Cy5.5 maleimide is a thiol reactive dye which is capable of selective labeling of proteins which contain sulfhydryl groups. Cy5.5 maleimide has near infrared emission that makes it suitable for bioimaging applications. This reagent can replace Alexa Fluor 680 and DyLight 680. For biomolecule labeling, the labeling reagent has low aqueous solubility, using of organic co-solvent to dissolve this molecular is necessary for efficient reaction. First, Cyanine dye should be dissolved in organic solvent and then added to a solution of biomolecule in appropriate aqueous buffer.

Cy5.5-NLP conjugate was synthesized by the NLP peptide and the Cy5.5-maleimide in 100 μL of anhydrous DMF, to which 10 μL of DIPEA was added. In the Her2/neu positive (SKBR-3 and BT474) or Her2/neunegative (MDA-MB-231) breast cancer cells, intracellular localization of Cy5.5-labeled peptide conjugates make the effects visible [1].

Reference:
[1]Cai, H. ; Singh, A.N.; Sun, X.; Peng, F. Synthesis and Characterization of Her2-NLP Peptide Conjugates Targeting Circulating Breast Cancer Cells: Cellular Uptake and Localization by Fluorescent Microscopic Imaging. Journal of Fluorescence, 2015, 25(1), 113–117.