title: Impact of the amino acid sequence on the conformation of side chain lactam-bridged octapeptides.
abstract: Synthetic helical peptides are valuable scaffolds for the development of modulators of protein-protein interactions involving helical motifs. Backbone-to-side chain or side chain-to-side chain constraints have been and still are intensively exploited to stabilize short α-helices. Very often, these constraints have been combined with backbone modifications induced by Cα-tetrasubstituted, β-, or γ-amino acids, which facilitate the α-peptide or α/β/γ-peptide adopting an α-helical conformation. In this work, we investigated the helical character of octapeptides that were cyclized by a Lys-Asp-(i,i + 4)-lactam bridge. We started with two sequences extracted from the helix-loop-helix region of the Id proteins, which are inhibitors of cell differentiation during development and in cancer. Nineteen analogs containing the lactam bridge at different positions and displaying different amino acid core triads (i + 1,2,3) as well as outer residues were prepared by solid-phase methodology. Their conformation in water and water/2,2,2-trifluoroethanol mixtures was investigated by circular dichroism (CD) spectroscopy. The cyclopeptides could be grouped in helix-prone and non-helix-prone structures. Both the amino acid core triad (i + 1,2,3) and the pendant residues positively or negatively affected the formation of a helical structure. Computational studies based on the NMR-derived helical structure of a cyclopeptide containing Aib at position (i + 2) of the triad were generally in agreement with the secondary structure propensity of the cyclopeptides observed by CD spectroscopy. In conclusion, the Lys-Asp-(i,i + 4)-lactam bridge may succeed or fail in the stabilization of short helices, depending on the primary structure. Moreover, computational methods may be valuable tools to discriminate helix-prone from non-helix-prone peptide-based macrolactams. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.
authors.affiliation: Department of Molecular Biology, University of Salzburg, Billrothstrasse 11, 5020, Salzburg, Austria.|Faculty of Chemistry and Biochemistry, Ruhr University Bochum, Universitätsstrasse 150, 44801, Bochum, Germany.
authors.forename: Saskia
authors.initials: S
authors.lastname: Neukirchen
authors.affiliation: Faculty of Chemistry and Biochemistry, Ruhr University Bochum, Universitätsstrasse 150, 44801, Bochum, Germany.|Institute of Pharmaceutical and Medicinal Chemistry, Heinrich Heine University Düsseldorf, Universitätsstrasse 1, 40225, Düsseldorf, Germany.
authors.forename: Viktoria
authors.initials: V
authors.lastname: Krieger
authors.affiliation: Department of Molecular Biology, University of Salzburg, Billrothstrasse 11, 5020, Salzburg, Austria.
authors.forename: Cornelia
authors.initials: C
authors.lastname: Roschger
authors.affiliation: Department of Molecular Biology, University of Salzburg, Billrothstrasse 11, 5020, Salzburg, Austria.
authors.forename: Mario
authors.initials: M
authors.lastname: Schubert
authors.affiliation: Department of Molecular Biology, University of Salzburg, Billrothstrasse 11, 5020, Salzburg, Austria.
authors.forename: Brigitta
authors.initials: B
authors.lastname: Elsässer
authors.affiliation: Department of Molecular Biology, University of Salzburg, Billrothstrasse 11, 5020, Salzburg, Austria.
authors.forename: Chiara
authors.identifier: http://orcid.org/0000-0002-7550-6896
authors.initials: C
authors.lastname: Cabrele
mesh_terms: D000595:Amino Acid Sequence; D002942:Circular Dichroism; D007769:Lactams / Q000737:chemistry*; D010455:Peptides / Q000737:chemistry*; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D060327:Solid-Phase Synthesis Techniques; D054457:Tissue Scaffolds / Q000737:chemistry*; D014270:Trifluoroethanol / Q000737:chemistry
chemical_list: D007769:Lactams; D010455:Peptides; D014270:Trifluoroethanol
keywords: NMR spectroscopy; circular dichroism; computational methods; cyclopeptides; lactam bridge; modeling; molecular dynamics; side chain cyclization