A Polyoxyethylene-Substituted Bimetallic Europium Helicate for Luminescent Staining of Living Cells

The homoditopic ligand $H_2L^{C3}$ has been designed to form neutral triple-stranded bimetallic helicates of overall composition [$Ln_2(L^{C3})_3$]. The grafting of the polyoxyethylene fragments ensures water solubility and also favors cell penetration while being amenable to further derivatization. The ligand $pK_a$ values have been determined by spectrophotometric titration and range from 3.5 (sum of the first two) to 10.3. The thermodynamic stability of the helicates is large at physiological pH ($\log\beta_{23}$ in the range 22–23). The ligand triplet state has an adequate energy (0–phonon transition at $\approx20 800 cm^{-1}$) for sensitizing the luminescence of $Eu^{III}$ (Q=11%). Analysis of the $Eu^{III}$ emission spectrum points to an overall pseudo $D_3$ symmetry for the metal environment. No significant effect of [$Eu_2(L^ {C3})_3$] is observed on the viability of several cancerous cell lines (MCF-7, HeLa, Jurkat, and 5D10). The cell imaging properties of the $Eu^{III}$ helicate are demonstrated for the HeLa cell line by luminescence microscopy. Bright $Eu^{III}$ emission is seen for helicate concentration $>50 \mu M$ and after 20– 30 min loading time. The helicate stains the cytoplasm and the permeation mechanism is likely to be endocytosis.