Protein-Modification of Lysine with 2-(2-Styrylcyclopropyl)ethanal

A new lysine‐reactive cyclopropropyl aldehyde for the covalent modification of proteins was developed. The reagent exploits a divinylcyclopropane‐cycloheptadiene rearrangement to render the initial condensation irreversible. A labelling study on eGFP demonstrated excellent chemoselectivity for the modification of amine‐nucleophiles with the possibility of subsequent modifications.