The coumarin moiety as chromophore of fluorescent ion indicators in biological systems

Fluorescent probes have evolved into an extremely useful tool for the detection of ions in biological systems. The design of ion indicators is based in the proper choice of the ion chelating group as well as the chromophore moiety. The chromophores of choice should fulfill a number of requirements concerning the photostability of the group, the range of the excitation and emission wavelengths of the indicators, the Stokes shift, the fluorescence quantum yield, the excitation and/or emission wavelength shift upon coordination of the probe with its target ion, the lipophilicity of the indicators, and their possible cell toxicity. Coumarin and its analogues have been extensively used in ion detection by incorporation of the coumarin chromophore in the larger indicator framework. Coumarins fulfill all the aforementioned requirements since they are relatively photostable and their excitation and emission maxima, in many cases, are long enough to minimise "background" fluorescence of cellular components, tissues and biological fluids. They exhibit Stokes shifts large enough to avoid significant overlap of the excitation and emission spectra, their fluorescence quantum yields allow for ion detection at low indicator concentrations, and they can be introduced to cells either by microinjection or as membrane permeable derivatives without causing cell death. Synthetic approaches, aiming at the optimisation of indicator properties, have extended the conjugated coumarin system either by introduction of substituents or by expansion of the heterocyclic system. In this review, the basic rationale for the selection of the particular coumarin analogues is analysed, synthetic pathways leading to the desired structures are presented, and properties and relative advantages in the use of these probes are described.