Sulfo-Cy5 dye is one of the most popular far-red fluorescent dyes. It is often a reagent of choice for protein and peptide labeling. Sulfo-Cy5 dye is a water-soluble, bright, far-red-fluorescent dye with excitation ideally suited for the 633 nm or 647 nm laser lines. Sulfo-Cy5 conjugates of antibodies, peptides, and proteins are pH insensitive from pH 4 to pH 10. A significant advantage to using long wavelength dyes such as Cy5 or AF 647 dye over other fluorophores is the low autofluorescence of biological specimens in this region of the spectrum.
The carboxylic acid of Sulfo Cy5 dye is a reagent of choice for the preparation of custom activated esters that often are not commercially available. Examples of such activated esters include sulfo-NHS, TFP (2,3,5,6-Tetrafluorophenol), STP (4-Sulfo-2,3,5,6-Tetrafluorophenol, Sodium Salt). Another common application for non-activated carboxylic acid is peptide modification during solid phase synthesis, which usually requires in-situ activation with peptide coupling regents, for example HATU. Cy5 Acid is also often used for control experiments, and for calibration.