Conformations of model compounds of proteins. I. Acetylglycine N‐methylamide

Infrared spectra in the region 4000–60 cm −1 have been measured for acetylglycine N ‐methylamide and its deuterium homologs, CD 3 CONHCH 2 CONHCH 3 , CH 3 ‐CONHCD 2 CONHCH 3 , CH 2 CONHCH 2 CONHCD 3 , and CH 3 CONDCH 2 CONDCH 3 . Normal frequencies have also been calculated for these molecules with various conformations. The spectra show that this compound has two crystalline modifications, form A and form B. The frequencies and their isotope shifts show that the molecular conformation (Ψ, ϕ) of form B is near (0, 120) and that of form A near (180, 120). The short‐range factors determining the conformation of peptide backbone having glycine residues are discussed.