An efficient method for site-selective boronation of peptides and proteins applied in magnified bacterial imaging

Borylated compounds have received tremendous attention in chemical biology and drug design. Motivated by this notion, we develop an efficient methodology for boron incorporation into peptides and proteins via chemoselective conjugation of cysteine residues under catalyst-, additive-, and metal-free conditions. By harnessing the optimum S_N2 displacement profile of bromomethyl boronate, we establish a methodology that enables the selective incorporation of methylboronic acid into various peptides, including bioactive constructs, as well as proteins with rapid kinetics (>10² M^-1s^-1) and excellent conversions at physiological pH. The method also leverages rapid access to bivalent boron conjugates, which, when applied to clinically used antimicrobial peptide UBI(29-41), augment its binding efficacy (40-fold) via covalent capture of the diols on bacterial surface glycans, mainly wall teichoic acid (and thus lipoteichoic acid). Notably, bis-boronated UBI exhibits selective staining of S. aureus over gram-negative bacteria and mammalian cells, alongside a 14-fold increase in serum half-life compared to native UBI.