Biological activity of the photoaffinity reagent

As photoaffinity reagents can be used in the same manner as the natural 

The investigator should be aware of a few possible pitfalls in binding studies and other assays. First, the reagent should be pure. This is of the utmost importance if the photoaffinity label is a derivative of the natural ligand and turns out to be less potent in its biological activity. In numerous cases where a derivative has appeared to be (say one hundred times) less active than its precursor, it has not been shown that the precursor was not a contaminant (say 1 %) of the derivative which was actually completely 

The stability of the reagent to the assay conditions should also be ascertained. The instability of azides and diazo compounds towards thiols was mentioned earlier, and it would be prudent to ensure that all new reagents whether they contain familiar photoactivatable groups or not are stable to the prevailing chemical and enzymatic conditions. In an attempt to label the cAMP receptor of D. discoideum, Wallace and Frazier (1979b) found that 8-N rcAMP was converted to NrAMP by the phosphodiesterase of a crude membrane preparation. The NrAMP specifically labeled actin and not the cAMP receptor. 

---