Cy5-PEG6-NHS ester

Cy5-PEG6-NHS ester is a PEG-based linker that couples a near-infrared cyanine dye to protein-targeting ligands via an NHS ester handle, enabling efficient formation of PROTAC constructs and related conjugates. Its PEG spacer is designed to improve solubility and provide conformational flexibility, which can support productive ternary complex formation. The dye functionality is useful for tracking synthesis, binding, and cellular uptake, while the NHS ester supports robust amide-bond formation under standard bioconjugation workflows. Detailed structural and reactivity considerations are provided below.

Structure: This molecule combines a cyanine fluorophore with a polyethylene glycol spacer terminating in an N-hydroxysuccinimide ester. It contains an activated carboxylate for amide coupling, ether linkages within the PEG chain, and aromatic and polymethine components characteristic of Cy dyes, yielding an amphiphilic, water-compatible conjugation scaffold.

Reactivity: The NHS ester reacts with primary amines (for example, lysine side chains or amino-functionalized ligands) to form stable amide bonds through nucleophilic acyl substitution. Conjugation is typically performed in mildly basic aqueous buffer or mixed aqueous solvent systems to maintain NHS ester reactivity while minimizing hydrolysis. Common catalysts are not required; careful control of pH and reaction time helps limit competing hydrolysis. Purification is often performed by chromatography or dialysis to remove unreacted dye and byproducts.