Studies on antimicrobial peptoids: effect of metal binding motif conjugation and polymer grafting Dasom Song

Abstract

Antimicrobial peptoids have been developed as promising alternatives to traditional antimicrobial peptides (AMPs) in response to urgent challenge of antimicrobial resistance (AMR). With enhanced stability and resistance to enzymatic degradation, peptoids offer significant advantages for therapeutic applications. Building on these benefits, numerous studies have focused on improving the activity of antimicrobial peptoids through various modifications and applications. Chapter 1 provides an overview of these advancements and highlights ongoing efforts to enhance their therapeutic potential. This chapter establishes a foundation for subsequent research, including the present study, which aims to further expand the functionality and effectiveness of antimicrobial peptoids in combating bacterial infections. Chapter 2 investigates the incorporation of catalytic metal-binding motifs—the amino terminal Cu(II) and Ni(II) binding (ATCUN) motif—into cationic amphipathic antimicrobial peptoids to enhance their activity. The ATCUN motif, upon binding to Cu(II), facilitates the generation of hydroxyl radicals through a Fenton-like reaction. These hydroxyl radicals, potent reactive oxygen species (ROS), play a crucial role in bacterial cell death. We synthesized a library of ATCUN-peptoid conjugates and evaluated their antimicrobial activity, cytotoxicity, and bacterial killing mechanisms. Additionally, we confirmed that these peptoid derivatives exhibit both antimicrobial and anti-inflammatory effects with low toxicity in a sepsis-induced mouse model, underscoring their potential as therapeutic agents. Chapter 3 focuses on the design and synthesis of antimicrobial peptoids conjugated with ZnDPA (zinc(II)-dipicolylamine) and its derivative, Zn2BPMP (BPMP; 2,6-bis[(bis(2-pyridylmethyl)amino)-methyl]-4- methylphenolate), to enhance selectivity for bacterial membranes. This chapter begins with an overview of bacterial cell membrane composition, emphasizing the anionic nature that distinguishes bacterial membranes from mammalian ones. The incorporation of ZnDPA complexes into peptoids is inteded to exploit this unique feature, enhancing antimicrobial activity while minimizing hemolytic activity against eukaryotic cells. The chapter details the design and synthesis of ZnDPA and Zn2BPMP conjugated antimicrobial peptoids to target the anionic characteristics of bacterial membranes, aiming to enhance selectivity and activity. The synthesized peptoid conjugates were evaluated through various antimicrobial assays against Gram-positive and Gram-negative bacteria, including multidrug-resistant strains. Additional studies were conducted to assess membrane interactions, hemolytic activity, and stability under physiological conditions. The findings highlight the effectiveness of ZnDPA and Zn2BPMP conjugation in improving the antimicrobial potential of peptoids, supporting their development as targeted agents against drug-resistant bacterial infections. Chapter 4 presents the synthesis and evaluation of antimicrobial peptoid-polymer conjugates (poly-PCs) designed to leverage multivalency for enhanced antimicrobial activity. Utilizing PET-RAFT polymerization, a series of antimicrobial poly-PCs with varied polymer chain lengths and peptoid content were synthesized to investigate how these structural variations influence biological activity. The antimicrobial poly-PCs were characterized through gel permeation chromatography (GPC) and NMR, followed by antimicrobial assays against E. coli and S. aureus. Hemolytic assays were also performed to assess toxicity towards mammalian cells. The findings demonstrate that shorter polymer backbones with moderate peptoid content generally show higher antimicrobial activity while balancing biocompatibility. This research underscores the potential of antimicrobial poly-PCs as customizable platforms for developing advanced antimicrobial materials with tunable properties, contributing to the broader field of peptoid conjugates for combating drug-resistant bacteria.