The photolysis of disulfide bonds is implicated in denaturation of proteins exposed to ultraviolet light. Despite this biological relevance in stabilizing the structure of many proteins, the mechanisms of disulfide photolysis are still contested after decades of research. Herein, we report new insight into the photochemistry of L-cystine in aqueous solution by femtosecond X-ray absorption spectroscopy at the sulfur K-edge. We observe homolytic bond cleavage upon ultraviolet irradiation and the formation of thiyl radicals as the single primary photoproduct. Ultrafast thiyl decay due to geminate recombination proceeds at a quantum yield of >80 % within 20 ps. These dynamics coincide with the emergence of a secondary product, attributed to the generation of perthiyl radicals. From these findings, we suggest a mechanism of perthiyl radical generation from a vibrationally excited parent molecule that asymmetrically fragments along a carbon-sulfur bond. Our results point toward a dynamic photostability of the disulfide bridge in condensed-phase.

二硫键的光解与暴露于紫外线下的蛋白质的变性有关。尽管在稳定许多蛋白质的结构方面具有这种生物学相关性，但经过数十年的研究，二硫键光解的机制仍然存在争议。在此，我们报告了通过在硫 K 边缘的飞秒 X 射线吸收光谱对 L-胱氨酸在水溶液中光化学的新见解。我们观察到紫外线照射下的均解键裂解和巯基自由基的形成作为单一初级光产物。由于双酯复合引起的超快巯基衰变在 20 ps 内以 >80 % 的量子产率进行。这些动态与次级产物的出现相吻合，这归因于苯乙烯基自由基的产生。从这些发现中，我们提出了一种从振动激发的母体分子产生苯硫自由基的机制，该母体分子沿碳硫键不对称地碎裂。我们的结果表明，凝聚相中二硫键的动态光稳定性。