Labeling reactive centers of various types in cells with specific site-directed probes is a common method to explore both function and biochemical modification of proteins. The popular click chemistry method of protein labeling employs use of a reaction between an azido group and an alkyne on complimentary pairs of a specific reactive probe and a labeling agent (i.e. a tag) such as biotin or a fluorophore. The Staudinger ligation is an alternative to the click chemistry reaction in which a phosphine-labeled molecule reacts with an azido group on the opposing molecule of interest. Phosphine biotin is a labeling reagent that selectively reacts with azido groups on modified proteins through the Staudinger ligation reaction. Modified proteins can be detected using common avidin-based biochemical techniques in whole cells or by blotting experiments following SDS-PAGE. For example, phosphine-biotin has been used successfully in conjunction with DAz-1 or DAz-2 to label and detect sulfenic acid sites in proteins.
