Elucidating the Reactivity of O 2 (a 1 Δ g ): A Study with Amino Acid Anions and Related Sulfur and Oxygen Anionic Species

Rate constants and product ions were determined for a series of anions reacting with singlet molecular oxygen O (a Δ ) at thermal energy using an electrospray ionization-selected ion flow tube. The 20 naturally occurring amino acids were used to produce corresponding deprotonated anions; only [Cys-H] and [Pro-H] were found to be reactive with O (a Δ ), generating OSCH CH(NH )CO + HO and C H NO + H O , respectively. The reaction of O (a Δ ) with [Cys-H] has a rate constant more than ten times larger than the reaction of O (a Δ ) with [Pro-H] . Furthermore, reactions of O (a Δ ) with carboxylic acid and thiol anions were carried out to elucidate the reactivity of the sulfur-containing functional groups. Potential energy surfaces and overall reaction exothermicities were calculated for representative reactions using density functional theory. Reactions in which attack occurs at the sulfur produce HCSO as an ionic product. Reactions of several carboxylic acid anions likely proceed through a hydroperoxide intermediate that is analogous to that calculated for reactions with amino acid anions at a higher collision energy. Overall, rate constants for reactions of carboxylic acid anions RC(O)O were found to be smaller for larger R groups.