The nonplanar peptide unit. IV. Geometry and nonplanar distortions of the cis‐peptide unit

Semiempirical quantum chemical calculations were carried out using the CNDO/2 and the INDO methods on the model compound cis‐N‐methylacetamide in order to get an insight into the problem of possible nonplanar distortions of the cis‐peptide unit. In addition, the crystal structure data of cyclic peptides containing cis‐peptide units were analyzed. These studies have indicated that the dihedral angles θN and Δω are correlated approximately by the relation Δω = −θN, whereas θC is small and is uncorrelated with Δω, as was found in the case of the trans‐peptide unit. Both theory and crystal data suggest that out‐of‐plane distortions at the nitrogen atom of the peptide unit were quite likely to occur and should be included in the conformational calculations. The average geometry for the planar cis‐peptide unit has also been obtained from the observed examples.