l-Cysteine Modified by S-Sulfation: Consequence on Fragmentation Processes Elucidated by Tandem Mass Spectrometry and Chemical Dynamics Simulations

Low-energy collision-induced dissociation (CID) of deprotonated l-cysteine S-sulfate, [cysS-SO₃]^-, delivered in the gas phase by electrospray ionization, has been found to provide a means to form deprotonated l-cysteine sulfenic acid, which is a fleeting intermediate in biological media. The reaction mechanism underlying this process is the focus of the present contribution. At the same time, other novel species are formed, which were not observed in previous experiments. To understand fragmentation pathways of [cysS-SO₃]^-, reactive chemical dynamics simulations coupled with a novel algorithm for automatic determination of intermediates and transition states were performed. This approach has allowed the identification of the mechanisms involved and explained the experimental fragmentation pathways. Chemical dynamics simulations have shown that a roaming-like mechanism can be at the origin of l-cysteine sulfenic acid.