Exploration and Applications of Boron Mediated Bioconjugation Chemistries

Abstract

Besides their broad applications in organic chemistry, boronic acids are also increasingly seen in a variety of biological applications. For instance, they have been used in therapeutic drugs for chemotherapy or probes for the detection of saccharides. One unique feature of boronic acids is that they are capable of forming reversible complexes with sugars or even amino acids. Importantly, they are known to have low inherent toxicity to human. In this work, we have focused on two important reactions involving boronic acids: boronate ester formation, in which boronic acids react with diols and iminoboronate formation in which boronic acids form dative bonds with neighboring amino groups. We have demonstrated the potential of these reactions in bacteria targeting or protein modification. We envisioned that boronic acids could be used as a great warhead to be included in the development of novel antibiotics to counter antibiotic resistance of bacteria, which has emerged to be a serious clinical problem. Different from conventional antibiotics, we decided to utilize reversible covalent chemistries in the design of bacterium binding probes. Inspired by the diol-rich environment on bacterial surface, the first strategy took advantage of the reaction between boronic acid and diols to form boronate esters. We have rationally designed a linear peptide containing five copies of the ‘Wulff-type’ boronic acids or bicyclic amphiphilic peptides with two copies of boronic acids. We examined their capability of binding to E. coli cells or their bactericidal activity against S. aureus. Furthermore, we established a synthetic peptide library (OBTC) incorporating the 2-acetylphenyl boronic acid (2-APBA) warhead to form iminoboronate with the target-lipid II, a precursor for the biosynthesis of peptidoglycan. Although this library failed to generate any potent peptide hits, it provided useful information regarding the development of a synthetic library as well as the screening process. As an extension of the iminoboronate chemistry, thiazolidinoboronate (TzB) attracted our attention for its unique reaction mechanism, superior kinetics and excellent selectivity towards N-terminal cysteine residues. In this work, we have proposed an additional acyl transfer following TzB formation to turn this reaction into an irreversible conjugation. The new reaction inherits the fast kinetics and outstanding selectivity from the TzB chemistry. Excitingly, the product of TzB mediated acyl transfer survived complex conditions such as SDS-PAGE and LC/MS. This reaction was also applied to modify two recombinant proteins with N-terminal cysteine residues or to create a C5C phage library with two distinct modifications. We have further extended the substrate from cysteine to diaminopropionic acid (Dap), serine and tris base. We were delighted to observe imidazolidino boronate (IzB) formation and oxazolidino boronate (OzB) formation, which led to the design of cysteineresponsive probes or peptides that can be spontaneously cyclized in neutral aqueous conditions.