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  • Title: Mechanism of thiolate-disulfide interchange reactions in biochemistry.
    Author: Bach RD, Dmitrenko O, Thorpe C.
    Journal: J Org Chem; 2008 Jan 04; 73(1):12-21. PubMed ID: 18052192.
    Abstract:
    Both density functional theory (DFT) (B3LYP) and CCSD ab initio calculations were employed in a theoretical investigation of the mechanism of thiolate-disulfide exchange reactions. The reaction pathway for degenerate thiolate-disulfide exchange reactions with dimethyl disulfide has been shown to proceed through a SN2-like transition structure that is very close in energy to the corresponding trisulfur anionic intermediate ([delta-S-S-Sdelta(-)]). When relatively small substituents are involved, the level of theory must be increased to CCSD to make this rather subtle mechanistic distinction. With the more sterically hindered exchange reaction involving t-butyl mercaptide and di-t-butyl disulfide, the potential energy surface exhibits a distinct preference for the S(N)2 displacement pathway with an activation barrier of 9.8 kcal/mol. When corrections for solvent polarity are included (COSMO), an S(N)2 mechanism is also implicated in both polar and nonpolar solvents. DFT studies on thiolate-disulfide exchange, when the substituent is a model peptide, strongly support the intermediacy of a trisulfur intermediate that lies 10.7 kcal/mol below isolated reactants. A well depth of this magnitude should provide a sufficient lifetime of the intermediate to accommodate the requisite conformational adjustments that accompanies formation of the new disulfide bond.
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