N-(m-PEG4)-N'-(PEG4-NHS ester)-Cy5

This PROTAC linker reagent, N-(m-PEG4)-N'-(PEG4-NHS ester)-Cy5, combines a flexible PEG spacer with a reactive NHS ester handle and a Cy5 fluorophore for tracking. Its ether-rich, water-compatible architecture supports effective linker solvation and reduced steric constraints between binding elements, while the NHS ester enables efficient conjugation to primary amines. The subsequent points describe its structure and practical reactivity for constructing labeled PROTACs for mechanistic studies.

Structure: The molecule is built from PEG-based repeating ether units that provide a flexible, hydrophilic spacer. It contains an NHS ester functional group for acyl transfer to amines and a Cy5 chromophore for fluorescence. Amide and ester functionalities govern polarity, stability, and conjugation behavior.

Reactivity: The NHS ester reacts with primary amines via nucleophilic acyl substitution, forming a stable amide bond. Typical conditions use mildly basic aqueous buffers to maintain amine nucleophilicity while limiting hydrolysis of the ester. No special catalysts are generally required; the key is controlling reaction time and solvent composition to balance coupling efficiency and NHS ester stability.