Structural and spectral properties of the disulfide linkage in proteins and other molecules

All-valence-electron calculations by the EH and CNDO/2 molecular orbital methods have been found to yield satisfactory predictions on the electronic structure of disulfides (RSSR compounds). Electron density maps help explain the origin of the barriers to internal rotation and show an interesting contrast between the EH and deorthogonalized CNDO/2 wave functions. Variation in the calculated strength of the SS bond as a function of dihedral angle correlates well with measured SS stretching frequencies. The semiempirical wave functions not only explain the conformational dependence of the longest wave-length transition of the disulfide chromophore, but also allow a new assignment of the higher bands to be made.