Oligopeptide and methods for producing conjugates thereof

An oligopeptide of formula P-C-Z or Z-C-P, wherein:

• P is a 8 to 800 amino acid peptide having no reduced cystein residue,
• C is a cystein residue,
• Z represents a 1-10 amino acid spacer, wherein the amino acid residues of Z are identical or different and wherein Z does not contain a cystein residue,
• characterized in that said cystein residue C is linked to a substance of interest through a maleimido compound of formula (I) bearing said substance of interest:
• wherein:
• B, B′1, B′2, and B″, identical or different, are independently single bonds or spacers selected from polyols, polyolefins, polyalkyls, vinyl polymers, polyaldehydes polyacid esters,
• D, D′ and D″, identical or different, are independently selected from amine, amide, amino-alcohol, urea, thiourea, carbamate, carbonate, ester, ether, thioether, aryl, heteroaryl, oxime groups,
• A is a single bond or a chelating agent,
• SI is the substance of interest,
• X′ is an acid, amine, amide, ester, ether, alkyl, alkenyl, alkynyl, aryl or heteroaryl function, and
• n=1 to 100.