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Competition of Charge- versus Radical-Directed Fragmentation of Gas-Phase Protonated Cysteine Sulfinyl Radicals

Love, Chasity B., Tan, Lei, Francisco, Joseph S., Xia, Yu
Journal of the American Chemical Society 2013 v.135 no.16 pp. 6226-6233
cysteine, dissociation, free radicals, gases, hydrogen, ions, oxygen
The fragmentation behavior of various cysteine sulfinyl ions (intact, N-acetylated, and O-methylated), new members of the gas-phase amino acid radical ion family, was investigated by low-energy collision-induced dissociation (CID). The dominant fragmentation channel for the protonated cysteine sulfinyl radicals (Sᴼ•Cys) was the radical-directed Cα–Cᵦ homolytic cleavage, resulting in the formation of glycyl radical ions and loss of CH₂SO. This channel, however, was not observed for protonated N-acetylated cysteine sulfinyl radicals (Ac-Sᴼ•Cys); instead, charge-directed H₂O loss followed immediately by SH loss prevailed. Counterintuitively, the H₂O loss did not derive from the carboxyl group but involved the sulfinyl oxygen, a proton, and a Cᵦ hydrogen atom. Theoretical calculations suggested that N-acetylation significantly increases the barrier (∼14 kcal mol–¹) for the radical-directed fragmentation channel because of its reduced capability to stabilize the thus-formed glycyl radical ions via the captodative effect. N-Acetylation also assists in moving the proton to the sulfinyl site, which reduces the barrier for H₂O loss. Our studies demonstrate that for cysteine sulfinyl radical ions, the stability of the product ions (glycyl radical ions) and the location of the charge (proton) can significantly modulate the competition between radical- and charge-directed fragmentation.