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  • Title: Cyclic beta-tetra- and pentapeptides: synthesis through on-resin cyclization and conformational studies by X-ray, NMR and CD spectroscopy and theoretical calculations.
    Author: Büttner F, Norgren AS, Zhang S, Prabpai S, Kongsaeree P, Arvidsson PI.
    Journal: Chemistry; 2005 Oct 21; 11(21):6145-58. PubMed ID: 16052654.
    Abstract:
    The solution-phase synthesis of the simplest cyclic beta-tetrapeptide, cyclo(beta-Ala)4 (4), as well as the solid-phase syntheses through side chain anchoring and on-resin cyclization of the cyclic beta3-tetrapeptide cyclo(-beta3hPhe-beta3hLeu-beta3hLys-beta3hGln-) (14) and the first cyclic beta3-pentapeptide cyclo(-beta3hVal-beta3hPhe-beta3hLeu-beta3hLys-beta3hLys-) (19) are reported. Extensive computational as well as spectroscopic studies, including X-ray and NMR spectroscopy, were undertaken to determine the preferred conformations of these unnatural oligomers in solution and in the solid state. cyclo(beta-Ala)4 (4) with no chiral side chains is shown to exist as a mixture of rapidly interchanging conformers in solution, whereas inclusion of chiral side chains in the cyclo-beta3-tetrapeptide causes stabilization of one dominating conformer. The cyclic beta3-pentapeptide on the other hand shows larger conformational freedom. The X-ray structure of achiral cyclo(beta-Ala)4 (4) displays a Ci-symmetrical 16-membered ring with adjacent C=O and N-H atoms pointing pair wise up and down with respect to the ring plane. CD spectroscopic examinations of all cyclic beta-peptides were undertaken and revealed results valuable as starting point for further structural investigations of these entities.
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