Size of the photolabile group

Aryl azides are rather bulky groups to append to a ligand, particularly to a small organic molecule, but it has proved possible in many cases to build the reactive group into the structure of a molecule, for instance 8-azido-cAMP and 4-azido-phenylalanine (Fig. 3.2). 

In other cases regions of molecules that are insensitive to structural perturbation have been derivatized (see below). 

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