Functionalized 2-Hydroxybenzaldehyde-PEG Modules as Portable Tags for the Engagement of Protein Lysine ϵ-Amino Groups

The formation of reversible-covalent interactions between a small-molecule ligand and its protein target is emerging as a general strategy to design binders with increased affinity. In this context, 2-hydroxybenzaldehyde (2HB) has been recently proposed as suitable electrophilic tag to engage primary amines, such as the ϵ-amino group of lysine residues, in remarkably stable imines. Lys residues are often expressed in high amounts on protein surfaces and in the proximity of ligand binding sites, and a fine-tuning of the chemical connection between the ligand and 2HB is fundamental for the affinity gain. Herein we report the synthesis of four 2HB tags functionalized with a short polyethylene glycol (PEG) spacer and a suitable reactive ‘handle’ (alkyne, azide, carboxylic acid and amine, respectively) for their conjugation to virtually any type of small molecule ligand.