Radical Formation in the Gas-Phase Ozonolysis of Deprotonated Cysteine

Although the deleterious effects of ozone on the human respiratory system are well‐known, many of the precise chemical mechanisms that both cause damage and afford protection in the pulmonary epithelial lining fluid are poorly understood. As a key first step to elucidating the intrinsic reactivity of ozone with proteins, its reactions with deprotonated cysteine [Cys−H]− are examined in the gas phase. Reaction proceeds at near the collision limit to give a rich set of products including 1) sequential oxygen atom ion reactions to yield cysteine sulfenate, sulfinate and sulfonate anions, and significantly 2) sulfenate radical anions formed by ejection of a hydroperoxy radical. The free‐radical pathway occurs only when both thiol and carboxylate moieties are available, implicating electron‐transfer as a key step in this reaction. This novel and facile reaction is also observed in small cys‐containing peptides indicating a possible role for this chemistry in protein ozonolysis. Gas‐phase reactions: Deprotonated cysteine reacted with ozone to give a rich set of products including sequential oxygen atom ion reactions to yield cysteine sulfenate, sulfonate, and sulfonate anions, and significantly sulfenate radical anions formed by ejection of a hydroperoxy radical (see picture). The free‐radical pathway occurred only when both thiol and carboxylate moieties were available.