Zuzana Barbieriková

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.

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).

Photoinduced Oxidation of Sterically Hindered Amines in Acetonitrile Solutions and Titania Suspensions (An EPR Study)

The reactions of sterically hindered amines (SHA) were investigated in acetonitrile solutions and TiO2 suspensions upon exposure to monochromatic radiation, λ = 365 nm, by means of in situ EPR spectroscopy. The formation of singlet oxygen, as one of the possible oxidation agents for SHA, in these systems is affected significantly by solvent used and the experimental conditions. Experiments in homogeneous media evidenced alternative pathways for the SHA oxidation with a variety reactive oxygen species involved. In anhydrous acetonitrile solutions containing KO2, the SHA oxidation was negligible not only in the dark but also on continuous exposure. However, the presence of water, even at low concentrations, led to the transformation of O2•− to singlet oxygen and hydrogen peroxide, which served as a source of hydroxyl radicals. These species participated in oxidation of SHA resulting in the generation of nitroxide radicals. To investigate the influence of different competitive reactions of SHA with other ROS formed upon TiO2 photoexcitation, a series of experiments using different additives (e.g. KO2, H2O2, NaN3, dimethylsulfoxide, methanol as organic cosolvents) under air or argon were performed. The detailed analysis of paramagnetic intermediates formed upon the irradiation of the studied systems was accomplished using EPR spin trapping technique.

Stable radical trianions from reversibly formed sigma-dimers of selenadiazoloquinolones studied by in situ EPR/UV-vis spectroelectrochemistry and quantum chemical calculations.

The redox behavior of the series of 7-substituted 6-oxo-6,9-dihydro[1,2,5]selenadiazolo[3,4-h]quinolines and 8-substituted 9-oxo-6,9-dihydro[1,2,5]selenadiazolo[3,4-f]quinolines with R(7), R(8) = H, COOC(2)H(5), COOCH(3), COOH, COCH(3), and CN has been studied by in situ EPR and EPR/UV-vis spectroelectrochemistry in dimethylsulfoxide. All selenadiazoloquinolones undergo a one-electron reduction process to form the corresponding radical anions. Their stability strongly depends on substitution at the nitrogen atom of the 4-pyridone ring. The primary generated radical anions from N-ethyl-substituted quinolones are stable, whereas for the quinolones with imino hydrogen, the initial radical anions rapidly dimerize to produce unusually stable sigma-dimer (σ-dimer) dianions. These are reversibly oxidized to the initial compounds at potentials considerably less negative than the original reduction process in the back voltammetric scan. The dimer dianion can be further reduced to the stable paramagnetic dimer radical trianion in the region of the second reversible reduction step. The proposed complex reaction mechanism was confirmed by in situ EPR/UV-vis cyclovoltammetric experiments. The site of the dimerization in the σ-dimer and the mapping of the unpaired spin density both for radical anions and σ-dimer radical trianions with unusual unpaired spin distribution have been assigned by means of density functional theory calculations.

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.

UVA-induced effects of 2,6-disubstituted 4-anilinoquinazolines on cancer cell lines

Five 2,6-substituted 4-anilinoquinazolines were evaluated for their ability to generate superoxide radical anion and singlet oxygen upon UVA irradiation and to induce cytotoxic/phototoxic effects on cancer cell lines L1210, HeLa and HT-29. The formation of radical intermediates, especially reactive oxygen species, upon UVA photoexcitation of the studied derivatives was monitored by indirect techniques of EPR spectroscopy. For all 4-anilinoquinazolines the photoinduced generation of superoxide radical anion was evidenced using spin trapping agent 5,5-dimethyl-1-pyrroline N-oxide, and the presence of (1)O2 was detected by the oxidation of 4-hydroxy-2,2,6,6-tetramethylpiperidine to the paramagnetic species 4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl. The confirmed photoinduced activation of molecular oxygen via both Type I and Type II photooxidation mechanisms indicates potential phototoxic responses in cells. Biological results showed that derivatives I-V initiated different cytotoxic/phototoxic effects dependent on their concentration, time of treatment and the character of the cell line. UVA irradiation increased the cytotoxic activity of all tested 4-anilinoquinazoline derivatives. The highest cytotoxicity/phototoxicity on all tested cancer cells was induced by N,2-diphenyl-quinazolin-4-amine (derivative III). This most effective derivative emerged as the potent photosensitizer, which possesses a significant antiproliferative activity and DNA damage in L1210 cells increased by UVA irradiation. In addition derivative III induced programmed cell death in leukemia cells through mitochondrial/caspase 9/caspase 3-dependent pathway.

Anodic oxidation of selenadiazoloquinolones in alkaline media

Newly synthesized derivatives of 6‐oxo‐6,9‐dihydro[1,2,5]selenadiazolo[3,4‐h]quinoline variously substituted at position 7 (R = H, COOH, COCH3, CN, COOC2H5 and COOCH3) are established in strongly alkaline aqueous solutions (0.1 M NaOH; pH ∼ 13) as N(9)‐deprotonated structures, but in less alkaline solutions (0.001 M NaOH; pH ∼ 11) the N(9)‐protonated oxo tautomeric forms dominate. Upon their anodic oxidation in alkaline solutions, the selenadiazole ring is replaced, forming instead the paramagnetic species analogous to the ortho semiquinone radical anions as monitored by in situ EPR spectroscopy. The quantum chemical calculations for two representative selenadiazoloquinolones (R = H and COOH) and their anodic oxidation products presented are in agreement with experiments. Copyright

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.

Redox processes of 2,6-dichlorophenolindophenolate in different solvents. A combined electrochemical, spectroelectrochemical, photochemical, and theoretical study

Cyclic voltammetric and EPR/UV-vis-NIR spectroelectrochemical studies were performed to examine the cathodic reduction of 2,6-dichlorophenolindophenolate (DCIP) in proton-donating aqueous and methanol solutions, as well as in aprotic dimethylsulfoxide (DMSO), and to characterize the paramagnetic species generated upon the DCIP reduction. In situ EPR and UV-vis-NIR spectroelectrochemistry confirmed the formation of the radical anion DCIP•– in DMSO and methanol. The same radical anion was found also in the reaction system consisting of KO2 mixed under argon with DCIP in DMSO or methanol, evidencing the electron transfer from superoxide radical anion to DCIP. The expected radical anion DCIP•– was not detected in the photoexcited suspensions DCIP/TiO2/DMSO under argon, which indicates fast consecutive reactions of photogenerated DCIP•– in the vicinity of TiO2 surface. The reduction of blue-color DCIP to the final colorless product DCIPH2 can be realized in multiple reaction pathways determined mainly by the proton-donating capacity of the solvent. Following the calculated total DFT energies, the oxygen on the indophenol moiety represents the first proton acceptor site for DCIP, DCIP•–, as well as for DCIP2– species.

Protonation and electronic structure of 2,6-dichlorophenolindophenolate during reduction. A theoretical study including explicit solvent

AbstractProtonation in the two-electron/two-proton reduction processes of 2,6-dichlorophenolindophenolate (DCIP) is investigated combining density functional theory (DFT) and molecular dynamics (MD) methods. DCIP (anion), DCIP•– (radical anion), and DCIP2− (dianion) are considered, including the electronic structure analysis from the prospective of quantum theory of atoms and molecules (QTAIM). It is shown that oxygen on the indophenolate moiety and nitrogen are the first and/or the second proton acceptor sites and their energetic order depends on the total charge of the system. MD simulations of differently charged species interacting with the solvent molecules have been performed for methanol, water, and oxonium cation (H3O+). Methanol and water molecules are found to form only hydrogen bonds with the solute irrespective of its charge. The calculated pKa values show that the imino group of DCIPH− is a weaker acid than water. While in the case of DCIP (and DCIP•–) plus oxonium cation, proton transfer from the solvent to the solute was evidenced for both aforementioned acceptor sites. In addition, MD simulations of bulks containing 15 and 43 molecules of water around the DCIP molecule have been performed, revealing the formation of 2–4 hydrogen bonds. Graphical Abstract2,6-Dichlorophenolindophenolate interacts with solvent molecules (water, oxonium cation and methanol). Hydrogen transfer and electronic structure are studied by DFT and molecular dynamics methods