Fmoc-NH-pentanoic acid-NHS-SO3Na

This PROTAC linker, Fmoc-NH-pentanoic acid-NHS-SO3Na, is designed to provide a robust, amide-forming handle alongside a sulfonate functionality that supports aqueous compatibility during conjugation workflows. Its activated NHS ester chemistry enables efficient coupling to primary amines commonly found in PROTAC warheads or targeting ligands, while the Fmoc-protecting group supports controlled synthetic sequencing. The combination of spacer flexibility and water-tolerant sulfonate character makes it useful for assembling bifunctional degraders; the following sections describe its structure and reactivity in detail.

Structure: The linker contains an Fmoc-protected amino group, a flexible pentanoic acid-derived spacer, and an NHS-activated ester for acyl transfer. A sulfonate sodium salt imparts strong ionic character, improving solubility and maintaining charge balance in polar media. It features an Fmoc carbamate, an activated ester, and a sulfonate functionality suitable for stepwise PROTAC synthesis.

Reactivity: The NHS ester reacts with primary amines via nucleophilic acyl substitution, forming stable amide bonds while releasing NHS. Suitable conditions typically use mildly basic aqueous or mixed solvent systems that preserve NHS ester integrity and promote amine nucleophilicity. Common base choices include bicarbonate or carbonate buffers, with coupling carried out at temperatures compatible with NHS stability. No special catalysts are generally required; careful control of pH and stoichiometry helps minimize hydrolysis.