Radiation-induced reduction of quinoxalin-2-one derivatives in aqueous solutions

Abstract

Quinoxaline-2-one derivatives have been proposed as potential drugs in treatments of various diseases since some of them showed a variety of pharmacological properties. The kinetics and spectral characteristics of the transients formed in the reactions of hydrated electrons (e(aq)(-)) with quinoxalin-2-(1H)-one (Q) and its methyl derivative, 3-methyl quinoxalin-2-(1H)-one (3-MeQ) were studied by pulse radiolysis in aqueous solutions at pH ranging from 5 to 14. The transient absorption spectra recorded in the reactions of (e(aq)(-)) with Q and 3-MeQ at pH 7 consisted of a broad, almost flat band in the range 390-450 nm and were assigned to the respective protonated radical anions (QH(center dot)/3-MeQH(center dot)) at N4 atom in a pyrazin-2-one ring. On the other hand, the transient absorption spectra recorded in the reactions of (e(aq)(-)) with Q and 3-MeQ at pH 13 are characterized by a broad band with a much better pronounced maximum at lambda(max) = 390 nm and higher intensity (in comparison to that at pH 7) and were assigned to the respective radical anions (Q(center dot-)/3-MeQ(center dot-)). Both forms are involved in the prototropic equilibrium with the pK(a) located at pH >= 13.5. The rate constants of the reactions of (e(aq)(-)) with Q and 3-MeQ were found to be at pH 7 (2.6 +/- 0.1) x 10(10) M-1 s(-1) and (2.1 +/- 0.1) x 10(10) M-1 s(-1) and at pH 13 (1.6 +/- 0.1) x 10(10) M-1 s(-1) and (1.3 +/- 0.1) x 10(10) M-1 s(-1), respectively. Semi-empirical quantum mechanical calculations reproduce fairly well the spectral features of the experimental absorption spectra and show that protonated radical anions at nitrogen atom (N4) in both molecules are the most stable hydrogenated radicals.