β- and γ-Peptides: Structural Investigations on Heterotypic Coiled-Coil Assemblies of an α/β/γ-Chimera with all-α-Peptides

Abstract

Predicting the behavior of foldamer assemblies with all-α-peptides in higher-ordered arrangements remains a challenge. However, there is a strong interest and need to develop a deeper and more thorough knowledge to access foldamer designs that undergo interactions with natural peptide motifs. In this dissertation, the endeavors of investigating higher-ordered interactions between foldamers and all-α-peptides were continued in a well-defined tetrameric coiled-coil model system. First, two Cys/Phe-motifs identified in previous work as complementary binding partners of an α/β/γ-chimera were compared using CD-spectroscopy and cryogenic transmission electron microscopy (cryo-TEM). Systematic substitutions were performed on cysteine and analyzed by CD-spectroscopy to determine differences between the two different Cys/Phe-motif-containing complementary sequences in the formation of helical bundles with the α/β/γ-chimeric sequence. High-resolution structural data were obtained using a variety of analytical methods to investigate the favored side chain packing in the higher-ordered helical fold. The analytical techniques applied are NMR spectroscopy, vibrational sum-frequency generation (SFG) spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, and X-ray crystallography. In the course of this thesis, the thiol region was examined for the first time by vibrational SFG-spectroscopy at the air/water interface for shifts due to H-bonding. In addition, the first crystal structure of a heterotypic coiled-coil assembly of an α/β/γ-chimera with an all-α-peptide was generated, providing the first high-resolution packing information of the heterotypic helical bundle in a solid state. Another part of this work deals with the construction of a library of chimeric peptides with iterative substitutions that increase the number of β/γ-modules. These chimeras were then evaluated for stability when complemented with corresponding all- α-peptide sequences involving the Cys/Phe-motifs. This investigation revealed a restriction of intermolecular interactions sensitive to a variation in the number of backbone residues. Finally, a β/γ-peptide consisting of five β/γ-modules was analyzed for structural features by CD-spectroscopy.