Development of New Double-Stranded Phenylalanyl Chelators Using η–χ Diagrams and Binding Constants for Chelators and Lanthanide Ions

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We examined the interaction between several ligands and alkaline (M+) or lanthanide (Ln3+) ions using η–χ diagrams. η and χ represent absolute hardness and absolute electronegativity, respectively. Based on the diagrams, new double-stranded amino acid chelators (1) conjugated to Cat (catechol) were synthesized. They had two cavities coordinating hard metal ions, such as Li+ and K+, and soft metal ions, such as Lu3+ and Eu3+. 1 formed a solvated molecular complex, 1–Ln3+, with a molar ratio of 1 : 1 at a binding constant (Kb) of 104—106 with Y3+, La3+, Eu3+, and Lu3+, respectively. With a chemical hard ion, Li+, we found that the Kb of the 1–Eu3+ complex increased. The optimized geometries of 1–Lu3+ and 1–La3+ complexes were obtained from B3LYP correlation functions using Stuttgart-Dresden-Bonn (SDD) Effective Core Potential (ECP) basis sets for Lu3+ and La3+ ions. The double-stranded chelator may prove useful for studying the selective separation of lanthanide ions, and the development of functional peptides.