The Conformation of <i>cyclo</i>(−d-Pro−Ala<sub>4</sub>−) as a Model for Cyclic Pentapeptides of the dL<sub>4</sub> Type

The conformation of the cyclic pentapeptide <i>cyclo</i>(−d-Pro−Ala<sub>4</sub>−) in solution and in the solid state was reinvestigated using modern NMR techniques. To allow unequivocal characterization of hydrogen bonds, relaxation behavior, and intramolecular distances, differently labeled isotopomers were synthesized. The NMR results, supported by extensive MD simulations, demonstrate unambiguously that the preferred conformation previously described by us, but recently questioned, is indeed correct. The validation of the conformational preferences of this cyclic peptide is important given that this system is a template for several bioactive compounds and for controlled “spatial screening” for the search of bioactive conformations.