Mal-PEG1-NHS ester

Mal-PEG1-NHS ester is a heterobifunctional PEG-based linker designed for constructing PROTACs and related targeted protein degradation conjugates through NHS-ester chemistry. Its PEG spacer improves aqueous compatibility and can help tune linker flexibility and solubility, while the maleimide handle enables orthogonal coupling to thiols. This combination supports efficient, modular synthesis of degraders and facilitates stepwise assembly of protein-binding and E3-ligase–recruiting components; detailed structural and reactivity considerations are provided below.

Structure: The linker comprises a maleimide moiety connected to a short polyethylene glycol spacer terminated with an N-hydroxysuccinimide ester. It contains an activated carboxylate (NHS ester) for amide bond formation and a strained alkene (maleimide) for selective thiol addition. The PEG segment provides hydrophilicity and conformational flexibility.

Reactivity: The NHS ester reacts with primary amines to form stable amide bonds under mildly basic aqueous or mixed solvent conditions, typically using buffered systems compatible with NHS stability. The maleimide undergoes Michael-type addition with thiols (including cysteine or reduced thiol-containing ligands) to yield a thioether linkage. For PROTAC assembly, orthogonal sequencing is commonly used: install the amine partner first via NHS chemistry, then couple the thiol partner to the maleimide, avoiding conditions that promote maleimide hydrolysis or thiol oxidation.