Types of photoactivatable groups

Westheimer and his associates have used carbenes in their studies (Singh et al. 1962 Shafer et al. 1966 Vaughan and Westheimer 1969 Browne et al. 1971 Hexter and Westheimer 1971). Carbenes are capable of reacting not only with polar residues but also with hydrophobic residues. Carbenes have been generated from diazoacetates and diazomalonate derivatives. Diazoacetates were initially employed since they may be photolyzed at 370 nm. This minimizes the potential for photochemical decomposition of the protein - one of the important limitations of photoaffinity labels. 

The reaction of greatest biochemical significance is represented by process 3 in which the carbene reacts with a tyrosine residue of another chymotrypsin molecule. This finding is consistent with the known ability of chymotrypsin to form dimers at acidic pH and indicates that this approach is capable of delineating the nearest 

One approach to the rearrangement problem for carbenes has been partially solved with the use of ethyl 2-diazomalonyl esters which are not as susceptible to rearrangements as diazoacetates (Hexter and Westheimer 1971). Vaughan and Westheimer (1969) have prepared the nitrophenyl ester of ethyl 2-diazomalonate and used it to acylate the reactive serine of trypsin. Photolysis at 253 nm has yielded a glutamic acid residue which presumably arises from the insertion of the 

Cooperman and his associates have used the ethyl diazomalonyl group in the synthesis of a series of reversibly bound photoaffinity labels (Brunswick and Cooperman 1971, 1973 Cooperman and Brunswick 1973). The synthesis and use of these compounds will be described below. 

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