Cyclic β‐Tetra‐ and Pentapeptides: Synthesis through On‐Resin Cyclization and Conformational Studies by X‐Ray, NMR and CD Spectroscopy and Theoretical Calculations

The beauty of the wide functionality of proteins and peptides in Nature is determined by their ability to adopt three-dimensional structures. This thesis describes artificial molecules developed to mimic secondary structures similar to those found crucial for biological activities.In the first part of this thesis, we focused on post-translational modifications of a class of unnatural oligomers known as β-peptides. Through the design and synthesis of a glycosylated β3-peptide, the first such hybrid conjugate was reported. In this first report, a rather unstable 314-helical structure was found. Subsequently, a collection of six new glycosylated β3-peptides was synthesized with the aim to optimize the helical stability in water.The ability of natural proteins, i.e. lectins, to recognize the carbohydrate residue on these unnatural peptide backbones was investigated through a biomolecular recognition study.The second part of this thesis concerns the design of conformationally homogeneous scaffolds, which could be of importance for biomedical applications. In paper V, four- and five-membered cyclic all-β3-peptides were investigated for this purpose. In a subsequent paper, a completely different strategy was employed; herein, the ability of a single β2-amino acid to restrict the conformational freedom of a cyclic α-peptide was studied. In the third part of this thesis, we synthesized and investigated the folding propensities of novel backbone modified oligomers, i.e. β-peptoids (N-substituted β-Ala) with α-chiral side chains.The collective results of these studies have established the procedures required for synthesis of glycosylated β-peptides and deepened our understanding of the factors governing folding among such oligomers. Moreover, it was established that β-amino acids can be a useful tool to increase conformational stability of cyclic peptides.

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