Creating Cleaner Chemical Reactions That Run on Oxygen and Produce Only Water as Waste

Oxygen provides the driving force for many vital chemical reactions. Two well-known examples are respiration, the process that allows living things to break down nutrients to release energy, and combustion, reactions that produce heat and light. However, oxygen has an unusual electronic configuration that tends to slow down reactions with organic matter, so reactions often need help getting started. In nature, specialized proteins called enzymes can activate oxygen when needed by first rearranging its electrons.

Now a team led by Penn State researchers has developed a synthetic mimic of one of these enzymes. This mimic carries out an enzyme-like reaction. Just like its natural counterpart, this reaction produces only water as waste. The research could be used in sustainable industrial chemical synthesis and pharmaceutical drug development.

A paper describing the synthetic enzyme was published in the Journal of the American Chemical Society.

Jonathan Kuo, assistant professor of chemistry at Penn State Eberly College of Science and leader of the research team, said, "Oxygen is a wonderful molecule; it's produced through photosynthesis, and therefore it's essentially solar energy stored in chemical form."

"That's the energy that lets you light a campfire. When oxygen reacts with organic matter, it can turn into a reaction that gets out of control, which we call combustion. Fortunately, the oxygen molecule — dioxygen, a bonded pair of oxygen atoms — has an unusual electronic configuration, which makes it difficult to start a fire. This gives us plenty of oxygen that we can breathe without worrying about everything organic spontaneously combusting."

The mimic developed by Kuo and his team inserts one oxygen atom from dioxygen into an organic chemical building block called an aromatic ring. Aromatic rings are typically stable structures that are difficult to modify. The researchers designed the mimic to help decipher exactly how these enzymes work. Their ultimate goal is to develop a more sustainable chemical infrastructure that promotes desired reactions without producing harmful byproducts.

Kuo said, "The reaction that this enzyme carries out only needs oxygen and produces only water as waste. So the research could also help build a foundation for developing environmentally clean chemistry."

The researchers examined the enzyme's active site — the part of the enzyme where the reaction takes place — and determined which structural features would need to be recreated in a synthetic mimic.

Kuo said, "The enzyme targets a compound called catechol. Catechol can be produced from benzene, a fundamental aromatic compound derived from petroleum; both are used in the industrial production of chemicals. The six-carbon rings of benzene and catechol tend to be stable. As a result, most of the chemicals we produce from these substances also contain this six-membered ring. This limits the diversity of chemicals we can use in the production of plastics, polymers, and synthetic fibers."

In laboratory experiments, the researchers showed that the synthetic enzyme mimic could perform the function of the naturally occurring enzyme known as extradiol dioxygenase. This mimic expands catechol's six-carbon ring, adding an oxygen atom to form a seven-atom ring. Because the seven-atom ring is less stable, it broadens the scope of possible synthetic modifications. The researchers stated that this work could form the basis for new and diverse chemical compounds. Kuo said, "Having a functional enzyme mimic is important because we can test hypotheses about which chemical steps are required. For example, in our mimic we used a metal ion that isn't natural. Enzymes normally use iron, cobalt, or manganese, but we used iridium. So the reaction doesn't have to be specific to the metal ions found in nature. Iridium is a noble metal like gold, which means it resists unwanted reactions with oxygen. This low reactivity with oxygen makes it easier to create enzyme mimics. For example, iron-based mimics can react unexpectedly with air — think of rust! Using iridium can allow synthetic mimics to last longer or be created faster."

Every reaction carried out by the enzyme mimic consumes one oxygen molecule and produces one water molecule as waste. The research team noted that this atom-efficient reaction could enable the design of sustainable and environmentally friendly enzymes, because these enzymes essentially produce no waste at all.

Kuo said, "Throughout history, chemists have mostly focused on the question, 'Can we carry out reactions to get the chemicals we want?' and the answer to that question is almost always 'yes.'"

"The modern form of this question asks whether we can get what we want while leaving nothing behind. We need an entirely new road map. Nature offers the only known blueprint for a sustainable chemical infrastructure. Conducting fundamental research to understand exactly how nature accomplishes this — which is the goal of this project — could ultimately enable us to produce chemicals and other materials in a way that can compete with nature's circularity."

In addition to Kuo, the Penn State research team included Alexander G. Arnette, the paper's first author and a graduate student in chemistry, and Alexey Silakov, associate professor of chemistry.