Site-Specific Bioconjugation and Multi-Bioorthogonal Labeling via Rapid Formation of a Boron-Nitrogen Heterocycle.

Precise control of covalent bond formation in the presence of multiple functional groups is pertinent in the development of many next-generation bioconjugates and materials. Strategies derived from bioorthogonal chemistries are contributing greatly in that regard; however, the gain of chemoselectivity is often compromised by the slow rates of many of these existing chemistries. Recent work on a variation of the classical aldehyde/ketone condensation based on ortho-carbonylphenylboronic acids has uncovered markedly accelerated rates compared to those of the simple carbonyl counterparts. The products of these reactions are distinct, often in the form of boron-nitrogen heterocycles. In particular, we have shown that 2-formylphenylboronic acid (2fPBA), when coupled with an α-amino-hydrazide, produces a unique zwitterionic and stable 2,3,1-benzodiazaborine derivative. In this work, we apply this chemistry to generate chemically defined and functional bioconjugates, herein illustrated with immunoconjugates. We show that an antibody and a fluorophore (as payload) equipped with the relevant reactive handles undergo rapid conjugation at near-stoichiometric ratios, displaying a reaction half-life of only ∼5 min with 2 equiv of the linker payload. Importantly, the reaction can be extended to multicomponent labeling by partnering with the popular strain-promoted azide-alkyne cycloaddition and tetrazine- trans-cyclooctene (Tz-TCO) ligation. The mutual orthogonality to both of these chemistries allows simultaneous triple bioorthogonal conjugations, a rare feat thus far that will widen the scope of various multilabeling applications. Further collaboration with the Tz-TCO reaction enables rapid one-pot synthesis of a site-specific dual-payload antibody conjugate. Altogether, we envision that the 2fPBA-α-amino-hydrazide ligation will facilitate efficient assembly of diverse bioconjugates and materials, enabling access to more complex modalities via partnership with other orthogonal chemistries.