Having shown that tagging a protein at a single site with different lanthanoid complexes delivers outstanding structural information at a selected site of a protein (such as active sites and ligand binding sites), we now present a simple way by which different lanthanoid complexes can be assembled on a highly solvent-exposed cysteine residue. Furthermore, the chemical assembly is selective for selenocysteine, if a selenocysteine residue can be introduced into the protein of interest.