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The nonplanar peptide unit. IV. Geometry and nonplanar distortions of the cis ‐peptide unit
Author(s) -
Kolaskar A. S.,
Sarathy K. P.
Publication year - 1980
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.1980.360190710
Subject(s) - cndo/2 , chemistry , dihedral angle , peptide , unit (ring theory) , crystallography , crystal structure , geometry , stereochemistry , molecule , mathematics , hydrogen bond , organic chemistry , biochemistry , mathematics education
Abstract 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 θ N and Δ ω are correlated approximately by the relation Δ ω = −θ N , whereas θ C is small and is uncorrelated with Δ ω , 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.