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Microbes can help the petrochemical sector to produce long-lasting hydrocarbons.

If the petrochemical sector is to ever wean itself off oil and gas, it must identify sustainably sourced chemicals that can be seamlessly integrated into existing processes for producing fuels, lubricants, and polymers.

The apparent solution is to make such molecules biologically, but microbial products differ from fossil fuel hydrocarbons in two important ways: they contain too much oxygen and have too many other atoms dangling off the carbons. To function in existing synthetic processes, microbial hydrocarbons must often be de-oxygenated (or reduced in chemical terms) and stripped of superfluous chemical groups, all of which consumes energy.

Microbes have been designed by a team of scientists from the University of California, Berkeley, and the University of Minnesota to produce hydrocarbon chains that are easier to deoxygenate and use less energy — basically just the sugar glucose that the bacteria eat, plus a little heat.

The method enables the microbial manufacture of a wide range of compounds currently produced from oil and gas, including lubricants generated from medium-chain hydrocarbons, which have between eight and ten carbon atoms in their chain.

"Part of the problem with using glucose as a feedstock for making molecules or driving the chemical industry is that the fossil fuel structures of petrochemicals are so different — they're usually fully reduced, with no oxygen substitutions," said Michelle Chang, a UC Berkeley chemistry and chemical and biomolecular engineering professor. "Bacteria know how to generate all these complex compounds with all these functional groups poking out of them, just like all natural products," she says, "but producing petrochemicals, which we're used to utilising as chemical industry precursors, is a bit of a difficulty."

"This is one step toward deoxygenating these microbial products, and it allows us to start creating things that can replace petrochemicals using only glucose from plant biomass, which is more sustainable and renewable," she explained. "We'll be able to avoid petrochemicals and other fossil fuels this way."

Others have devised microbial techniques for building shorter and longer chains, up to roughly 20 carbons, but the bacteria were programmed to generate medium-length hydrocarbon chains, which has never been done before. However, Chang claims that the process may easily be altered to generate chains of different lengths, such as short-chain hydrocarbons used as precursors to the most common polymers, such as polyethylene.

their findings were published in the journal Nature Chemistry.

Simple linear chains of carbon atoms with a hydrogen atom connected to each carbon make up fossil hydrocarbons. However, microbially generated precursors that are oxygenated and have carbon atoms adorned with many additional atoms and tiny molecules cannot simply be substituted by chemical procedures geared for turning these into high-value products.

Chang and her team, which included former UC Berkeley postdoctoral researchers Zhen Wang and Heng Song as co-first authors, searched databases for enzymes that might synthesise medium-chain hydrocarbons in order to induce bacteria to generate something that could replace these fossil fuel precursors. They were also looking for an enzyme that could add a unique chemical group called carboxylic acid to one end of a hydrocarbon, converting it to a fatty acid.

Overall, the researchers forced E. coli bacteria to ferment glucose and create the necessary medium-chain fatty acid by inserting five different genes into the bacteria. The additional enzymatic reactions were independent of or orthogonal to the bacteria's existing enzyme pathways, which worked better than tinkering with the bacteria's complicated metabolic network.

"We discovered novel enzymes that could actually synthesise these mid-size hydrocarbon chains that were orthogonal to bacterial fatty acid production." This allows us to run it independently, and it consumes less energy than the original synthase pathway," Chang explained. "You have your route chewing through all the sugar to produce higher conversions and a high yield, while the cells consume enough glucose to survive."

The final step of creating a medium-chain fatty acid primed the product for easy conversion to olefins, which are precursors to polymers and lubricants, via a catalytic reaction. The UC Berkeley team collaborated with a Minnesota group led by Paul Dauenhauer, who demonstrated that removing the carboxylic acid from the final microbial products — 3-hydroxyoctanoic and 3-hydroxydecanoic acids — using a simple acid-based catalytic reaction called Lewis acid catalysis (named after UC Berkeley chemist Gilbert Newton Lewis) produced the olefins heptene and nonene, respectively. Lewis acid catalysis consumes a fraction of the energy required by redox reactions to remove oxygen from natural products and produce pure hydrocarbons.

"Professor Chang's group created biorenewable compounds that were ideal raw materials for catalytic refining," said Dauenhauer, who refers to these precursor molecules as bio-petroleum. "We were able to easily convert these molecules to larger, more valuable compounds using metal nanoparticle catalysts since they had just enough oxygen."This allowed us to fine-tune the distribution of molecular goods as needed, just like we could with conventional petroleum products, except we were employing renewable resources this time."

Heptene, which has seven carbons, and nonene, which has nine, can be used as lubricants directly, or they can be cracked down to smaller hydrocarbons and used as precursors to plastic polymers like polyethylene or polypropylene, or they can be linked together to form even longer hydrocarbons like those found in waxes and diesel fuel.

Chang explained, "This is a general approach for synthesising target compounds, regardless of chain length." "You also don't have to reengineer an enzyme system every time you want to modify a functional group, the length of the chain, or the number of branches."

Despite their metabolic engineering feat, Chang stated that the long-term and more sustainable goal would be to completely redesign processes for synthesising industrial hydrocarbons, such as plastics, so that they are optimised to use the types of chemicals that microbes normally produce, rather than modifying microbial products to fit into existing synthetic processes.

"There's a lot of curiosity around the topic, 'What if we look at whole new polymer structures?'" she added. "Can we use fermentation to generate monomers from glucose for polymers with qualities similar to those we use today, but not the same structures as polyethylene or polypropylene, which are difficult to recycle?"

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