Expanding the fluorescence toolkit: molecular design, synthesis and biological application of unnatural amino acids.

Fluorescence imaging has become an indispensable tool in modern biology, enabling the visualisation of dynamic molecular processes with spatial and temporal precision. Traditional strategies rely heavily on the conjugation of large, extrinsic fluorophores, such as green fluorscent protein or organic dyes, through linkers to proteins or peptides of interest. While sometimes effective, these bulky labels can interfere with native protein structure, function, and interactions, limiting their utility in studying sensitive or compact biological systems. In contrast, fluorescent unnatural amino acids that can be site-specifically incorporated into proteins with minimal perturbation, provide high-resolution insights into molecular behaviour while preserving biological integrity. This perspective focuses on recent advances in the synthesis and application of fluorescent unnatural amino acids, particularly through the structural modification of natural aromatic-containing residues such as phenylalanine, tyrosine, and tryptophan. Additionally, the incorporation of small-molecule fluorophores as amino acid side chains to create versatile probes is also discussed. The utility of these fluorescent amino acids in biological chemistry is highlighted through their application in imaging protein dynamics, measuring enzyme activity and studying biomolecular interactions. Together, these advances establish fluorescent amino acids as a valuable and evolving toolkit for minimally invasive biological imaging.