The latter reaction path dominates when primary amines are present in the sample. When an aryl azide is exposed to UV light (250 to 350 nm), it forms a nitrene group that can initiate addition reactions with double bonds, insertion into C–H and N–H sites, or subsequent ring expansion to react with a nucleophile (e.g., primary amines). Squiggle bonds represent a labeling reagent or one end of a crosslinker having the reactive group. Diazirines (lower left) are a newer class of compounds that are growing in popularity and availability for protein research.
![photolinker ii photolinker ii](https://pubs.rsc.org/image/article/2010/AY/c001661f/c001661f-f1.gif)
Psoralen (bottom right) reacts almost exclusively with RNA or DNA, being used to label nucleic acids or to crosslink and investigate the interaction of proteins with nucleic acids. Traditionally, varieties of aryl azides (also called phenyl azides, top two rows) were the most widely used. Selected photo-reactive chemical groups used in protein crosslinking. The most useful of these for protein biology research are aryl azides and diazirines. Photoreactive groups that have been incorporated into crosslinking and labeling compound for use in bioconjugate techniques include aryl azides, azido-methyl-coumarins, benzophenones, anthraquinones, certain diazo compounds, diazirines, and psoralen derivatives. This feature makes them particularly useful in capturing protein interactions (see discussion below). Additionally, many of these groups will conjugate to any one of several common functional groups in proteins that they encounter during the brief time when they are activated. Most importantly, they make it possible to add reagents at an early step in an experiment and then to initiate crosslinking (by exposure to UV light) at some later step that coordinates with the particular biological condition of interest. Photochemical reactive groups have certain advantages over strictly thermochemical reagents for crosslinking and labeling applications with biological samples and experiments.