Solid‐state geometry and conformation of linear, diastereoisomeric oligo-prolines

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.