Properties of Gaseous Deprotonated L-Cysteine S-Sulfate Anion [cysS-SO₃]^-: Intramolecular H-Bond Network, Electron...

L-cysteine S-sulfate, Cys-SSO₃H, and their derivatives play essential roles in biological chemistry and pharmaceutical synthesis, yet their intrinsic molecular properties have not been studied to date. In this contribution, the deprotonated anion [cysS-SO₃]^- was introduced in the gas phase by electrospray and characterized by size-selected, cryogenic, negative ion photoelectron spectroscopy. The electron affinity of the [cysS-SO₃]^• radical was determined to be 4.95 ± 0.10 eV. In combination with theoretical calculations, it was found that the most stable structure of [cysS-SO₃]^- (S) is stabilized via three intramolecular hydrogen bonds (HBs); i.e., one O-H⋯⋯N between the -COOH and -NH₂ groups, and two N-H⋯⋯O HBs between -NH₂ and -SO₃, in which the amino group serves as both HB acceptor and donor. In addition, a nearly iso-energetic conformer (S) with the formation of an O-H⋯⋯N-H⋯⋯O-S chain-type binding motif competes with S in the source. The most reactive site of the molecule susceptible for electrophilic attacks is the linkage S atom. Theoretically predicted infrared spectra indicate that O-H and N-H stretching modes are the fingerprint region (2800 to 3600 cm^-1) to distinguish different isomers. The obtained information lays out a foundation to better understand the transformation and structure-reactivity correlation of Cys-SSO₃H in biologic settings.