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, SO(2) and SO(2)H(*), 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 intramolecular oxidation and provide more evidence that care must be taken when analyzing not only methionine-containing peptides but also small sulfides.