Fluorescein-PEG6-NHS ester

Fluorescein-PEG6-NHS ester is a PEGylated, NHS-activated linker that couples a fluorescent fluorescein reporter to primary amines under standard bioconjugation conditions. In PROTAC workflows, it enables visualization and tracking of conjugation efficiency, facilitates optimization of linker–ligand assembly, and supports mechanistic studies of targeted degradation by providing a robust fluorescence handle. The detailed structural and reactivity characteristics are provided below.

Structure: The molecule combines a fluorescein fluorophore with a polyethylene glycol spacer terminated by an N-hydroxysuccinimide ester. It contains an NHS ester functional group for acyl transfer, ether linkages within the PEG chain, and aromatic/heteroaromatic motifs typical of fluorescein, providing water-compatible, flexible, and conjugation-ready architecture.

Reactivity: The NHS ester reacts with primary amines via nucleophilic acyl substitution, forming a stable amide bond while releasing the NHS leaving group. Suitable conditions typically employ aqueous or mixed aqueous buffers at mildly basic pH, with careful control of moisture and amine concentration. Common coupling solvents include water or buffered alcohol/water mixtures; catalysts are generally unnecessary. After coupling, unreacted ester is removed by desalting or chromatography to minimize hydrolysis and background labeling.