Dissociative Photoionization of Dimethyl Disulfide and Dimethyl Diselenide using Imaging PEPICO (iPEPICO): Structure and Energetics

摘要

The folding behaviors and stabilities of proteins are strongly affected by the disulfide bonds between their cysteine residues, thus determining their tertiary and quaternary structures. Dimethyl disulfide (DMDS) is one the simplest molecules containing this disulfide bond. DMDS is also an important precursor in the atmospheric sulfur chemistry, contributing to the formation of acid rain. Disulfide bonds also play an important role in possessing strong bonding interactions in coal and petroleum extracts. Selenium is an essential trace element in humans, and is present in proteins in the form of selenocysteine and selenomethionine to give diselenide bonds, similar to the disulfide bonds. These selenoproteins are antioxidising agents and act by eliminating peroxides from the organism. They are also involved in cancer prevention and inflammation protection. Sulfur and selenium have many common characteristics. Actually, in living organisms, selenium usually accompanies or substitutes sulfur thanks to its comparable physicochemical properties. Numerous experimental and theoretical studies on the dissociation of DMDS and a few on that of dimethyl diselenide have been performed. Because of the ease with which disulfides are oxidized to cations, an understanding of the energetics surrounding simple disulfide ions (e.g., CH_3SSCH_3~+) will lead to an improved recognition of those factors influencing and ultimately controlling conformational mobility and reactivity of macromolecules containing disulfide bonds. Previous studies on dimethyl diselenides are mostly theoretical, and one can safely say that almost no reliable experiments on the energetics of diselenides has been performed. In this study we aim to investigate the accurate thermochemistry, fragmentation pathways and dissociation rates of energy-selected dimethyl disulfides and dimethyl diselenides.