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An Assessment of Computational Methods for Calculating Accurate Structures and Energies of Bio-Relevant Polysulfur/Selenium-Containing Compounds

Department of Chemistry and Biochemistry, University of Windsor, Windsor, ON N9B 3P4, Canada
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Author to whom correspondence should be addressed.
These authors contributed equally.
Molecules 2018, 23(12), 3323; https://doi.org/10.3390/molecules23123323
Received: 27 November 2018 / Revised: 12 December 2018 / Accepted: 12 December 2018 / Published: 14 December 2018
(This article belongs to the Section Computational and Theoretical Chemistry)
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. View Full-Text
Keywords: DFT; cysteine polysulfide; reactive sulfur species; gas phase basicity; proton affinity; bond dissociation enthalpy; hydrogen affinity; thermochemistry DFT; cysteine polysulfide; reactive sulfur species; gas phase basicity; proton affinity; bond dissociation enthalpy; hydrogen affinity; thermochemistry
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Nikoo, S.; Meister, P.J.; Hayward, J.J.; Gauld, J.W. An Assessment of Computational Methods for Calculating Accurate Structures and Energies of Bio-Relevant Polysulfur/Selenium-Containing Compounds. Molecules 2018, 23, 3323.

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