4-oxo-4H-quinolizine-3-carboxylic acids as Mg2+ selective, fluorescent indicators

In this paper, we present the synthesis of a new series of substituted 4-oxo-4H-quinolizine-3-carboxylic acids and the evaluation of their fluorescent response upon complexation with Mg2+ and Ca2+. This has led to the development of the first Mg2+-selective, ratioable fluorescent indicators. We found that 4-oxo-4H-quinolizine-3-carboxylates are excellent fluorophores which show a strong fluorescent response to Mg2+ but not to Ca2; the latter metal ion often causing interference during Mg2+ measurements with previously described indicators. The dissociation constants of the studied fluorophores-around 1.0 mM-are of the same order of magnitude as is usually observed for intracellular Mg2+.