Cy5.5 NHS Ester (also sold under sulfo-Cyanine5.5 NHS Ester 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 NHS Ester 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 produce 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.
The NHS ester (succinimidyl ester) of Cy5.5 is the most popular tool for conjugating this dye to a protein or antibody. NHS esters can be used to label to the primary amines (R-NH2) of proteins, amine-modified oligonucleotides, and other amine-containing molecules.