Flavonols and Crown-Flavonols as Metal Cation Chelators. The Different Nature of Ba2+ and Mg2+ Complexes

The derivatives of 3-hydroxyflavone exhibit excited-state intramolecular proton transfer (ESIPT) reaction with significant (60−80 nm) shifts of fluorescence spectra between normal and phototautomer forms. This fact makes these compounds attractive as fluorescence probes in analytical chemistry, biophysics, and molecular biology. Different flavonol derivatives, including 4‘-(monoaza-15-crown-5)flavonol, were synthesized, and their absorption and fluorescent spectra were studied in acetonitrile in the presence of different concentrations of Mg2+ and Ba2+ ions. It was shown that the general feature of flavonols is the ability to form two types of complexes with alkaline-earth cations: the low-stability “external” and high-stability chelating complexes. On the formation of the complexes, parent flavonols and their 4‘-dialkylamino derivatives undergo different perturbations of their electronic structures. 4‘-(Monoaza-15-crown-5)flavonol forms two types of complexes with both Mg2+ and Ba2+ ions; the sequence of steps in formation of Ba2+ and Mg2+ complexes is different.