Sulfo-Cy3 NHS ester is a sulfonated, hydrophilic and highly water soluble dye. For labeling proteins and peptides, no organic co-solvent is needed. Sulfo-Cy3 NHS ester is ideal for proteins with low solubility and proteins prone to denaturation. Non-sulfonated Cy3 NHS ester is also available. In life science, the sulfonate groups make the Cy dyes soluble in water and also reduce fluorescence-quenching which can arise from dye–dye interactions.

In vitro, sulfo-Cy3 NHS ester can react with green-emitting QDs ligand-exchanged and LA/His-PIMA-PEG-NH2 to provide QD-dye conjugates [1].

Reference:
[1] Reid, D. A.; Keegan, S.; Leo-Macias, A.; Watanabe, G.; Strande, N.T.; Chang, H.H.; Oksuz, B.A.; Fenyo, D.; Lieber, M.R.; Ramsden, D.A.; Rothenberg, E. Organization and dynamics of the nonhomologous end-joining machinery during DNA double-strand break repair. Proceedings of the National Academy of Sciences of the U.S.A., 2015, 112(20).