Phage-Encoded Bismuth Bicycles: Instant Access to Targeted Bioactive Peptides

Genetically encoded libraries play a crucial role in discovering structurally rigid, high-affinity macrocyclic peptide ligands for therapeutic applications. This study represents the first genetic encoding of peptide-bismuth and peptide-arsenic bicyclic peptides in phage display. We introduce bismuth tripotassium dicitrate (gastrodenol) as a water-soluble Bi(III) reagent for phage library modification and in situ bicyclic peptide preparation, eliminating the need for organic co-solvents. Additionally, we explore As(III) as an alternative thiophilic element used analogously to our previously introduced class of peptide-bismuth bicycles. The modification of phage libraries and peptides with these elements is instantaneous and entirely biocompatible, offering an advantage over conventional alkylation-based methods. In a pilot display screening campaign aimed at identifying ligands for the biotin-binding protein streptavidin, we demonstrate the enrichment of bicyclic peptides with dissociation constants two orders of magnitude lower than those of their linear counterparts, underscoring the impact of structural constraint on binding affinity.