Cy5.5 Maleimide (also sold under sulfo-Cyanine5.5 Maleimide name) is a bright and photostable near-IR dye that is spectrally similar to Alexa Fluor® 680, DyLight 680™, and IRDye® 680 dye. Cy5.5 Maleimide is completely water-soluble and often a reagent of choice for labeling antibodies, peptides, and proteins that are sensitive toward organic co-solvents. Cy5.5 conjugates are pH insensitive from pH 4 to pH 10 and produces minimal autofluorescence of biological specimens in this region of the spectrum. Fluorescence of this long-wavelength Cyanine dye is not visible to the human eye but is readily detected by most imaging systems.
Maleimide is the most popular sulfhydryl-reactive reactive group for conjugating the dye to a thiol group on a protein, oligonucleotide thiophosphate, or low molecular weight ligand. The maleimide group specifically and efficiently reacts with reduced thiols (sulfhydryl groups, –SH) at pH 6.5 to 7.5 to form stable thioether bonds.