Bismuth–Selenopeptides Combine Potent Bioactivity with Exceptional Kinetic Inertness

Abstract Bismuth peptides and proteins are emerging as versatile tools for medicinal chemistry and chemical biology. Bismuth(III) binds three cysteine residues in peptides and proteins with remarkable selectivity. While the thermodynamic stability of these bismuth complexes is outstanding, their kinetic lability imposes limitations. Introducing bismuth selenopeptides, we demonstrate that selenocysteine binds bismuth with substantially higher kinetic stability than cysteine. This effect was quantified by directly comparing a peptide containing three cysteine residues with an identical peptide containing three selenocysteines. Bismuth selenopeptides are not only inert to strong chelators such as ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) but also to the natural metal‐binding protein transferrin present in human plasma. We further demonstrate biological utility by developing bismuth selenopeptides that selectively bind and inhibit unrelated target proteins. To extend this concept to a more complex system, we investigated the human epidermal growth factor (EGF), a small protein comprising three disulfide bonds. We established that precisely two bismuth atoms bind to the six cysteine or selenocysteine residues in EGF and seleno‐EGF, respectively. In the presence of EDTA, bismuth seleno‐EGF remains fully intact, unlike its cysteine‐based analog, consistent with observations from smaller peptides. Structural models confirm full preservation of the native EGF fold.