Visualizing biologically active small molecules in cells using click chemistry

Natural products and synthetic small molecules can be used to perturb, dissect and manipulate biological processes, thereby providing the basis for drug development. Over the past decades, the evolution of molecular biology protocols and microscopy techniques has made it possible to visually detect proteins in living systems with valuable spatiotemporal resolution, in which dynamic topological information has proved to be insightful. By contrast, although small molecules have become essential for biological studies, general methods to track them in cells remain underexplored. In this Review, we discuss how bioorthogonal chemistry, and click chemistry in particular, can be exploited to label and visualize almost any biologically active small molecule in cells and tissues. We review recent developments, highlighting cases in which visualizing small molecules has provided crucial mechanistic insights. This methodology is facile to implement, is versatile and is illuminating.