Catalyst-Free Biotin Conjugation Onto Poly(Styrene-co-Butadiene) Mediated by an Orthogonal Agent Comprising Nitrile N-Oxide and Blocked Isocyanate.

Bioconjugates are compounds synthesized by covalently linking biomolecules with a functional fragment, forming a broad class of systems that encompasses pharmaceuticals, molecular probes, and macromolecular materials such as bioconjugated polymers. Achieving such covalent linkage requires the introduction of suitable functional groups for ligation into each fragment. Herein, we report the development of an orthogonal agent as a novel molecular ligation tool that integrates an oxime functionality, which serves as a precursor of a nitrile N-oxide, together with a blocked isocyanate functionality. Treatment of the orthogonal agent with iodosobenzene (PhIO) generates a nitrile N-oxide, which undergoes a catalyst-free [3+2] cycloaddition reaction to an alkene-containing molecule. Subsequent thermal treatment at approximately 120°C produces an isocyanate that reacts with a nucleophilic molecule without the need for a catalyst, enabling sequential chemical ligation between alkenes and nucleophiles. We further demonstrate the utility of this orthogonal agent through the preparation of biotin-conjugated poly(styrene-co-butadiene) (SB) resin. The polymer structure was characterized by 1H NMR and IR spectroscopy, along with a colorimetric assay using horseradish peroxidase (HRP)-conjugated streptavidin. These results highlight that the present orthogonal agent is a versatile and effective ligation tool for catalyst-free bioconjugation of alkene-containing common polymers with biomolecules.