Abstract
The heavier chalcogens sulfur and selenium are important in organic and inorganic chemistry, and the role of such chalcogens in biological systems has recently gained more attention. Sulfur and, to a lesser extent selenium, are involved in diverse reactions from redox signaling to antioxidant activity and are considered essential nutrients. We investigated the ability of the DFT functionals (B3LYP, B3PW91, ωB97XD, M06-2X, and M08-HX) relative to electron correlation methods MP2 and QCISD to produce reliable and accurate structures as well as thermochemical data for sulfur/selenium-containing systems. Bond lengths, proton affinities (PA), gas phase basicities (GPB), chalcogen–chalcogen bond dissociation enthalpies (BDE), and the hydrogen affinities (HA) of thiyl/selenyl radicals were evaluated for a range of small polysulfur/selenium compounds and cysteine per/polysulfide. The S–S bond length was found to be the most sensitive to basis set choice, while the geometry of selenium-containing compounds was less sensitive to basis set. In mixed chalcogens species of sulfur and selenium, the location of the sulfur atom affects the S–Se bond length as it can hold more negative charge. PA, GPB, BDE, and HA of selenium systems were all lower, indicating more acidity and more stability of radicals. Extending the sulfur chain in cysteine results in a decrease of BDE and HA, but these plateau at a certain point (199 kJ mol−1 and 295 kJ mol−1), and PA and GPB are also decreased relative to the thiol, indicating that the polysulfur species exist as thiolates in a biological system. In general, it was found that ωB97XD/6-311G(2d,p) gave the most reasonable structures and thermochemistry relative to benchmark calculations. However, nuances in performance are observed and discussed.
Highlights
The chalcogens sulfur and selenium have long been known to play key roles in a diverse array of important physiological and biological processes including enzymatic mechanisms [1,2], signaling [3,4,5], and mediation and repair of oxidatively damaged biomolecules [6,7,8]
We began by using the broadest variety of DFT methods (B3LYP, B3PW91, ωB97XD, M06-2X, M08-HX) and range of basis sets (6-31G(d) variety of DFT methods (B3LYP, B3PW91, ωB97XD, M06-2X, M08-HX) and range of basis sets (6to 6-311++G(3df,3pd)) used in this study to obtain optimized structures for CH3 XXH and CH3 XX−
We found that when the sulfur atom is in the middle of the chain, it has a larger Mulliken charge compared to selenium (−0.23 vs. −0.13) in the anionic species
Summary
The chalcogens sulfur and selenium have long been known to play key roles in a diverse array of important physiological and biological processes including enzymatic mechanisms [1,2], signaling [3,4,5], and mediation and repair of oxidatively damaged biomolecules [6,7,8]. In addition to being found within three of the 22 proteinogenic amino acids—cysteine (Cys), methionine (Met), and selenocysteine (Sec)—they are found in many essential metabolites (e.g., thiazole). This is due to their ability to possess a broad range of oxidation states as well as bonding environments, and they can often undergo reversible redox [9]. With Recently, its oxidized disulfide form, GSSG it has increasingly been [10]. Recognized that reactive sulfur species (RSS) comprise a rich. It hasofincreasingly been important recognizedspecies that reactive sulfur species (RSS) hydrogen comprise asulfide rich and and diverse range physiologically (Scheme 1) [11] 22 of of 17 glutathione (GSH), which can mediate the cellular redox environment through interconversion with cells is glutathione (GSH), mediate the cellular redox environment through interconversion its oxidized disulfide form,which GSSGcan [10].
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