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Chemical ‘Sleuths’ Hunt for Life’s Building Blocks in Space

Researchers have reconstructed the chemical steps that lead to the formation of complex hydrocarbons in space, presenting routes to forming 2-D carbon-based nanostructures in a mix of heated gases.

(CN) – Researchers have reconstructed the chemical steps that lead to the formation of complex hydrocarbons in space, presenting routes to forming 2-D carbon-based nanostructures in a mix of heated gases.

The team’s findings, published Monday in the journal Nature Astronomy, could help explain the existence of pyrene – a polycyclic aromatic hydrocarbon – and similar chemical compounds in some meteorites.

“This is how we believe some of the first carbon-based structures evolved in the universe,” said co-author Musahid Ahmed, a scientist at Lawrence Berkeley Lab’s Chemical Sciences Division.

“Starting off from simple gases, you can generate one-dimensional and two-dimensional structures, and pyrene could lead you to 2-D graphene. From there you can get to graphite, and the evolution of more complex chemistry begins.”

Pyrene is composed of 16 carbon atoms and 10 hydrogen atoms. The team found that the same heated chemical processes that promote the formation of pyrene are also relevant to the creation of soot particles and combustion processes in vehicle engines.

The research adds to earlier work that examined hydrocarbons with smaller molecular rings that have also been found in space, including in Saturn’s moon Titan – benzene and naphthalene, namely.

“When these hydrocarbons were first seen in space, people got very excited. There was the question of how they formed,” said co-lead author Ralf I. Kaiser, a chemistry professor at the University of Hawaii at Manoa.

For example, were they formed purely through reactions in a mix of gases, or did they assemble on a watery surface?

Ahmed noted that astronomers and chemists exchange ideas and information as they investigate the story of how life’s chemical precursors developed in the universe.

“We talk to astronomers a lot because we want their help in figuring out what's out there, and it informs us to think about how it got there,” he said.

Kaiser said physical chemists, on the other hand, can help illuminate reaction mechanisms that can lead to the synthesis of certain molecules in space.

Pyrene is a part of a family called polycyclic aromatic hydrocarbons (PAHs), which are estimated to represent about 20 percent of all carbon in the Milky Way Galaxy. PAHs are organic molecules that are made up of a sequence of fused molecular rings. To study how these rings arise in space, researchers try to synthesize these and other surrounding molecules known to exist in the universe.

“You build them up one ring at a time, and we've been making these rings bigger and bigger,” said co-author Alexander M. Mebel, a chemistry professor at Florida International University. “This is a very reductionist way of looking at the origins of life: one building block at a time.”

The team examined the chemical reactions resulting from a combination of a complex hydrocarbon known as the 4-phenanthrenyl radical – which features a molecular structure that includes a sequence of three rings, 14 carbon atoms and nine hydrogen atoms – with the chemical compound acetylene, which has two carbon atoms and two hydrogen atoms.

Chemical compounds needed for the research were not commercially available according to Felix Fischer, whose lab prepared the samples.

“These chemicals are very tedious to synthesize in the laboratory,” said Fischer, an assistant professor of chemistry at the University of California, Berkeley.

At Berkeley Lab’s Advanced Light Source facility, the team injected the gas mixture into a microreactor that warmed the sample to a high temperature to mimic the proximity of a star. The ALS generates beams of light – from X-ray wavelengths to infrared – to aid a range of experiments by in-house and visiting scientists.

The blend of gases was shot out of the microreactor through a small nozzle at supersonic speeds, arresting the active chemistry within the warmed cell. The researchers then focused a beam of vacuum ultraviolet light from the synchrotron on the heated gas mixture, which bumped off electronics, a process known as ionization.

The team then examined the occurring chemistry using a charged-particle detector that calculated the different arrival times of particles that formed following ionization. These arrival times featured the telltale attributes of the parent molecules. These experimental measurements, along with Mabel's theoretical calculations, enable the team to behold the intermediate steps of the chemistry at work and to verify the production of pyrene in the reactions.

Mebel’s work showed how pyrene, a four-ringed molecular structure, could emerge from a compound known as phenanthrene, a three-ringed structure.

These theoretical measurements can aid the study of a variety of phenomena, “from combustion flames on Earth to outflows of carbon stars and the interstellar medium,” he said.

Kaiser added, “Future studies could study how to create even larger chains of ringed molecules using the same technique, and to explore how to form graphene from pyrene chemistry.”

Other experiments conducted by team members at the University of Hawaii will investigate what occurs when scientists mix hydrocarbon gases in icy conditions and simulate cosmic radiation to determine whether that may cause the creation of life-bearing molecules.

“Is this enough of a trigger?” Ahmed asked. “There has to be some self-organization and self-assembly involved" to produce life forms.

“The big question is whether this is something that, inherently, the laws of physics do allow.”

The research was funded by the U.S. Department of Energy's Office of Science, UC Berkeley, the University of Hawaii, Florida International University and the National Science Foundation.

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