Computational study on structures, thermochemical properties, and bond energies of disulfide oxygen (S-S-O)-bridged CH _3SSOH and CH _3SS(=O)H and radicals

摘要

Cleavage of disulfide bonds is a common method used in linking peptides to proteins in biochemical reactions. The structures, internal rotor potentials, bond energies, and thermochemical properties (δ _fH°, S°, and Cp(T)) of the S-S bridge molecules CH _3SSOH and CH _3SS(=O)H and the radicals CH _3SS =O and C H _2SSOH that correspond to H-atom loss are determined by computational chemistry. Structure and thermochemical parameters (S° and Cp(T)) are determined using density functional Becke, three-parameter, Lee-Yang-Parr (B3LYP)/6-31++G (d, p), B3LYP/6-311++G (3df, 2p). The enthalpies of formation for stable species are calculated using the total energies at B3LYP/6-31++G (d, p), B3LYP/6-311++G (3df, 2p), and the higher level composite CBS-QB3 levels with work reactions that are close to isodesmic in most cases. The enthalpies of formation for CH _3SSOH, CH _3SS(=O)H are -38.3 and -16.6 kcal mol ~(-1), respectively, where the difference is in enthalpy RSO-H versus RS(=O)-H bonding. The C-H bond energy of CH _3SSOH is 99.2 kcal mol ~(-1), and the O-H bond energy is weaker at 76.9 kcal mol ~(-1). Cleavage of the weak O-H bond in CH _3SSOH results in an electron rearrangement upon loss of the CH _3SSO-H hydrogen atom; the radical rearranges to form the more stable CH _3SS· = O radical structure. Cleavage of the C-H bond in CH _3SS(=O)H results in an unstable [CH _2SS(=O)H]* intermediate, which decomposes exothermically to lower energy CH _2 = S + HSO. The CH _3SS(=O)-H bond energy is quite weak at 54.8 kcal mol ~(-1) with the H-C bond estimated at between 91 and 98 kcal mol ~(-1). Disulfide bond energies for CH _3S-SOH and CH3S-S(=O)H are low: 67.1 and 39.2 kcal mol ~(-1).