Lanthanide ion luminescence probes. Characterization of metal ion binding sites and intermetal energy transfer distance measurements in calcium-binding proteins. 1. Parvalbumin.

Eu(III) laser excitation spectroscopy of the 7F0 leads to 5D0 transition reveals spectral features characteristic of the occupation of the CD and EF Ca(II)-binding sites of parvalbumin by Eu(III) ions. In addition, at pH 6.5, a signal attributable to binding at a third site is observed. This feature is abolished upon lowering the pH to 3.8. At pH 6.5, the appearance of this feature correlates with the decrease in luminescence intensity during titrations of parvalbumin with Eu(III) or Tb(III) after more than approximately 1.8 equiv of either of these ions has been added. Eu(III) ions in the primary sites coordinate zero to two H2O molecules while the third site involves about three coordinated H2O molecules. Parvalbumins in which mixed pairs of lanthanide ions, Ln(III), occupy the CD and EF sites were prepared. Nonradiative energy transfer between Eu(III) and Tb(III) acting as luminescent donors and various other Ln(III) ions serving as acceptors was observed by monitoring the excited-state lifetimes of the donor ions using a pulsed dye laser apparatus. With the assumption of a Förster-type dipole--dipole mechanism, inter-binding-site distance estimates were made from our measurements and are in reasonable agreement with the distance (11.8 A) obtained by X-ray crystallography, especially when Eu(III) is the donor. R0 values (critical distances for 50% energy transfer) in H2O solution range from 9.2 A for the Tb(III)--Ho(III) donor-acceptor pair down to 5.7 A for the Eu(III)--Ho(III) pair.