Optical rotatory properties of L‐cystine conformational isomers. Theoretical analysis

The near ultraviolet chiroptical properties of L ‐cystine conformational isomers are examined on a static, “one‐electron” model in which the disulfide moiety is the chromophoric group and the atoms of the L ‐alanyl fragments are treated as perturbers. The zeroth order chromophoric wave functions are calculated on a semiempirical molecular orbital model in which excited states are constructed in the virtual orbital‐configuration interaction approximation. The perturbing environment is represented by point charges located at the atomic centers of the L ‐alanyl fragments. Chromophore–perturber interactions are expressed as charge–multipole moments with only the charge–dipole and charged–quadrupole terms being retained in the calculations. Vicinal contributions to the rotatory strengths of the five lowest energy disulfide transitions are computed for 30 conformational isomers of the L ‐cystine dizwitterion. The results provide support for the view that vicinal or peripheral effects can account entirely for the observed near ultraviolet (λ > 230 nm) chiroptical properties of L ‐cystine and its derivatives and that these properties are diagnostic of conformational features external to the disulfide moiety.