Solid‐state geometry and conformation of linear, diastereoisomeric oligoprolines

We have carried out a systematic analysis of the solid‐state conformational preferences of a number of linear homo‐oligoprolines (to the tetramer) by ir absorption and x‐ray diffraction. The peptides present different chiral sequences (tacticities), various types (urethane and amide) of N‐protecting groups, and free and blocked C‐termini (which imply different capabilities of forming H‐bonds). The following conclusions can be drawn: (i) values for the geometry of the prolyl residue and the peptide bond in the cis and in the trans conformations are proposed; (ii) in general the conformational angles φ and ψ in the linear homo‐oligoprolines have values appropriate for the polyproline II structure (conformation F); (iii) the pyrrolidine ring shows various types of puckering with no apparent relation to the backbone conformation; (iv) Pro‐Pro peptide bonds generally take the trans conformation, the few cases of cis conformation being formed by Pro residues of different chirality; (v) the single H‐bond donor — OH, when present, is always bonded to H‐acceptors, which can be either the urethane or the amide or the peptide carbonyl but never the carbonyl group of the — COOH moiety.