Toward Ideal Proximity-Driven Reactions in Chemical Biology.

Chemical reactions in living systems are not governed solely by the intrinsic reactivity of functional groups or by the frequency of productive intermolecular collisions. Nature uses molecular recognition and spatial organization to control when and where chemical reactions occur. By selectively assembling reaction partners, biological systems increase the local concentrations, precisely orient reactive functionalities, and thereby achieve remarkable rates and selectivities under highly dilute conditions. Building on the long-established principles of molecular templation, proximity-driven reactions provide a framework for translating molecular recognition into controlled chemical reactivity in biological settings. This perspective does not seek to provide a comprehensive review of template-directed synthesis. Instead, it asks how the established principles of molecular recognition and templation are being used for the specific purposes of chemical biology, taking nucleic acid- and peptide-templated reactions, protein-based recognition platforms, proximity-driven bioconjugation, and bioorthogonal ligation strategies as case studies. I highlight how molecular templates accelerate bond formation, improve site-selectivity, and enable transformations that remain inaccessible through conventional solution-phase chemistry in the presence of competing biomolecules. Particular emphasis is placed on the quantitative coupling between three parameters that are usually discussed in isolation: the dissociation constant, which fixes the concentration window in which a template can operate; the effective molarity, which fixes the rate gain available once the complex has assembled; and the dissociation rate constant of the product complex, which fixes the ceiling on turnover frequency. Where the literature provides numbers for these quantities, they are given explicitly, and systems that achieve genuine template turnover are discussed in detail rather than treated as an aspiration. Despite substantial progress, many current systems remain limited by molecular complexity, irreversible or overly strong recognition, and the lack of efficient template regeneration. I propose that an "ideal" proximity-driven reaction should combine selective and reversible substrate recruitment, productive preorganization, rapid and chemoselective bond formation, minimal perturbation of the biological environment, and efficient template release and turnover. Future advances may therefore arise from integrating programmable molecular recognition with reversible binding and next-generation bioorthogonal ligation chemistries.