The introduction of multiple labels onto biomolecules is a challenge. We report herein the synthesis of vinyl sulfone derivatized bifunctional tag single-attachment-point reagents (BTSAP) bearing biotin and a fluorescent tag and their applications in proteins for the introduction of multiple labels by means of the Michael-type addition of the electrophilic vinyl sulfone group. These BTSAP reagents were easily synthesized by a two-step chemical ...