CRYSTAL-STRUCTURE AND SPECTROSCOPIC CHARACTERIZATION OF A LUMINESCENTEUROPIUM CHELATE

We report the crystal structure at 1 Angstrom resolution of europium b ound to diethylenetriaminepentaacetate covalently coupled to an organi c chromophore, carbostyril 124 (DTPA-cs124; carbostyril 124 = 7-amino- 4-methyl-2(1H)-quinolinone). The complex is highly luminescent in solu tion and in the crystal form. The Eu-DTPA-cs124 crystallizes in the sp ace group C2/c with two molecules in the asymmetric unit. The two mole cules form a dimer which is stabilized by pi-stacking of two carbostyr ils and by the ligation of the carbonyl oxygen of one cs124 to the eur opium of the second half of the dimer. This ligation removes the last water bound to the primary coordination sphere of europium, resulting in no waters bound to the lanthanide. The crystal structure also shows the amide oxygen between the cs124 and the DTPA is ligated to the met al. Spectroscopic measurements of the europium luminescence in the cry stal show that 66% of the luminescence is centered around 617 nm with a full width at half-maximum of 2.5 nm. In solution, 48% of the europi um luminescence intensity is in the 617 nm peak, with a fwhm of 3.5 nm . The europium lifetime in the crystal structure is 0.90 ms; the lifet ime of the monomeric Eu-DTPA-cs 124 is 0.62 ms in H2O solution and 2.4 ms in D2O solution. The differing lifetimes can be explained by the w ell-known effects of H2O on nonradiative europium emission rates and b y postulating an electric (crystal) field surrounding the Eu which is more anisotropic in the crystal than in the solution, creating a faste r radiative rate of Eu excited state deactivation. This greater anisot ropy is expected on the basis of the crystal structure. The crystal st ructure also suggests improvements in the linkage of cs124 to DTPA whi ch may improve the energy transfer between the organic chromophore and the europium and sharpen the Eu emission spectrum in solution.