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

Abstract

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 (ΔfH°, S°, and Cp(T)) of the S–S bridge molecules CH3SSOH and CH3SS(=O)H and the radicals CH3SS•=O and C•H2SSOH 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 CH3SSOH, CH3SS(=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 CH3SSOH 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 CH3SSOH results in an electron rearrangement upon loss of the CH3SSO–H hydrogen atom; the radical rearranges to form the more stable CH3SS· = O radical structure. Cleavage of the C–H 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 54.8 kcal mol−1 with the H–C bond estimated at between 91 and 98 kcal mol−1. Disulfide bond energies for CH3S–SOH and CH3S–S(=O)H are low: 67.1 and 39.2 kcal mol−1. Copyright © 2011 John Wiley & Sons, Ltd.