Fmoc-NH-PEG3-NHS ester

Fmoc-NH-PEG3-NHS ester is a bifunctional PEG-based linker designed for efficient conjugation in PROTAC assembly workflows. It combines an Fmoc-protected amine handle for controlled coupling with an NHS-ester electrophile that readily forms stable amide bonds with primary amines. This dual functionality supports modular synthesis, improves solubility and spacer behavior, and enables attachment of targeting ligands or E3-recruiting moieties. The structure and reactivity considerations are described in detail below.

Structure: The molecule contains an Fmoc carbamate-protected amino group linked to a short poly(ethylene glycol) chain terminated by an N-hydroxysuccinimide ester. It features aromatic fluorenylmethoxycarbonyl functionality, ether-rich PEG segments, and an activated carboxylate ester designed for nucleophilic acyl substitution.

Reactivity: The NHS ester reacts with primary amines to form amide linkages via nucleophilic acyl substitution, typically under mildly basic conditions to promote amine deprotonation while preserving NHS ester integrity. Common approaches use anhydrous or low-water organic/aqueous mixtures to balance solubility and hydrolysis control. No special catalysts are generally required; careful pH control, rapid mixing, and protection from moisture are key to minimizing NHS hydrolysis during PROTAC conjugation steps.