Conformational properties of disulphide bridges. 2. Rotational potentials of diethyl disulphide

Abstract

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 minima 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 ΔH298 = 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.