Engineering Molecular Architectures of Antimicrobial Peptide (AMP)-Polymer Conjugates

Antimicrobial peptides (AMPs) are promising alternatives to conventional antibiotics for treating infections caused by drug-resistant bacteria, yet many peptides are limited by toxicity to mammalian cells and instability in biological environments. To balance the hydrophobic, cationic character for bacterial killing and the neutral, hydrophilic character for biocompatibility, neutral hydrophilic polymers with various architectures have shown great potential as AMP carriers. For example, attaching AMP to the end of a polyethylene glycol (PEG) chain results in linear AMP-PEG conjugates, often with improved biocompatibility of AMPs but markedly reduced antimicrobial activity. As the linear AMP-PEG conjugates may assemble into micelles with PEG shells and AMP cores in solution, we speculate that non-linear PEG conjugates may offer opportunities to present AMP in different ways that better enhance antimicrobial performance, leveraging the benefits of the great biocompatibility of PEG. In this thesis, we investigated how attaching cytokine-derived AMPs to neutral PEG configured in linear, star-shaped, and comb-like architectures affects the presentation of AMPs in solution and therefore the functional performance (i.e., antimicrobial activity, proteolytic stability, and toxicity to mammalian cells). To access conjugates with similar compositions but distinct architectures, we adapted well-established conjugation reactions in some cases, and in others, we used controlled polymerizations. To understand the structure-property-performance relationship, we studied the solution properties (e.g., secondary structure, size, zeta potential, and morphology) of the free peptide and the conjugates at a constant peptide equivalent concentration. In Chapter 2, we prepared linear and star-shaped conjugates with various arm numbers and arm lengths from a helical AMP, stapled P9, and found displaying P9 with a high density on short 8-arm star-shaped conjugates promoted assemblies that retained the antimicrobial activity of the free peptide. In Chapter 3, we synthesized comb-like stapled P9-PEG conjugates with various polymer backbone lengths and side chain lengths and studied their solution behavior. We also explored the synthesis and design rules for the conjugates of an unstructured, cysteine-containing AMP L8 with PEG in Chapter 4. We envision the synthetic strategies, characterization methods, and design rules in this thesis can be helpful to the application of AMPs and be extended to guide the molecular design and synthesis of other therapeutic peptide-polymer conjugates.