The influence of the type of substituents and the solvent on the interactions between different coumarins and selected TEMPO analogues - Fluorescence quenching studies

The interactions between a group of different coumarins {7-hydroxycoumarin, 7-methoxycoumarin, 7-hydroxy-6-methoxycoumarin, 7-amino-4-methylcoumarin, 7-hydroxy-4-methylcoumarin, 7-hydroxy-4-(trifluoromethyl)coumarin} and 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) and its C4-substituted derivatives (4-hydroxy-TEMPO, 4-methoxy-TEMPO, 4-carboxy-TEMPO, 4-acetamido-TEMPO, 4-oxo-TEMPO) have been studied mainly in aqueous solutions with the use of spectroscopic techniques - namely UV-Vis absorption as well as steady-state and time-resolved fluorescence spectroscopy - and supported by theoretical calculations of molecular radii and ionization potentials. Since it has been found that the fluorescence intensity of the chosen coumarins is efficiently decreased by the nitroxides, the mechanism of fluorescence quenching has been determined. The work constitutes a continuation of our previous experiments on the interactions between different coumarin derivatives and 4-hydroxy-TEMPO (J Fluoresc 24, 713-718, 2014). The biological significance of this work is proven by the fact that TEMPO nitroxides have already many applications and their detection and quantitative determination under physiological conditions might help to understand the mechanism of an oxidative stress.