Computational study on structures, thermochemical properties, and bond energies of disulfide oxygen (S-S-O)-bridged CH3SSOH and CH3SS(=O)H and radicals

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 (?fH degrees, S degrees, and Cp(T)) of the SS bridge molecules CH3SSOH and CH3SS(=O)H and the radicals CH3SS=O and CH2SSOH that correspond to H-atom loss are determined by computational chemistry. Structure and thermochemical parameters (S degrees and Cp(T)) are determined using density functional Becke, three-parameter, LeeYangParr (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 CBSQB3 levels with work reactions that are close to isodesmic in most cases. The enthalpies of formation for CH3SSOH, CH3SS(=O)H are -38.3 and -16.6kcalmol-1, respectively, where the difference is in enthalpy RSOH versus RS(=O)H bonding. The CH bond energy of CH3SSOH is 99.2kcalmol-1, and the OH bond energy is weaker at 76.9kcalmol-1. Cleavage of the weak OH bond in CH3SSOH results in an electron rearrangement upon loss of the CH3SSOH hydrogen atom; the radical rearranges to form the more stable CH3SS.=O radical structure. Cleavage of the CH bond in CH3SS(=O)H results in an unstable [CH2SS(=O)H]* intermediate, which decomposes exothermically to lower energy CH2=S+HSO. The CH3SS(=O)H bond energy is quite weak at 54.8kcalmol-1 with the HC bond estimated at between 91 and 98kcalmol-1. Disulfide bond energies for CH3SSOH and CH3SS(=O)H are low: 67.1 and 39.2kcalmol-1. Copyright (c) 2011 John Wiley & Sons, Ltd.