Design, synthesis and self-assembly properties of $\alpha$-peptide building block containing aib residues

Abstract

Hierarchical self-assembly has shown great potential in the preparation of high dimensional nano/micro- biomimic materials. Numerous design strategies for building blocks have been employed to create well-defined structures. The incorporation of functionalities such as amphiphilic or aromatic groups is a well-known design principle for short $\alpha$ -peptide building blocks but most morphologies still remain in low dimensional. Therefore, the achievement of high dimensional morphologies from unmodified and flexible $\alpha$ -peptidic building blocks remains an outstanding challenge. It has been demonstrated that discrete and homogeneous two or three dimensional structures could be derived from $\beta$ -peptidic scaffolds, due mainly to the restricted conformational space of $\beta$ -peptides. Thus the same design principle should be applicable to $\alpha$ -peptides. In this thesis, conformationally constrained $\alpha$ -heptamer that contain both helix-promoting Aib ($\alpha$ -amino isobutyric acid) and Leu residues to drive hydrophobic interactions has been synthesized and characterized in the solution and solid states using NMR spectroscopy, single crystal X-ray and powder X-ray diffraction techniques. It was observed that this short $\alpha$ -peptide, which effectively populates helical secondary structures, could self-assemble to form well-defined and homogeneous two dimensional plate structures while an alanine homologue control peptide gave rise only to heterogeneous aggregated spheres. This study demonstrates that new and potentially useful building blocks are available from $\alpha$ -amino acid peptides through comprehensive molecular-level design. It is envisioned that this study will widen the choices of suitable building blocks for the design of new and useful self-assembled materials.