Fmoc-NH-PEG5-NHS ester

Fmoc-NH-PEG5-NHS ester is a bifunctional PEG-based linker designed for efficient assembly of PROTAC constructs, combining an orthogonally protected amine handle with an activated N-hydroxysuccinimide ester for rapid coupling to primary amines on targeting ligands or scaffolds. Its PEG character supports favorable solubility and spatial presentation of conjugated partners, while the Fmoc group enables controlled stepwise synthesis. The following sections describe its structure and the practical reactivity considerations used in PROTAC linker chemistry.

Structure: The molecule contains an Fmoc-protected nitrogen attached to a flexible polyethylene glycol chain terminating in an NHS ester. The key reactive moiety is the N-hydroxysuccinimide carbonate-like ester linkage, supported by ether-rich PEG segments and aromatic carbamate functionality, providing conformational flexibility and aqueous compatibility.

Reactivity: The NHS ester undergoes nucleophilic acyl substitution with primary amines to form stable amide bonds under mildly basic conditions. Typical PROTAC synthesis workflows employ anhydrous or low-water organic cosolvents and buffered aqueous mixtures to balance solubility and minimize hydrolysis. The Fmoc group can be removed using base in orthogonal steps, enabling sequential conjugation while preserving the amide-forming reactivity of the NHS ester.