Green Chemistry Creating Chemical Reactions that Run on Oxygen, Produce Only Water as Waste

Chemists have built an artificial enzyme that turns one of nature's most stubborn molecules — the stable six-carbon ring found in plastics and fuels — into something far more reactive, using nothing but oxygen and leaving only water behind.

Oxygen provides the driving force for many vital chemical reactions. Two well-known examples include respiration, the process that allows living things to breakdown food to release its energy, and combustion, reactions that produces heat and light. But oxygen has an unusual electronic configuration that tends to slow reactions with organic matter, so reactions often need help to get started. In nature, specialized proteins called enzymes can activate oxygen on demand by first rearranging its electrons. Now, a team led by Penn State researchers has developed a synthetic mimic of one of these enzymes, driving an enzyme-like reaction. This reaction, like its analog in nature, produces only water as waste. The research could potentially be used in sustainable industrial chemical syntheses and pharmaceutical drug development.

“Oxygen is a marvelous molecule; it is produced by photosynthesis making it essentially the sun’s energy stored in chemical form,” said Jonathan Kuo, assistant professor of chemistry in the Penn State Eberly College of Science and the leader of the research team.

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“This energy is why you can have a campfire. When oxygen reacts with organic matter, it can become a runaway reaction; we call that combustion. Luckily for us, the oxygen molecule, which occurs as a bonded pair of oxygen atoms referred to as dioxygen, has an unusual electronic configuration, which makes it hard to get the fire started. So, we have plenty of oxygen to breathe, without worrying about everything organic spontaneously combusting.”

The mimic built by Kuo and his team inserts an oxygen atom from dioxygen into an organic chemical building block, known as an aromatic ring — structures that are typically stable and hard to alter. The researchers built the mimic to help decipher exactly how these enzymes work. Their ultimate goal is to develop a more sustainable chemical infrastructure, promoting desired reactions without producing harmful byproducts.

“The reaction driven by this enzyme requires only oxygen and produces only water as waste,” Kuo said. “As such, the research could also help build the foundation for the development of environmentally clean chemistry.”

The researchers studied the active site of the enzyme — the location in the enzyme where the reaction takes place — and decided which structural features might need to be replicated in a synthetic mimic.

“The enzyme targets a compound called catechol,” Kuo said. “Catechol can be made from benzene, a basic aromatic compound derived from petroleum; both are used in the industrial production of chemicals. The six-carbon ring of benzene and catechol tend to be stable, so most chemicals we make from either material also contain that six-membered ring. That limits what chemicals we can produce for manufacturing plastics, polymers, and synthetic fibers.”

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In laboratory experiments, the researchers demonstrated that the synthetic enzyme mimic can perform the function of the naturally occurring enzyme, known as extradiol dioxygenase. It expands catechol’s six-carbon ring and inserts an atom of oxygen creating a ring of seven atoms. The seven-atom ring is less stable, widening the possible synthetic manipulations. This work could be the basis for new and diverse chemical compounds, the researchers explained.

“Having a functional enzyme mimic is important because we can test hypotheses for what exact chemical steps are required,” Kuo said. “For example, we used a non-natural metal ion for our mimic. The enzymes normally use iron, cobalt, or manganese, but we used iridium. So, the reaction is not necessarily specific to the metal ions found in nature. Iridium is a noble metal, like gold, which means it resists unwanted reactions with oxygen. This reduced oxygen reactivity makes it easier to build enzyme mimics. Iron-based mimics, for example, can react unexpectedly with air — think about rust! Using iridium may allow synthetic mimics to last longer or be built faster.”

Each reaction of the enzyme mimic costs one molecule of oxygen and produces one molecule of water as waste. The research team said this atom efficient reaction could allow the design of enzymes that are sustainable and environmentally friendly, as they produce essentially no waste.

“For most of history, chemists have focused on ‘can we perform reactions to obtain the chemicals we want,’ and the answer is almost always ‘yes,’” Kuo said. “A modern version of the question asks if we can get what we want but leave nothing behind. We need a whole new playbook. Nature provides the only known blueprint for a sustainable chemical infrastructure. Doing the basic research to understand precisely how nature accomplishes this — which is the goal of this project — could eventually allow us build chemicals and other materials in a way that rivals the circularity of the nature.”

Original Article: Mimicking Extradiol Dioxygenase Reactivity on Iridium; Journal of the American Chemical Society; DOI:10.1021/jacs.5c23353

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