A europium complex that selectively stains nucleoli of cells

A europium complex selectively staining the nucleolus of NIH 3T3, HeLa, and HDF cells is reported. This complex possesses not only the advantage of the long lifetime of europium emission (0.3 ms), but also a chromophore that allows excitation at a relatively long wavelength (lambda(max) = 384 nm) and gives rise to an acceptable quantum yield (9%). The complex can be used both in live cell and fixed cell imaging, giving an average intracellular concentration on the order of 0.5 mu M. Strong binding to serum albumin has been demonstrated by examination of the analogous gadolinium complex, studying relaxivity changes with increasing protein concentration. The intracellular speciation of the complex has been examined by circularly polarized emission spectroscopy and is consistent with the presence of more than one europium species, possibly protein bound.