title: Self-Assembly and Bioactivity of Peptides for Therapeutic Applications
Creators: name: Edwards-Gayle, Charlotte
Publisher: name: University of Reading
Year: 2020

description: Peptides are an attractive platform to make new and novel constructs for therapeutic applications due to their biocompatibility, biodegradability and biofunctionality. Peptides that self-assemble may have enhanced in vivo stability and may be able to self-deliver. Furthermore, peptides can be modified to enhance their in vivo stability through lipidation, PEGylation and other modifications. This thesis aims to explore the self-assembly and bioactivity of small oligopeptides, telechelic tyrosine functionalised PEGylated peptides and peptide hormones with potential therapeutic applications. With the rise in cases of antimicrobial resistance, there is a great amount of interest in the development of alternate medicines to combat these issues. Short cationic peptides are beginning to show great potential in this area, due to their ‘multi-hit’ strategy and relatively cheap cost of synthesis. The self-assembly and lipid interactions of a group of surfactant-like peptides (SLPs) and peptide bola-amphiphiles containing arginine and alanine are examined. It is apparent that the size of the hydrophobic block causes increased aggregation propensity and increased structural ordering. The cytocompatibility of the SLPs and bola-amphiphiles is measured, revealing that cytotoxicity is not linked to molecular weight. The SLPs are more cytocompatibile than the bola-amphiphiles. These peptides show activity at cytocompatibile concentrations against P.aeruginosa, a bacterial species on the World Health Organisations list of species that require new treatments. Further to this, the self-assembly and bioactivity of two short symmetrical AMPs, consisting of arginine and phenylalanine was examined. These peptides were found to have weak selfassembly behaviour, but strong interactions with many different Pseudomonas bacteria, with particularly strong interactions with P.aeruginosa. The peptides alter the structure of liposomes based on the composition of the P.aeruginosa membrane, with the most active peptide, R4F4, completely disrupting vesicle formation. The peptide R4F4 disrupts biofilm formation and interacts with cyclic diguanylate (c-di-GMP), a regulator of biofilm formation and dispersion in the species. Materials functionalised with tyrosine may be of interest for enzyme responsive materials. The self-assembly of tyrosine functionalised telechelic PEO-star conjugates at native pH and pH 12 was examined These conjugates were found to self-assemble into a mixture of spherical globules and fibres. The self-assembly of the lower molecular weight conjugate was disrupted by pH adjustment. The larger conjugate formed mixed long straight fibres and smaller globules when adjusted to pH 12. These conjugates did not form hydrogels and were proved to be cytocompatible at sufficiently high concentration. Self-assembled hormones may have prolonged half-lives and increased stability in vivo. Peptide hormone oxytocin has found to be heat unstable, affecting its performance in developing nations. Probing conditions for selfassembly may improve the stability. Thus, the assembly of the peptide hormone oxytocin and analogues was examined. The self-assembly of oxytocin was inconclusive when examined by biophysical techniques. Analogue carbetocin was found to not aggregate at native pH. Oxytocin was found to form a self-standing β-sheet gel at 2 wt% and pH 12, whereas carbetocin appeared slightly crystalline under these conditions. A lipidated variant of oxytocin was found to be insoluble and to assemble into irregular aggregates in ethanol. Overall, this thesis examines through biophysical techniques and bioassays, different peptides with a range of potential therapeutic applications, contributing to the understanding of short antimicrobial peptides.