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Cyclo(D‐Pro‐L‐Pro‐D‐Pro‐L‐Pro): Structural properties and cis/trans isomerization of the cyclotetrapeptide backbone

Combinations of L‐ and D‐proline residues are useful compounds for finding new structures and properties of cyclic peptides. This is demonstrated with one striking example, the cyclic tetrapeptide c(D‐Pro‐L‐Pro‐D‐Pro‐L‐Pro). For this molecule composed of strictly alternating D‐ and L‐configurated residues, a highly symmetrical structure is expected, which should be an optically inactive meso‐form. Cyclization of the enantiomeric pure linear precursor D‐Pro‐L‐Pro‐D‐Pro‐L‐Pro, however, yields a racemic mixture of two enantiomeric cyclotetrapeptides, both with twofold symmetry and a cis–trans–cis–trans sequence of the peptide bonds. Remarkably, this formation of a racemate was not caused by racemization, but by cis/trans isomerization of all peptide bonds in the ring. This process may occur in the linear precursor during the ring formation (cyclization of conformers with trans–cis–trans or cis–trans–cis arrangement of the amide bonds) as well as in the enantiomeric pure cyclic tetrapeptide at higher temperature. In the latter case, an all‐cis structure should exist as the intermediate, which can form a cis–trans–cis–trans sequence in two equivalent ways, leading finally to two enantiomeric cyclotetrapeptides. In the first one, the cis peptide bonds are attributed to the L‐residues and the trans peptide bonds to the D‐residues; in the second one, the cis bonds belong to the D and the trans bonds to the L‐residues. The mixture of these two enantiomers does not crystallize in the racemic form, but in enantiomeric pure separate crystals. The structural properties could be proved by 1H‐ and 13C‐nmr spectroscopy and x‐ray analysis. The cis/trans isomerization process was confirmed by optical rotation measurements and CD spectroscopy, as well as DREIDING model studies. Calorimetric measurements in the solid state suggest the existence of the expected all‐cis intermediate. The backbone conformation of the 12‐membered medium‐sized ring shows only slight deviations—up to 6° —from the planarity of the peptide bonds. On the other hand, the four pyrrolidine rings show different types of puckering of the Cγ or the Cβ atoms.