Photoreactive groups aryl azides

Reported structures for homobifunctional aryl azides include a biphenyl derivative and a naphthalene derivative (Mikkelsen and Wallach, 1976), a biphenyl derivative containing a central, cleavable disulfide group (Guire, 1976), and a compound containing a central l,3-diamino-2-propanol bridge between phenyl azide rings that are nitrated (Guire, 1976). The only commercially available homobifunctional photoreactive crosslinker is BASED. 

Of the following amine-reactive and photoreactive crosslinkers, the overwhelming majority use an aryl azide group as the photosensitive functional group. Only a few use alternative photoreactive chemistries, particularly perfluorinated aryl azide, benzophenone, or diazo compounds. For general background information on photoreactive crosslinkers, see Das and Fox (1979), Kiehm and Ji (1977), Vanin and Ji (1981), and Brunner (1993). 

Although photoreactive aryl azides are relatively inert to thermochemical reactions prior to photolysis, they are not stable in the presence of sulfhydryl compounds, which can reduce the azide functional group to an amine. Avoid, therefore, the use of reduc-tants such as DTT or 2-mercaptoethanol before the photolyzing step, as these can react with the aryl azide within minutes (Staros et al., 1978). Avoid also amine-containing... 

N-Hydroxysuccinimidyl-4-azidosalicylic acid (NFIS-ASA) is a heterobifunctional reagent containing an NHS ester on one end and a photoreactive aryl azide group on the... 

The use of PAL in enzyme modifications was first described in 1962, when Westheimer and coworkers reported on the use of a diazoacetyl group to inactivate chymotrypsin [29]. Considerable research on the development of new PAL reagents has taken place ever since [27, 28, 30-36], but only a few photophores, which largely meet the above-mentioned requirements, are being used nowadays in A/BPs. These are aryl azides (first reported use in 1969) [37], diazirines (1973) [38] and benzophenones (1973) [39]. The chemistry of these three photoreactive groups after photolysis as well as their use in recently reported A/BPs will be discussed in Sects. 3.1-3.5. 

UV photolysis of the powdered crystals of several aryl azides has been found to give azo compounds, from dimerization of the nitrenes, mostly in yields of >91% The only exceptions noted were p-(A -methylacetamido)phenyl azide, which also gives a product from insertion of the nitrene into a C-H bond of the methyl group, and 2-azidobiphenyl, which gives comparable amounts of the azo product and carbazole. Monitoring these solid-state photoreactions by ESR revealed that the arylnitrenes have extremely long half-lives, compared with those of nitrenes in the gas-phase or solution, and clearly the crystalline... 

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