CONFORMATIONAL PROPERTIES OF DISULFIDE BRIDGES .2. ROTATIONAL POTENTIALS OF DIETHYL DISULFIDE

The conformational properties of diethyl disulphide, a model compound for the disulphide bridges in peptides and proteins, were studied with ab initio methods. Stationary point structures were optimized at the HF/6-31G* level with consideration of electron correlation in subsequent single-point MP2 calculations. The six energy minima were also optimized at the MP2/6-31G* level with calculation of zero-point vibrational frequencies and thermal corrections. Additional single-point MP2 energy calculations employed larger basis sets up to 6-311G(2d,p). With positive disulphide chirality, the global energy minimum is a 'spiral' conformation with gauche + C-C-S-S torsion angles. The further stability order for energy minims deviates from previous ab initio results. In particular, the extended trans,trans conformer is subject to a significant relative destabilization on inclusion of electron correlation in the calculations and is only the fifth most stable energy minimum with estimated ab initio DELTAH298 = 5.13 kJ mol-1. The results presented are relevant for the discussion of conformational properties for the structurally equivalent disulphide bridges in polypeptides and calculations of relative energies with molecular mechanics methods.