates the power of a new computational model and framework, built on over a decade of collaborative research, which has been made freely available, that provides insights into light-driven reactions and can be used by the scientific community to refine and optimize future light-driven chemistry. Helena Keller, the lead author, is enthusiastic about the next steps.

"We are in a really exciting place now, where the capabilities of using computational methods to understand complex systems like this are becoming more and more accessible. The better we understand how to control processes at the smallest scales—like at the level of individual electron transfers—the closer we get to revolutionizing the way we produce energy and materials for the good of the world," she says.