Stereoselectivity in Liquid Aerosols through Acid-Mediated Epoxide Ring-Opening Reactions

Stereoisomers in aqueous atmospheric aerosols are assumed to be present in equal concentrations, as aerosols are most commonly mixtures of simple inorganic salts with a range of organic species that lack obvious pathways to induce stereoselectivity. Further, since submicron aerosols are highly acidic (pH ∼ 0-3), acid-driven nucleophilic reactions are a crucial pathway for forming key condensed-phase products. Herein, we show strong diastereoselectivity during the ring-opening reaction of β-isoprene epoxydiol (β-IEPOX) isomers by sulfate to form methyltetrol sulfates (MTS, e.g., 1,3,4-trihydroxy-2-methylbutan-2-yl sulfate) in acidic aqueous particles. These key epoxides are formed from isoprene (2-methyl-1,3-butadiene) oxidation and subsequent reactive uptake to aerosols, forming condensed-phase species (i.e., secondary organic aerosol, SOA). Diastereomers and regioisomers were quantified using hydrophilic interaction liquid chromatography coupled with high-resolution mass spectrometry. Acid-driven concerted attack of sulfate at the epoxide-ring tertiary site was the dominant pathway (∼93%) for MTS formation, while other pathways─concerted substitution at the secondary carbon and acid-driven stepwise substitution─contributed <10%. Computational chemistry simulations revealed a ∼15 kcal/mol lower energy barrier for concerted backside attack of sulfate to the epoxide compared with attack from the opposite face, in line with observed stereoselectivity. Major MTS diastereomers formed from trans-β-IEPOX and cis-β-IEPOX isomers were racemic mixtures of (2R,3S)/(2S,3R)-2-MTS and (2R,3R)/(2S,3S)-2-MTS, respectively, with similar diastereoselectivity. Analysis of MTS-derived products indicates that stereochemical differences are likely propagated through continued condensed-phase secondary chemistry, which may have significant implications for climate-relevant aerosol properties. Overall, this stereoselectivity within seemingly simple liquid aerosols has important implications for condensed-phase reactions in aqueous aerosols.