Characterization of Oxidation Products from HOCl Uptake by Microhydrated Methionine Anions Using Cryogenic Ion Vibrational Spectroscopy

The oxidation of the amino acid methionine (Met) by hypochlorous acid (HOCl) to yield methionine sulfoxide (MetO) has been implicated in both the interfacial chemistry of tropospheric sea spray aerosols and the destruction of pathogens in the immune system. Here, we investigate the reaction of deprotonated methionine water clusters, Met–·(H2O)n, with HOCl and characterize the resulting products using cryogenic ion vibrational spectroscopy and electronic structure calculations. Capture of the MetO– oxidation product in the gas phase requires the presence of water molecules attached to the reactant anion. Analysis of its vibrational band pattern confirms that the sulfide group of Met– has indeed been oxidized. Additionally, the vibrational spectrum of the anion corresponding to the uptake of HOCl by Met–·(H2O)n indicates that it exists as an “exit-channel” complex in which the Cl– product ion is bound to the COOH group following the formation of the SO motif.