Properties of Gaseous Deprotonated L-Cysteine S-Sulfate Anion [cysS-SO(3)](−): Intramolecular H-Bond Network, Electron Affinity, Chemically Active Site, and Vibrational Fingerprints

L-cysteine S-sulfate, Cys-SSO(3)H, 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-SO(3)](−) was introduced in the gas phase by electrospray and characterized by size-selected, cryogenic, negative ion photoelectron spectroscopy. The electron affinity of the [cysS-SO(3)](•) 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-SO(3)](−) (S(1)) is stabilized via three intramolecular hydrogen bonds (HBs); i.e., one O−H⋯⋯N between the –COOH and –NH(2) groups, and two N−H⋯⋯O HBs between –NH(2) and –SO(3), in which the amino group serves as both HB acceptor and donor. In addition, a nearly iso-energetic conformer (S(2)) with the formation of an O−H⋯⋯N−H⋯⋯O−S chain-type binding motif competes with S(1) 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-SSO(3)H in biologic settings.