UV photochemistry of the L-cystine disulfide bridge in aqueous solution investigated by femtosecond X-ray absorption spectroscopy

Despite the biological relevance of the disulfide bond as the motive, which stabilizes the tertiary structure of many proteins, its photostability, UV-induced bond cleavage mechanisms and secondary photochemistry are still contested after decades of research. In this study, we employed femtosecond X-ray absorption spectroscopy to unravel the photochemistry of the aliphatic disulfide bridge of the semi-essential proteinogenic amino acid L-cysteine (L-cystine) in aqueous solution. We observe homolytic bond cleavage upon UV irradiation and the emergence of thiyl radicals as the single primary photoproduct and its ultrafast decay due to geminate recombination at remarkably high quantum yield in excess of 80% within 20 ps. These dynamics coincide with the emergence of a secondary product, attributed to the perthiyl radical. More than 70% of broken disulfide bridges form within the first nanosecond after bond cleavage. From these observations, we establish a dynamic photostability of the disulfide bridge and a mechanism of perthiyl radical formation from a 'hot' ground-state parent molecule that asymmetrically fragments along a carbon-sulfur bond, resolving long-standing questions in the photochemistry of disulfide bridges in condensed phase.