Development of new double-stranded phenylalanyl chelators using eta-chi diagrams and binding constants for chelators and lanthanide ions.

We examined the interaction between several ligands and alkaline (M(+)) or lanthanide (Ln(3+)) ions using eta-chi diagrams. eta and chi 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 Lu(3+) and Eu(3+). 1 formed a solvated molecular complex, 1-Ln(3+), with a molar ratio of 1 : 1 at a binding constant (K(b)) of 10(4)-10(6) with Y(3+), La(3+), Eu(3+), and Lu(3+), respectively. With a chemical hard ion, Li(+), we found that the K(b) of the 1-Eu(3+) complex increased. The optimized geometries of 1-Lu(3+) and 1-La(3+) complexes were obtained from B3LYP correlation functions using Stuttgart-Dresden-Bonn (SDD) Effective Core Potential (ECP) basis sets for Lu(3+) and La(3+) ions. The double-stranded chelator may prove useful for studying the selective separation of lanthanide ions, and the development of functional peptides.