N-(m-PEG9)-N'-(PEG5-acid)-Cy5

This PROTAC linker is designed to connect a targeting ligand to an E3-recruiting moiety while providing a hydrophilic, flexible spacer environment and a functional handle for conjugation. Its PEG-based architecture supports favorable solubility and reduces steric constraints that can impair ternary complex formation. In PROTAC workflows, it enables modular assembly of degraders and facilitates analytical tracking when used with Cy5-containing constructs. Detailed structural and reactivity considerations are provided below.

Structure: The linker comprises a PEG-rich, ether-containing scaffold featuring multiple ethylene glycol repeat units that impart high hydrophilicity and conformational flexibility. It is terminated by an acid functionality suitable for amide coupling and incorporates a Cy5 fluorophore for optical characterization. The framework includes ether linkages, amide-forming groups, and conjugation-compatible aromatic components.

Reactivity: The acid terminus is typically used for amide-bond formation with primary or activated amine-bearing partners under standard coupling conditions. Mechanistically, carbodiimide-mediated activation followed by nucleophilic acyl substitution is commonly employed to generate stable amide linkages. Suitable solvents include polar aprotic media, often with bases to promote coupling efficiency. No special catalyst is required beyond coupling reagents compatible with PEG and Cy5 stability.