Azido-PEG4-NHS-ester

Azido-PEG4-NHS-ester is a bifunctional PEG-based linker designed for modular PROTAC assembly, combining an NHS-ester handle for efficient coupling to primary amines with an azide group for orthogonal bioorthogonal conjugation. Its flexible polyethylene glycol spacer can improve solubility and spatial presentation of warheads, while the azide enables downstream click chemistry to connect targeting and recruiting elements. The points below describe its structure and practical reactivity considerations in PROTAC workflows.

Structure: The linker contains a polyethylene glycol segment that provides conformational flexibility and hydrophilicity, flanked by an N-hydroxysuccinimide activated ester and a terminal azide. Key functional groups include an NHS ester for acyl transfer and an organic azide suitable for cycloaddition chemistry.

Reactivity: The NHS-ester reacts with primary amines to form stable amide bonds via nucleophilic acyl substitution, typically under mildly basic aqueous buffer conditions that preserve azide integrity. For PROTAC construction, the azide is commonly retained for later conjugation using copper-catalyzed or strain-promoted azide–alkyne cycloaddition, with appropriate solvent systems and catalyst/ligand choices to balance reaction rate and protein/ligand compatibility.