Dana Dvoranová

EPR study of photoinduced reduction of nitroso compounds in titanium dioxide suspensions

Abstract The radical intermediates produced upon UV irradiation of deoxygenated alcoholic titanium dioxide suspensions of nitrosobenzene, nitrobenzene, 2-nitrosotoluene, 2,3,5,6-tetramethylnitrosobenzene, 3,5-di-bromo-4-nitrosobenzenesulfonate (sodium salt), 2,4,6-tri-t-butyl-nitroso-benzene, and 2-methyl-2-nitrosopropane were investigated using in situ EPR technique. Nitrosobenzene is efficiently photoreduced in TiO2 suspensions (toluene/alcohol, 1:1 (v/v)) forming exclusively one stable radical intermediate corresponding to C6H5N OH species. The formation of this radical species is consistent with the proposed photocatalytic reduction mechanism, occurring from the primary generated nitrosobenzene mono-anion by the hydrogen abstraction from surroundings. The origin of hydrogen added to the nitroso group was demonstrated by the photocatalytic experiments using deuterated methanol, where the production of C6H5N OD was established. Additionally, an identical radical C6H5N OH was detected, when nitrobenzene was reduced under analogous experimental conditions. The photoinduced electron transfer from TiO2 to nitroso compounds is accompanied by alcohol oxidation via the photogenerated titanium dioxide valance band holes forming alkoxy and hydroxyalkyl radicals. Production of hydroxyalkyl radicals ( CH2OH, CH(OH)CH3, C(OH)(CH3)2) with redox potentials suitable for a direct electron transfer to nitroso compounds represents an alternative reaction pathway for their reduction. On the other hand, the investigated nitroso derivatives are efficient spin-trapping agents, therefore, formation of nitroxyl radical spin adducts was observed in the photocatalytic experiments. The EPR spectra monitored upon irradiation of substituted nitrosobenzene derivatives in alcoholic TiO2 suspensions reveal the correlation between nitrosobenzene derivative first step reduction potentials and yield of radical species produced.

Radical Intermediates in Photoinduced Reactions on TiO2 (An EPR Spin Trapping Study)

The radical intermediates formed upon UVA irradiation of titanium dioxide suspensions in aqueous and non-aqueous environments were investigated applying the EPR spin trapping technique. The results showed that the generation of reactive species and their consecutive reactions are influenced by the solvent properties (e.g., polarity, solubility of molecular oxygen, rate constant for the reaction of hydroxyl radicals with the solvent). The formation of hydroxyl radicals, evidenced as the corresponding spin-adducts, dominated in the irradiated TiO2 aqueous suspensions. The addition of 17O-enriched water caused changes in the EPR spectra reflecting the interaction of an unpaired electron with the 17O nucleus. The photoexcitation of TiO2 in non-aqueous solvents (dimethylsulfoxide, acetonitrile, methanol and ethanol) in the presence of 5,5-dimethyl-1-pyrroline N-oxide spin trap displayed a stabilization of the superoxide radical anions generated via electron transfer reaction to molecular oxygen, and various oxygen- and carbon-centered radicals from the solvents were generated. The character and origin of the carbon-centered spin-adducts was confirmed using nitroso spin trapping agents.

Characterization of titanium dioxide photoactivity following the formation of radicals by EPR spectroscopy

In order to find ways to characterize oxygen-saturated aqueous TiO2 suspensions, the formation of photo-induced free radicals was followed by EPR spectroscopy, using as indicators N-oxide and nitrone spin trapping agents, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), 3,3,5,5-tetramethyl-1-pyrroline N-oxide (TMPO), α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POB N), 4-(N-methylpyridyl)-N-tert-butylnitrone (MePyBN), as well as semi-stable free radicals, 4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl (TEMPOL), cation radical of 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), diammonium salt (ABTS) and 1,1-diphenyl-2-picrylhydrazyl (DPPH). DMPO and TMPO are efficiently oxidized to the EPR-silent products via radical in termediates. Conversely, the nitrone spin traps (POBN and MePyBN) showed selective formation of hydroxyl radical spin adducts upon continuous irradiation of oxygenated TiO2 suspensions. Their concentrations increased proportionally with the amount of photocatalyst and irradiation time. The EPR spectrum of the semi-stable free radicals TEMPOL, ABTS·+ or DPPH is gradually eliminated during irradiation, and this system represents a simple technique for the evaluation of TiO2 activity.

Photoinduced Superoxide Radical Anion and Singlet Oxygen Generation in the Presence of Novel Selenadiazoloquinolones (An EPR Study)

Novel 7‐substituted 6‐oxo‐6,9‐dihydro[1,2,5]selenadiazolo[3,4‐h]quinoline (SeQ(1–6)) and 8‐substituted 9‐oxo‐6,9‐dihydro[1,2,5]selenadiazolo[3,4‐f ]quinoline derivatives (SeQN(1–5)) with R7, R8 = H, COOC2H5, COOCH3, COOH, COCH3 or CN were synthesized and their spectral characteristics were obtained by UV/Vis spectroscopy. Ultraviolet A photoexcitation of the selenadiazoloquinolones in dimethylsulfoxide or acetonitrile resulted in the formation of paramagnetic species coupled with molecular oxygen activation generating the superoxide radical anion or singlet oxygen, evidenced by electron paramagnetic resonance spectroscopy. The cytotoxic/photocytotoxic impact of selenadiazoloquinolones on murine and human cancer cell lines was demonstrated using the derivative SeQ5 (with R7 = COCH3).

Copper(II) complexes with new fluoroquinolones: Synthesis, structure, spectroscopic and theoretical study, DNA damage, cytotoxicity and antiviral activity.

Copper(II) complexes with fluoroquinolones in the presence of the nitrogen donor heterocyclic ligands 1,10-phenanthroline have been considered in detail. The phenanthroline moiety was introduced into the ligand environment with the aim to determine whether the nuclease activity is feasible. All suitable X-ray structures of the complexes under study reveal a distorted square pyramidal coordination geometry for Cu(II) atom. The conformational and spectroscopic (FT-IR and UV-visible) behavior has been analyzed and has been interpreted with respect to B3LYP/6-311G* calculations including molecular dynamics. The ability of the complexes to cleave DNA was studied by agarose gel electrophoresis with plasmid DNA pBSK+. The results have confirmed that the complexes under study behave as the chemical nucleases. Nuclease like activity in the absence of hydrogen peroxide allows us to deduce an interaction of the complexes with the DNA resulting in the conversion of supercoiled circular DNA to the nicked form. The DNA cleavage activity enhanced by the presence of hydrogen peroxide demonstrates the participation of reactive oxygen species, such as superoxide radical anions and hydroxyl radicals which presence was confirmed independently using the standard radical scavenging agents. It has been suggested that the radical formation through the Fenton/Haber-Weiss reaction is mediated by the redox cycling mechanisms with the participation of cupric/cuprous ions. Cytotoxic activity was evaluated as the 50% cytotoxic concentration (CC50). The potential effects of tested compounds on replication of murine gammaherpesvirus 68 (MHV-68) under in vitro conditions were also evaluated. However, no antiviral activity against MHV-68 was observed.

Fused-ring derivatives of quinoxalines: spectroscopic characterization and photoinduced processes investigated by EPR spin trapping technique.

10-Ethyl-7-oxo-7,10-dihydropyrido[2,3-f]quinoxaline derivatives, synthesized as promising biologically/photobiologically active compounds were characterized by UV/vis, FT-IR and fluorescent spectroscopy. Photoinduced processes of these derivatives were studied by EPR spectroscopy, monitoring in situ the generation of reactive intermediates upon UVA (λmax = 365 nm) irradiation. The formation of reactive oxygen species and further oxygen- and carbon-centered radical intermediates was detected and possible reaction routes were suggested. To quantify the investigated processes, the quantum yields of the superoxide radical anion spin-adduct and 4-oxo-2,2,6,6-tetramethylpiperidine N-oxyl generation were determined, reflecting the activation of molecular oxygen by the excited state of the quinoxaline derivative.

Conformational, spectroscopic, and molecular dynamics DFT study of precursors for new potential antibacterial fluoroquinolone drugs.

Biological activity, functionality, and synthesis of (fluoro)quinolones is closely related to their precursors (for instance 3-fluoroanilinoethylene derivatives) (i.e., their functional groups, conformational behavior, and/or electronic structure). Herein, the theoretical study of 3-fluoroanilinoethylene derivatives is presented. Impact of substituents (acetyl, methyl ester, and ethyl ester) on the conformational analysis and the spectral behavior is investigated. The B3LYP/6-311++G** computational protocol is utilized. It is found that the intramolecular hydrogen bond N-H···O is responsible for the energetic preference of anti (a) conformer (anti position of 3-fluoroanilino group with respect to the C═C double bond). The Boltzmann ratios of the conformers are related to the differences of the particular dipole moments and/or their dependence on the solvent polarity. The studied acetyl, ethyl ester, and methyl ester substituted fluoroquinolone precursors prefer in the solvent either EZa, ZZa, or both conformers equally, respectively. In order to understand the degree of freedom of rotation of the trans ethyl ester group, B3LYP/6-311G** molecular dynamic simulations were carried out. Vibrational frequencies, electron transitions, as well as NMR spectra are analyzed with respect to conformational analysis, including the effect of the substituent. X-ray structures of the precursors are presented and compared with the results of the conformational analysis.

Building a library of monofluorinated anilino-methylidenes and spectroscopic studies of their properties

The synthesis of monofluorinated anilinomethylidenes is described. These compounds can serve as building blocks for biologically active fluoroquinolones. The structures of the target compounds have been proved by 1H, 13C, IR, UV, and HR-MS analysis. In addition, 19F NMR spectra of the products were recorded to study the influence of the fluorine atom on the aminomethylidene substituent.Graphical abstract.

UVA-Induced Processes in the Aqueous Titanium Dioxide Suspensions Containing Nitrite (An EPR Spin Trapping Study)

Abstract Application of TiO2 photocatalytic systems for water purification and remediation is based on the generation of short-lived reactive oxygen species able to destroy a variety of contaminants, upon the ultra-bandgap irradiation of TiO2 particles in the aerated aqueous media. However the recently more profound presence of inorganic nitrogen compounds can affect these processes due to the complex photochemical behavior of the nitrite and nitrate in aqueous solutions. The effect of the nitrite present in the titanium dioxide suspensions was monitored via the reactive radical intermediates detected by EPR spin trapping technique. Various spin trapping agents were applied to follow the changes in the behavior of the system caused by the nitrite upon UVA irradiation and the limits of the spin trapping technique itself were also considered. The competition reaction of the photogenerated holes and hydroxyl radicals with the nitrite was revealed as the dominant process occurring in the studied systems.

Synthesis and Herbicidal Activity of New Hydrazide and Hydrazonoyl Derivatives

Three new hydrazide and five new hydrazonoyl derivatives were synthesized. The chemical structures of these compounds were confirmed by 1H-NMR, IR spectroscopy and elemental analysis. The prepared compounds were tested for their activity to inhibit photosynthetic electron transport in spinach chloroplasts and growth of the green algae Chlorella vulgaris. IC50 values of these compounds varied in wide range, from a strong to no inhibitory effect. EPR spectroscopy showed that the active compounds interfered with intermediates Z•/D•, which are localized on the donor side of photosystem II. Fluorescence spectroscopy suggested that the mechanism of inhibitory action of the prepared compounds possibly involves interactions with aromatic amino acids present in photosynthetic proteins.