Site-Specific Labeling of Proteins with Near-IR Dyes

Convenient labeling of proteins is important for observing its function under physiological conditions.  In tissues particularly, heptamethine cyanine dyes (Cy-7) are valuable because they absorb in near infrared (NIR) region (750 – 900 nm) where light penetration is maximal.  In this work, we found Cy-7 dyes with a meso-Cl functionality covalently binding to proteins with free Cys residues under physiological conditions (aqueous environments, at near neutral pH, and 37 °C).  It transpired that the meso-Cl of the dye was displaced by free thiols in protein, while nucleophilic side-chains from amino acids like Tyr, Lys, and Ser did not react.  This finding shows a new possibility for convenient and selective labeling of proteins with near-IR fluorescent probes.