Conformational mimicry II. An obligatory cis amide bond in a small linear pep tide

The structure of Z‐Proψ[CN₄]‐Ala‐OBzl has been determined by X‐ray crystallographic techniques. The structure crystallizes in space group P2₁ with cell constants a = 22.176(3) Å, b = 6.141(1)Å, c = 8.275(1) Å, β= 98.31(1), and Z = 2. The structure has been refined to a residual of 0.038 for 2538 independent data. The amide bond between the prolyl and alanyl residues is cis, a result of the presence of the tetrazole ring system, as is the urethane bond linking the benzyloxycarbonyl and the prolyl groups. A comparison of the structures in this study to other structures containing cis amide bonds shows that the tetrazole ring system, when incorporated into peptides, mimics a cis amide bond. Changes in the distance between the α‐carbons adjacent to the tetrazole rings in the linear peptide as compared with the bicyclic diketopiperazine required a reassessment of the conformational mimicry with the cis amide bond.