Site-specific conjugation of single domain antibody fragments (vhh) using equilibrium transfer alkylation reagent (etac)

A method for providing a polypetide with a detectable label, wherein the polypetide comprises at least 5 amino acids of which a first amino acid is C (Cysteine) and at least one second amino acid is selected from the group consisting of S (Serine), T (Threonine), Y (Tyrosine), K (Lysine), H (Histidine) and R (Arginine) characterized by the steps

• a) initiating Michael addition between a first and a second amino acid by providing a reagent according to formula (I)
• With
• R1=-Biotin, Thiamine, Peptide with 2-30 amino acids, oligonucleotides with 6-100 nucleotides, -DBCO, DBCO, -N3,N3, -Fluorophore, -Fluorescent protein
• R2=substituted or unsubstituted aromatic residues
• R3=H, F, NO2, alkyl with 1 to 5 carbon atoms
• R4=direct bond, substituted or unsubstituted alkyl, amine or amide residue with 1 to 20 carbon atoms
• b) Adding a nucleophilic agent to remove the Michael addition products between two second amino acid by retro-Michael addition.