Harnessing the Power of Boronic Acids

Abstract

The imminent threat of antibiotic resistant pathogens that have emerged in clinical settings over the past several decades demands novel solutions in the form of both species- and/or strain-specific diagnostic technologies and treatments. Such new developments would aid in the improved management of bacterial infections by accurate diagnosis and targeted bacterial killing, which would mitigate the continued spread of antibiotic resistance as a result of broad-spectrum antibiotic application The cell surface of bacteria presents a unique opportunity towards development of these modalities, as bacterial cell walls possess both universal and unique features that can be targeted by chemical functionalities without the requirement of cell penetration. This work has sought to take advantage of naturally existing and non-natively installed bacterial cell wall chemical functionalities for which we can develop novel chemoselective chemistries and unique peptides that incorporate those chemical functionalities to enable targeted, biocompatible methods of bacterial labeling and targeting. We initially began these endeavors with the goal of improving upon existing readily reversible iminoboronate chemistry with acetylphenyl boronic acid (APBA), which selectively labels bacteria that contain amine-presenting cell wall lipids (e.g. PE and Lys-PG). In our efforts to improve upon the binding potency of this chemical motif, we synthesized a panel of APBA analogues with varying substituents to modulate amine-binding affinity. We additionally characterized these analogues capacity to form thiazolidinoboronates with free and N-terminal cysteine. Furthermore, we applied an APBA dimer presenting phage library towards identification of potent and selective APBA-presenting peptide binders of 1) a cationic antimicrobial peptide (CAMP) implicated in cancer, human beta defensin 3 (hBD3), and 2) colistin-resistant strains of bacteria that attain their resistance through a variety of different mechanisms. This high-throughput technology afforded identification of peptides that are indeed protein or species/strain selective binders, thus enabling targeted labeling of these important biomolecules. In our continued efforts to identify highly potent and selective bacterial targeting chemistries, we also developed an irreversible chemistry that enables the incorporation of chemical motifs, APBA and semicarbazide, into the cell walls of bacteria through cell wall remodeling mechanisms, which then undergo rapid conjugation with fluorescent and turn-on fluorescent reactive partners. While this alternative approach to bacterial detection requires a primary cell-wall incorporation step, the incorporation and subsequent labeling of these chemical motifs are both highly efficient, which enhances the potency of this bacterial labeling approach The chemical approaches to targeted bacterial labeling herein highlight our ability to develop several species- and strain-selective bioorthogonal chemical probes towards the goal of discovering targeted bacteria binding modalities. Beyond identification of such targeted bacterial binding molecules, we hope to translate these findings into effective, narrow-spectrum antibiotics, which is an endeavor currently being pursued in our laboratory.