2-Nitrophenyl aryl sulfides undergo both intramolecular and electrospray-induced intermolecular oxidation of sulfur: An experimental and theoretical case study

Aromatic sulfides bearing a nitro group undergo sulfur oxidation upon electrospray ionization in the positive-ion mode. For example, 2-nitrophenyl phenyl sulfide, its para nitro isomer, and its chloro and methyl substituted analogs pick up an oxygen atom to afford [M+H+O]⁺ and [M+Na+O]⁺ ions upon ESI. Elemental-composition determination and tandem mass spectrometry confirm the reactions. Another oxidation of the sulfur, by the ortho nitro group of the [M+H]⁺ ions, occurs as intramolecular oxygen-transfer processes, evidenced by characteristic losses of SO, SO₂ and SO₂H{radical dot}, the latter yielding the carbazole radical cation, and the generation of the aryl-SO⁺ product ion. The intramolecular oxidation via oxygen transfer from the nitro group to the sulfur was corroborated by molecular modeling. The results substantiate both inter- and intra-molecular oxidation and provide more evidence that care must be taken when analyzing not only methionine-containing peptides but also small sulfides.