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

Abstract

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.

Intra-molecular reactions between cysteine sulfinyl radical and a disulfide bond within peptide ions

“…The sulfinyl radical appeared to be the least reactive relative to the thiyl and perthiyl radical, which agreed with studies in the condensed phase [19][20][21]. The low reactivity was in-part due to the delocalization of the spin between the sulfur and oxygen atom within SO Cys as shown by theoretical calculations [22]. This chemical property of SO Cys makes it difficult to further characterize its reaction products due to low reaction yields (typically <1%) and expand the scope of investigation to other reactions.…”

“…1d). The spectrum is dominated by the neutral loss of 62 Da (CH 2 SO), a signature loss fragmentation channel for the sulfinyl radical ion, giving rise to a glycyl radical ion [9,22,32]. Radical-directed fragmentation of the thus formed glycyl radical ion leads to sequential losses of 59 Da (from glutamic acid side chain) and 58 Da (from lysine side chain) [9].…”

Integration of online digestion and electrolytic reduction with mass spectrometry for rapid disulfide-containing protein structural analysis

“…In addition, the resulting protein/peptide thiols often need to be protected due to the possibility of being reoxidized prior to MS analysis. Besides chemical reduction, other novel approaches include the cleavage of disulfide bonds via laser-based ionization [4–7], ultraviolet photodissociation [8–10], negative ion dissociation [11–14], electron-capture dissociation (ECD)[15], electron-transfer dissociation (ETD)[16–20], plasma-induced oxidation [21], or using new ion chemistry [22–28]. An alternative way for reducing disulfide bonds without involving chemical reductants is electrolytic reduction [29,30].…”

Tautomerization and Dissociation of Molecular Peptide Radical Cations

“…For example, regiospecific radical generation has been demonstrated from nitrate esters, diazo derivatives, peroxycarbamate derivatives, and diazirine rings . A variety of peptide ions with different radical sites can be formed at atmospheric pressure (AP) through ion–radical reactions . Furthermore, the formation of molecular peptide radical ions (i. e., M •+ , [M + H] •2+ , [M – 2H] •– ) can also be achieved through dissociative electron transfers of ternary metal–ligand–peptide complexes, which have been studied extensively and applied by our and other research groups …”