STEREOCHEMICAL STUDIES OF CYCLIC PEPTIDES .4. ENERGY CALCULATIONS OF CYCLIC DISULPHIDE CYSTEINYLCYSTEINE

On the basis of stereochemical criteria, allowed conformations of cyclic disulphide cysteinylcysteine were computed. The structure was possible only with a nonplanar cis-peptide unit. The energy of each allowed conformation was evaluated by using potential function for nonbonded interactions, electrostatic interactions, dihedral angle distortions and bond angle variations. The minimum energy conformation had a value of Δω = -12° for the peptide unit and an energy equal to -3.55 kcal/mole. A similar minimum energy conformation occurred at Δω = +14° and had an energy value of -2.92 kcal/mole. In view of the lower energy and smaller nonplanar nature of the peptide unit, the conformation with negative Δω is expected to be more stable. The study has revealed the importance of nonplanar distortion of the peptide unit and the need for distortion of covalent bond angles in cyclic peptides. © 1969.