text: In the absence of the original amino acid (-leucine) in an environment, -photo-leucine is used just as its naturally occurring analog in the protein processing mechanisms of the cell. Therefore, it can be used as a substitute for leucine in the primary structure of the protein. This property of photo-leucine is very useful for studying protein-protein interactions (PPIs), due to the fact that the photo-leucine molecule, because of its molecular structure, participates in the covalent cross-linking of proteins in the protein-protein interaction (PPI) domains when it is activated by ultraviolet (UV) light. This fact allows to determine and describe stable and transient protein interactions within cells without using any additional chemical cross-linkers, which could damage the cell structure being studied.
  The study of these protein-protein interactions is important because they are crucial in organizing cellular processes in space and time. In fact, interest in protein-protein interactions is not confined only to basic research: many of these interactions involved in viral fusion or in growth-factor signaling are promising targets for antiviral and anticancer drugs. Photo-affinity labeling is a powerful tool to identify protein targets of biologically active small molecules and to probe the structure of ligand binding sites, reason due to which photo amino acids, including photo-leucine, are so useful.