Size-Dependent Ultrafast UV Photochemistry of Aliphatic Disulfides in Solution.

We have investigated the photochemistry of dimethyl disulfide (DMDS) and glutathione disulfide (GSSG) in solution upon ultraviolet (UV) excitation in comparison to L-cystine by femtosecond X-ray absorption spectroscopy at the sulfur K-edge. For these aliphatic disulfides, the very similar differential absorption spectra reveal unequivocally a pair of geminate thiyl radicals as the single primary reaction product with a formation time constant of about 100 fs. We find size-dependent majority yields for disulfide recombination with DMDS reforming ten-fold slower and at lower quantum yield than L-cystine while GSSG recombines even faster than L-cystine at a yield of nearly 80 % within 2 ps after excitation. We attribute the much lower recombination rates and yields of DMDS to larger product separation after homolytic S-S bond cleavage due to the smaller thiyl radical mass and correspondingly larger mutual acceleration. Furthermore, we observe the delayed formation of at least one additional reaction product for these disulfides that we attribute to perthiyl radicals. The ultrafast recombination of L-cystine and GSSG at majority yield in condensed phase suggests a dynamic S-S bond integrity which may be exploited in natural systems to mitigate UV radiation damage, and which could be of interest in materials design.