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

Abstract

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 Mg 2+ and Ba 2+ 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 Mg 2+ and Ba 2+ ions; the sequence of steps in formation of Ba 2+ and Mg 2+ complexes is different.

Design of Cation‐Selective Synthetic Fluorescent Indicators

“…Consistent with the disruption of ESIPT, addition of Mg 2+ produces a new blue-shifted emission band (Fig. 18.25) [56]. The absorption spectrum shows a strong red shift, indicating increased charge-transfer character of the Mg 2+ coordinated species MgL.…”

Proton Transfer Reactions in the Excited Electronic State

“…The functional derivatives of 3-hydroxyflavone and 3-hydroxybenzofuranochromone dyes have found application as molecular sensors in ion sensing [120], micelle [121] and phospholipid vesicle [122,123] research, in ligand binding [124] and the studies of conformational changes in proteins [125] and in cell imaging [126]. For these dyes, on change in their physical environment (polarity, hydrogen bonding, local electric fields), the switching between two different colours of emission, usually violet-green and orange-red, is commonly observed, and this can provide a strong response in sensing.…”

Fluorescence sensing of intermolecular interactions and development of direct molecular biosensors

“…Derivatives of these dyes for covalent labeling of amino and SH-groups were also obtained (Klymchenko et al, 2004a), which can be used in various types of protein and peptide biosensors. Their azacrown ether derivatives exhibit dramatic spectral responses to the binding of bivalent cations (Roshal et al, 1998;Roshal et al, 1999). Therefore these reporters can provide a link between chemical sensing and biosensing.…”