DBCO-NHS Ester (or ADIBO-NHS Ester/DIBAC-NHS Ester), is an amine-reactive building block used for the modification of amine-containing molecules in organic media. It reacts with primary amines (e.g. side chain of lysine residues or aminosilane-coated surfaces) at neutral or slightly basic pH to form covalent bonds. Through copper-free/strain-promoted Click chemistry reactions, it can react with an azide-labelled molecule to produce a stable conjugate (triazole). The short spacer arm adds a small extra mass (228.3 Da) to modified molecules. This product is not recommended for the labelling of proteins or any other biopolymers in aqueous buffers due to poor aqueous solubility. Because DBCOs and azides are absent from biological systems, there is minimal background labelling of cells/lysates.