A polyoxyethylene-substituted bimetallic europium helicate for luminescent staining of living cells

The homoditopic ligand H2LC3 has been designed to form neutral triple-stranded bimetallic helicates of overall composition [Ln(2)(L-C3)]. 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 approximate to 20800 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.