Chemical Investigation of Lipopeptides and Oxaboroles Toward the Development of Crop and Medical Antifungals

Abstract

Antimicrobial-resistant infections and environmentally damaging crop pathogens both represent pressing issues of the current age. Natural products have served as effective drugs and structural design inspiration for over 70% of approved medications and still hold immense therapeutic potential. It is necessary to investigate new biological systems to discover novel structural scaffolds with promising commercial applications. Moon snails (family Naticidae) lay their egg masses in the open ocean with no physical parental protection. It is hypothesized that the microbiota on the eggs provide chemical protection by producing natural products with diverse biological activities. The moon snail egg masses represent a promising, underexplored symbiotic system for natural product discovery. Herein, we investigated the natural product potential of microbiota isolated from egg masses of Neverita spp. using mass spectrometry-based discovery tools. This resulted in the discovery and complete characterization of the bokeelamides (Chapter 2), a family of lipopeptide siderophores with strong selectivity for FeIII over other metals tested and a pFe (iron affinity) of 25.8 ± 0.3. The bokeelamides exhibited no antimicrobial, antibiofilm, or hemolytic activity at concentrations up to 50 µM, but were found to be weakly cytotoxic. Next, we reported the first NMR characterization and absolute stereoconfiguration assignment of the kurstakins, known antifungal lipopeptides previously shown to play a role in the efficacy of crop biocontrol agents (Chapter 3). As part of this effort, we uncovered over 50 analogs through MS/MS and GCMS lipid analysis and >3,000 NCBI strains with biosynthetic potential for kurstakin production, predominantly from the Bacillus cereus group. Finally, a medicinal chemistry approach to drug discovery investigated the antimicrobial properties of the oxaboroles, structural analogs of the approved antifungal Tavaborole (Chapter 4). The oxaboroles were generally inactive against bacteria except for Escherichia coli, while halogen-substituted rings showed the greatest potency against both fungal and bacterial pathogens. The structure-activity relationship (SAR) against fungal pathogens revealed the importance of meta-substituted halogens on the benzyl ring, with analogs having minimum inhibitory concentrations (MICs) as low as 22 µM against Penicillium chrysogenum, while bulky non-halogen substituents abolished activity altogether. Interestingly, the SAR against Escherichia coli showed a preference rather for para-substituted halogenated benzyl rings. Further studies should develop this bacterial-target SAR and explore boronic acid compounds with increased sp3 character for future microbial targets. Overall, the work described expands our knowledge of lipopeptide natural products and synthetic oxaboroles, revealing both high iron-affinity siderophores and antifungal compounds with potential for agricultural and medicinal applications.