Toward Precision Bioconjugation: Chemical Strategies for Site-Selective Cysteine Conjugation.

The ability to access atomically tailored complex peptides and proteins provides powerful opportunities for dissecting molecular functions and advancing applications in chemical biology, therapeutics, and (bio)-materials science. Robust precision-engineering strategies are essential to construct well-defined protein architectures while preserving native folding and activity. To do so, chemoselective bioconjugation techniques have been developed to modify specific side chains of amino acids. This allowed for the selective introduction of functionalities on predetermined amino acids. However, ultimate control can be achieved only through site-selective modifications that precisely define both the nature of the linkage and the exact position of conjugation on elongated peptide sequences or fully assembled proteins. Cysteine residues are of particular interest, as their highly nucleophilic thiols offer excellent chemoselectivity and typically occur in low abundance in their reduced form. Here, we examine chemoselective transformations targeting cysteine residues that have been further refined to occur exclusively at predefined positions within a peptide or protein, thereby achieving a high degree of site-selectivity. This review focuses exclusively on chemical strategies for cysteine modification, offering guidance for future synthetic developments within the field of precision chemistry. Achieving this level of precision requires advanced chemical strategies that exploit the local environment of the targeted cysteine. One approach involves leveraging neighboring functional groups, for example, engaging the thiol together with the α-amine or carboxylate to enable selective N- or C-terminal modification, respectively. In such designs, the cysteine side chain may contribute through transient interactions, direct incorporation into the covalent linkage, or the stabilization of the desired product. Recently, a promising strategy has attracted increasing attention in which site-selectivity is enabled by temporary interaction with a proximal amine, thus being applicable to differentiate also between internal cysteines. Together, these strategies highlight that site-selective protein modification has evolved into a powerful tool for the rational design and functional control of complex biomolecules, redefining what is achievable in chemical biology, therapeutics, and biomaterials science. We anticipate that increasingly routine or user-friendly approaches such as the programmable TriTEx method will further accelerate the adoption of precision biomolecule conjugates in both research and industrial settings.