Ozone oxidation of cysteine in optically trapped aqueous micro‐droplets

Abstract

The ozone (O3) oxidation kinetics of cysteine in aqueous micro‐droplets at different acidities is investigated in this study via aerosol optical tweezers coupled with Raman spectroscopy. This study exploits the O3 oxidation of cysteine near the interface of micro‐droplets as a model system to elucidate the oxidation damage of amino acids in biosurfaces. For each optically trapped micro‐droplet, Raman spectroscopy is used to determine its droplet radius, concentrations of solutes, and droplet pH, as well as their time evolutions during the kinetics measurements. The bimolecular rate coefficients of the cysteine + O3 reaction measured in micro‐droplets are around 4 × 105 M−1 s−1 and 2 × 104 M−1 s−1 for pH ≈ 5 and 0.5, respectively. These results agree with the previous bulk measurements, indicating that the observed aerosol kinetics can be solely rationalized via diffusion‐limited kinetics. The results also indicate that a high‐ionic strength could enhance the cysteine + O3 reaction, particularly for the zwitterion form of cysteine. The results imply that when surfactant proteins in lung fluids are exposed to ambient O3, the cysteine residues in proteins will be attacked by O3 at first due to the high reactivity of the thiol moiety in cysteine.