N-(azide-PEG3)-N'-(Mal-PEG4)-Cy5

This N-(azide-PEG3)-N'-(Mal-PEG4)-Cy5 linker is engineered for modular PROTAC construction by combining an azide handle for bioorthogonal conjugation with a maleimide group for selective thiol coupling, while the PEG spacers improve solubility and reduce steric constraints. The Cy5 fluorophore enables tracking of linker incorporation and monitoring of conjugate formation in experimental workflows. Detailed structural and reactivity considerations are provided below to support robust, reproducible assembly of targeted protein degraders.

Structure: The molecule contains an azide-functional PEG segment and a maleimide-functional PEG segment connected to a Cy5 dye. It features ether-rich PEG linkers, an azide moiety, and a maleimide ring for electrophilic thiol addition, along with a polymethine chromophore for fluorescence.

Reactivity: PROTAC assembly using this linker typically relies on orthogonal chemistries: azide participation in copper-catalyzed or strain-promoted azide–alkyne cycloaddition, and maleimide coupling via Michael-type addition to accessible thiols on targeting ligands or intermediate scaffolds. Reactions are commonly performed under aqueous buffer conditions compatible with maleimide stability, with pH controlled to favor thiol reactivity while minimizing maleimide hydrolysis; fluorescent labeling can be validated by Cy5 readout.