Cyanine5.5 maleimide chloride is a CY dye compound characterized by two nitrogen atoms connected through an odd number of methylene groups, exhibiting long excitation and emission wavelengths, high molar extinction coefficients, strong water solubility, and simple synthetic accessibility. Cyanine5.5 maleimide chloride is widely employed for the fluorescent labeling of proteins, antibodies, and small molecules, and the labeling process can be achieved through direct conjugation reactions that serve as a valuable reference model in bioimaging and analytical chemistry research.

Cyanine5.5 maleimide chloride is a conjugated dye composed of the Cyanine5.5 fluorophore and a maleimide reactive group. Cyanine 5.5 is a near-infrared fluorescent cyanine dye widely used for in vivo small animal imaging. It can be employed to label proteins, antibodies, peptides, and nanoparticles. Maleimide is a commonly used group in bioconjugation reactions, capable of forming stable thioester linkages with sulfhydryl groups (-SH) through Michael addition, thereby achieving labeling. Although amino groups (such as the side chains of lysine and arginine) also exhibit reactivity, under neutral or slightly acidic buffer conditions, maleimide demonstrates significantly higher selectivity toward sulfhydryl groups (reacting over 1000 times faster than with amino groups), enabling rapid, highly selective, and high-yield labeling. Furthermore, sulfhydryl groups are widely present as functional groups in biomolecules, such as cysteine in proteins and disulfide bonds. Consequently, maleimide/thiol labeling has become the second most commonly used bioconjugation method after NHS/amine labeling.