Affinity Labeling with 4-Azidophthalimide (AzPI): Relation between Labeling Rate and Fluorescence Intensity

We recently developed 4-azidophthalimide (AzPI) as a compact fluorogenic photoreactive tag that can be attached to ligands to achieve selective fluorescence labeling of target proteins even in the presence of a large excess of non-target proteins. To further establish the utility of the AzPI tag, we focused here on streptavidin labeling with biotin AzPI conjugates, and evaluated the relation between the amount of covalently labeled streptavidin (labeling rate) and fluorescence intensity. The labeling rate was proportional to the fluorescence intensity under standardized photo-irradiation conditions. Prolongation of the photo-irradiation time led to a marked increase in the labeling rate, but this was accompanied by a gradual decrease in the fluorescence intensity, which appeared to be due at least in part to photo-induced degradation of the target streptavidin. These findings should be helpful for achieving sensitive fluorescence detection of target proteins by using the AzPI tag.