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The nonplanar peptide unit III. Quantum chemical calculations for related compounds and experimental X‐ray diffraction data
Author(s) -
Kolaskar A. S.,
Lakshminarayanan A. V.,
Sarathy K. P.,
Sasisekharan V.
Publication year - 1975
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.1975.360140515
Subject(s) - chemistry , dihedral angle , acetamide , cndo/2 , amide , crystallography , peptide , diffraction , formamide , nitrogen atom , molecule , group (periodic table) , organic chemistry , hydrogen bond , physics , biochemistry , optics
The possible nonplanar distortions of the amide group in formamide, acetamide, N ‐methylacetamide, and N ‐ethylacetamide have been examined using CNDO/2 and INDO methods. The predictions from these methods are compared with the results obtained from X‐ray and neutron diffraction studies on crystals of small open peptides, cyclic peptides, and amides. It is shown that the INDO results are in good agreement with observations, and that the dihedral angles θ N and Δω defining the nonplanarity of the amide unit are correlated approximately by the relation θ N = −2Δω, while θ C is small and uncorrelated with Δω. The present study indicates that the nonplanar distortions at the nitrogen atom of the peptide unit may have to be taken into consideration, in addition to the variation in the dihedral angles (ϕ,ψ), in working out polypeptide and protein structures.