Time-resolved europium(III) excitation spectroscopy: a luminescence probe of metal ion binding sites.

A laser-induced luminescence technique is introduced for probing the structure and equilibria of lanthanide complexes and lanthanide ion binding to macromolecules. The method involves the excitation of the 7F0 leads to 5D0 transition between nondegenerate levels in the europium(III) ion by means of an intense pulsed dye laser source. Excitation profits obtained by scanning the laser through the transition region reveal distinct peaks characteristic of individual europium(III) ion environments. The technique may be used to characterize the species present in complex equilibria in solution or to study europium(III) binding to macromolecules. Distinct europium(III) binding sites in thermolysin with long and short excited state lifetimes are observed.