Aerosols’ acidic conditions favor some diastereomers over others

A new study shows that volatile organic gases may undergo unexpected stereoselective reactions as they permeate into aerosol particles. The chiral composition of the tiny atmospheric droplets may in turn affect their physicochemical properties, which determine how they scatter light, influence cloud formation, and more (J. Am. Chem. Soc. 2026, DOI: 10.1021/jacs.6c07307).

The findings were something of a “white whale” for lead researcher Andy Ault. In acidic particles, many people assumed “that once these molecules go from the gas to the condensed phase, all stereochemistry gets scrambled and gets lost,” the University of Michigan chemist explains.

So in 2018, when Ault teamed up with Jason Surratt at the University of North Carolina at Chapel Hill to quantify the composition of the tiny droplets, their data came as a surprise.

Whether a real-world sample or one made in the laboratory, the stereochemical mixture in the aerosols “never looked like our synthesized standard,” Ault says. One diastereomer was always enhanced over the other. Ault was determined to find out why.

The researchers decided to use isoprene as a model reactant because the gas is one of the most widely emitted volatile organic compounds in the atmosphere. After it’s in the air, isoprene reacts with hydroxyl radicals to form an isoprene epoxydiol. When this molecule passes into a liquid aerosol droplet, sulfate—an ion commonly found in acidic aerosols—pops open the epoxide ring and traps the molecule in the condensed phase. “It’s stuck in particle jail for life,” Ault says. Sulfate-containing compounds, therefore, accumulate in aerosols, making them good targets to study.

How the sulfate pops the epoxide determines the molecule’s final conformation. So by carefully characterizing the final products in an aerosol, Ault figured his team could determine what caused the observed diastereoselectivity.

This is easier said than done. The researchers had to choose an analytical technique with diastereomeric selectivity —in this case, hydrophilic interaction liquid chromatography coupled with mass spectrometry, the same method that originally revealed the curious composition of diastereomers in Ault and Surratt’s aerosols. They also needed pure samples of specific isoprene epoxydiols to start with and further needed each diastereomer product, to compare against their reaction spectra. Finally, the scientists needed computational models to help reverse engineer the dominant ring-opening mechanism. “Pulling a collaboration together like this, there’s a lot of moving parts,” Ault says.

After years of work, Ault finally has an explanation for the weird ratio of diastereomers he saw back in 2018. The dominant mechanism is acid-driven but concerted and electronically controlled. It’s basically S_N2-like: the epoxide ring opens simultaneously with the sulfate binding because of a buildup of positive charge on the more crowded carbon atom. This introduces stereoselectivity at that carbon because a carbocation never forms. And the selectivity persists beyond single subunits to dimers and trimers.

Miriam Freedman, a chemist at Pennsylvania State University who was not involved in the research, thinks the work nicely highlights the intriguing reactions that occur at the extremely low-pH regimes characteristic of aerosols. “This is really, really interesting chemistry that is happening inside droplets in the atmosphere,” she says. The work is a triumph in synthesis and analysis, she adds. There aren’t many scientists making the molecules needed to carry out these experiments nor pursuing the detailed analysis required to understand stereochemistry in atmospheric processes.

Still, Freedman is curious how this research will translate to the real world. “If you end up having a mixture of the [starting] epoxides, you will get a mixture of the diastereomers,” Freedman adds. “If we go and extrapolate to physical properties, we need to be concerned with what that mixture is doing.”