Chiral sensing with circularly polarized luminescence probe molecules using allosteric effects of lanthanide complexes

Circularly polarized luminescence (CPL) from bis-diimine-dicarbocylic acid Eu(III) complexes such as [Eu(pda)2]− (pda = 1,10-phenanthroline-2,9-dicarboxylic acid) was investigated. The structures of the Eu(III) complexes are achiral, belonging to D2d point group in crystals. However, it exhibits CPL in aqueous solutions containing chiral amino acids such as arginine. Because the CPL is induced by intermolecular interaction with chiral agents, the phenomenon is called induced CPL (iCPL). The mechanism of the iCPL was elucidated from the results of iCPL measurements for solutions of various conditions, such as pH dependence, concentration dependence, and D/L amino acids mole ratio dependence. It was revealed that the CPL from [Eu(pda)2]− is induced by an allosteric effect, i.e., the initial association of amino acids to [Eu(pda)2]− causes a structural change in the [Eu(pda)2]−, which promotes associating another amino acid. Due to this effect, the intensity of the iCPL of [Eu(pda)2]− and its derivatives is very sensitive for the detection of chiral amino acids in a particular concentration region. By observing the iCPL of [Eu(pda)2]− in media containing chiral amino acids, an image sensing for the location and the chirality of the dispersed amino acids was demonstrated with a lab-made microscopic CPL measurement system.