Stable conformers of L-cysteine and L, L-cystine were explored using an automated and efficient conformational searching method. The Gibbs energies of the stable conformers of L-cysteine and L, L-cystine were calculated with G4 and MP2 methods, respectively, at 450, 298.15, and 150 K. By assuming thermodynamic equilibrium and the barrier energies for the conformational isomerization pathways, the estimated ratios of the stable conformers of L-cysteine were compared with those determined by microwave spectroscopy in a previous study. Equilibrium structures of 1: 1 and 2: 1 cystine-Fe complexes were also calculated, and the energy of insertion of Fe into the disulfide bond was obtained. (c) 2017 Elsevier B.V. All rights reserved.