Synthesis and Self Assembling Properties of Peptide-Based Hydrogels

Abstract

An octapeptide (HEFISTAH) derived from the antiparallel β-interface of the diaminopimelate decarboxylase protein homotetramer has been reported to form self-assembly into fibers but no gelation. The effect of addition of different functional groups has been investigated to understand the effect on self-assembling properties. In chapter two, the effect of installing different lipid chains (C6, C8, C10, and C16) on the N-terminus of peptide HEFISTAH has been investigated. Notably, the C8 peptide conjugate assembled into wide twisted nanoribbons and formed hydrogels, which to the best of our knowledge constitutes the first example of a peptide containing an eight carbon alkyl chain that demonstrates these properties, a space typically occupied by peptide amphiphiles with long lipid chains. Furthermore, this self-assembling lipopeptide exhibited pH and temperature stability with shear thinning properties suitable for biomedical applications. Importantly, in this work the application of the polystyrene-based sorbent Diaion™ HP20SS for the simple large-scale purification of self-assembling peptides is presented as an alternative to the use of time-consuming and labor-intensive reverse-phase high-performance liquid chromatography. In chapter three, the CLipPA (Cysteine Lipidation on a Peptide or Amino Acid) technology has been utilised as an efficient synthetic tool for the syntheses of self-assembling amphiphilic lipopeptides. CLipPA technology allowed for site-specific lipidation of an unprotected β-sheet forming octapeptide under mild conditions, using a “click” radical-initiated thiol-ene reaction. Notably, the peptide S-acylated with hexanoic acid (C6) self-assembled into a twisted nanofiber morphology and formed a mechanically strong hydrogel at 2 wt %. The self-assembling properties of the C6-S-acylated peptide was studied in detail. In chapter four, increased water solubility and long-range intermolecular ordering properties were introduced into the fluorescent organic molecule thiophene-diketopyrrolopyrrole (TDPP) via its conjugation to the octapeptide HEFISTAH. The octapeptide, and its TDPP mono- and cross-linked conjugates were synthesised using 9-fluorenylmethoxycarbonyl (Fmoc) based solid-phase peptide synthesis (SPPS). Unlike the unmodified peptide, the resulting mono-linked and cross-linked peptides showed a fibrous morphology and formed hydrogels at 4 wt% in water at neutral pH, but failed to assemble at pH 2 and pH 9. Further peptide characterization showed that the TDPP organic core enhances peptide self-assembly and that both peptides assembled into fibers with a parallel β-sheet structure. Furthermore, UV-vis spectroscopic analysis suggested that the TDPP molecules form H-type aggregates where the chromophores are likely to be co-facially packed, but rotationally and/or laterally offset from one another. The TDPP–TDPP intermolecular molecular coupling indicates that π–π stacking interactions are highly likely – a favourable sign for charge transport. The enhanced aqueous solubility and self-assembling properties of the TDPP–peptide conjugates allowed the successful preparation of thin films. Atomic force microscopy, X-ray diffraction and UV-vis spectroscopic analysis of these thin films revealed that the hybrid materials retained a fibrous morphology, β-sheet structures and strong intermolecular coupling between neighbouring TDPP molecules. These results open an exciting avenue for bio-organic materials development, through structural and electronic tuning of the TDPP core.