Development of soft-based double-stranded peptide chelators which selectively separate europium and lanthanum ions based on the hardness concept.

New double-stranded peptide chelators (1) conjugated Cat (2,3-dihydroxybenzoic acid) were synthesized and formed a molecular complex 1-Eu(3+) (or 1-Lu(3+)) with Eu(3+) and Lu(3+) but not La(3+). The double-stranded peptide chelator may prove to be useful tools for studying the selective separation of lanthanide ions.