Stereochemical criteria for polypeptides and proteins. IV. Standard dimensions for the cis‐peptide unit and conformation of cis‐polypeptides

Using potential energy formulas for variation of bond angles and for ω‐distortion, the conformation of minimum energy for the cis form of the petide unit has been worked out. This agrees very well with the corresponding set of atoms in the crystal structure of Leu‐Pro‐Gly and the dimensions of a standard cis peptide unit are given based on these. The conformational (ϕ,Ψ) map for a cis peptide unit has been worked out from contact criteria, both for a pair of linked units as well as for‐helices having constant (ϕ,Ψ). The small allowed region of the helical map contains the conformation experimentally observed for poly‐ L ‐proline I .