Engineering Boron Hot Spots for the Site-Selective Installation of Iminoboronates on Peptide Chains

Boronic acids (BAs) are a promising bioconjugation function to design dynamic materials as they can establish reversible covalent bonds with oxygen/nitrogen nucleophiles that respond to different pH, ROS, carbohydrates and glutathione levels. However, the dynamic nature of these bonds also limits the control over the stability and site-selectivity of the bioconjugation, which ultimately leads to heterogeneous conjugates with poor stability under physiological conditions. Here we disclose a new strategy to install BAs on peptide chains. In this study, a "boron hot spot" based on the 3-hydroxyquinolin-2(1H)-one scaffold was developed and upon installation on a peptide N-terminal cysteine, enables the site-selective formation of iminoboronates with 2-formyl-phenyl boronic acids (K_a of 58128±2 m^-1 ). The reaction is selective in the presence of competing lysine ϵ-amino groups, and the resulting iminoboronates, displayed improved stability in buffers solutions and a cleavable profile in the presence of glutathione. Once developed, the methodology was used to prepare cleavable fluorescent conjugates with a laminin fragment, which enabled the validation of the 67LR receptor as a target to deliver cargo to cancer HT29 cells.