Stable cysteine sulfenic acid: synthesis by direct oxidation of a thiol, crystallographic analysis, and elucidation of reactivities

A small-molecule cysteine sulfenic acid (Cys–SOH) was synthesized and isolated as stable crystals, for the first time, by utilizing a nanosized molecular cavity as a protective cradle. The cradled Cys–SOH was synthesized by direct oxidation of the corresponding cysteine thiol with H₂O₂ under basic conditions and its structure was established by X-ray crystallographic analysis. In the reaction of the cradled Cys–SOH with a thiol to produce the disulfide, a remarkable acceleration was observed upon the addition of an amine base. This suggests the important role of base in the reaction of Cys–SOH with thiols in biological systems. The cradled Cys–SOH was reduced to the cysteine thiol by dithiothreitol or triphenylphosphine. The high stability and sufficient reactivity of the cradled Cys–SOH indicate its usefulness as a small-molecule model compound for better understanding the chemical behavior of Cys–SOH in biological systems.