Boron’s Double Edge—Antibiotics, Toxins, and the Fine Line Between Them

Boron is a chemically distinctive bioelement whose electron-deficient structure enables reversible coordination with oxygen-rich functional groups such as diols and hydroxyls. This property allows boron to modulate molecular stability, conformation, and biological reactivity, giving rise to both beneficial pharmacological effects and toxicological outcomes. This review examines the dual biological role of boron through the framework of bioactive boron-containing natural products and natural compounds capable of forming reversible boron complexes. Particular attention is given to naturally occurring boron-containing antibiotics, including the polyketide macrodiolides boromycin, aplasmomycin, tartrolons, and hyaboron, where boron plays a direct structural and functional role in antimicrobial activity. These compounds demonstrate how boron coordination can influence ion transport, membrane interactions, and molecular assembly, contributing to potent antibacterial properties. Beyond intrinsically boron-containing metabolites, many natural antibiotics and toxins possess oxygen-rich architectures capable of forming transient borate complexes through vicinal 1,2-diol motifs. Examples include polyene macrolide antibiotics such as amphotericin B, fungichromin, and nystatin, as well as tetracyclines, rifamycins, and macrolides such as sorangicin A, where boron coordination may affect solubility, aggregation, ionophoric behavior, and biological selectivity. Similar chemistry is observed in marine neurotoxins and polyether toxins—including tetrodotoxin, saxitoxin derivatives, azaspiracids, pectenotoxins, ciguatoxins, and gambierones—whose hydroxyl-rich frameworks enable reversible interactions with boron species present in seawater. Such complexation may enhance aqueous stability and contribute to trophic transfer and bioaccumulation within marine ecosystems. By framing boron as a molecular “double edge,” this review integrates chemical, biological, and environmental perspectives to highlight how boron coordination can simultaneously enhance antimicrobial activity while influencing toxicity and ecological persistence. Recognizing the role of boron in shaping the activity of natural products provides new insight into antibiotic function, toxin behavior, and the broader impact of boron chemistry in biological systems.