Computational study on structures, thermochemical properties, and bond energies of disulfide oxygen (S–S–O)‐bridged CH 3 SSOH and CH 3 SS(=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 (Δ f H °, S°, and Cp( T )) of the S–S bridge molecules CH 3 SSOH and CH 3 SS(=O)H and the radicals CH 3 SS•=O and C•H 2 SSOH 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 3 SSOH, CH 3 SS(=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 3 SSOH 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 3 SSOH results in an electron rearrangement upon loss of the CH 3 SSO–H hydrogen atom; the radical rearranges to form the more stable CH 3 SS· = O radical structure. Cleavage of the C–H bond in CH 3 SS(=O)H results in an unstable [CH 2 SS(=O)H]* intermediate, which decomposes exothermically to lower energy CH 2  = S + HSO. The CH 3 SS(=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 3 S–SOH and CH3S–S(=O)H are low: 67.1 and 39.2 kcal mol −1 . Copyright © 2011 John Wiley & Sons, Ltd.