Probing the effects of backbone ester substitution on self-assembly and biological activity of short depsipeptides
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Hydrogel materials composed of self-assembled amphiphilic peptides show great promise for use as injectable, highly biocompatible biomaterials for tissue regeneration applications. However, peptides do not easily degrade naturally without the presence of proteolytic enzymes, which recognize specific peptide sequences and are specific to certain cell and tissue types. In this dissertation, we evaluate the self-assembly and bioactivity of backbone ester-containing depsipeptides that are degradable by alkaline or acid hydrolysis as the basis for hydrogel materials, in order to circumvent any inflammation and immunogenicity caused by peptide materials that persist in the body. The self-assembly of depsipeptides has not been widely explored, thus we first studied the self-assembly of a simple N-protected dipeptide and its depsipeptide analogue both experimentally and computationally to evaluate the relative importance of hydrogen-bonding interactions mediated by the single amide bond in driving and stabilizing self-assembly. We determined that amide-amide hydrogen bonding interactions are not strictly necessary for self-assembly. We next hypothesized that amide-mediated hydrogen bonding may not be necessary for mediating peptide-protein interactions. To test this hypothesis in a simple, well-characterized system, we synthesized a depsipeptide analogue of a peptide containing the Arg-Asp-Gly (RGD) sequence, which is found in extracellular matrix proteins and known to promote cell adhesion through binding of cell surface integrin proteins. As before, the RGD analogue was capable of self-assembly leading to hydrogel formation. However, we found that the depsipeptide did not possess an affinity for the protein high enough to influence cell behavior in the same manner as the peptide. These results suggest that backbone amide hydrogen bonding is crucial in mediating RGD-integrin interaction affinity. Based on these results and other studies in the literature suggesting that amide-to-ester mutations have a complex and context-dependent effect on peptide-protein interactions, further development of depsipeptide-based materials should focus on exploring alternate N-protecting groups that are likely to have higher biocompatibility while driving robust self-assembly, exploring in more depth the ability to tune degradation rates and mechanical properties using alternate side chain chemistries, and exploiting depsipeptide self-assembly and degradability for non-viral gene delivery.