Compositions and methods for modification of target molecules

A method for chemoselective modification of a target molecule to produce a modified target molecule, the method comprising:

• contacting a target molecule comprising a thiol moiety with a first biomolecule comprising a reactive moiety;
• wherein the first biomolecule comprising the reactive moiety is generated by reaction of a first biomolecule comprising a phenol moiety or a catechol moiety with a tyrosinase polypeptide;
• and
• wherein said contacting is under conditions sufficient for conjugation of the target molecule to the first biomolecule comprising the reactive moiety, thereby producing a modified target molecule, and
• wherein the modified target molecule is of formula (IV) or (IVA):
• wherein:
• Y1 is a first biomolecule,
• Y2 is a second biomolecule;
• L is an optional linker;
• and
• n is an integer from 1 to 3, and
• wherein Y1 and Y2 can be the same or different biomolecule wherein the first biomolecule or the second biomolecule comprises an antibody or antibody fragment.