Direct and selective tagging of cysteine residues in peptides and proteins with 4-nitropyridyl lanthanide complexes

A cysteine-selective tagging method in water is reported, based on rapid displacement of a pyridyl nitro-substituent in simple pyridines and lanthanide complexes. The conjugation reaction creates a short link between the tag and peptide, holding the peptide closer to the Ln3+ ion and with reduced flexibility compared to existing methods.