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.