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

Abstract

The reactions of sterically hindered amines (SHA) were investigated in acetonitrile solutions and TiO(2) 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 KO(2), 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 O(2)(•-) 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 TiO(2) photoexcitation, a series of experiments using different additives (e.g. KO(2), H(2)O(2), NaN(3), 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.

Spectroscopic characterization, photoinduced processes and cytotoxic properties of substitutedN-ethyl selenadiazoloquinolones

“…[30,31] The prepared N-ethyl selenadiazoloquinolones proved sufficient solubility in water, which enables the experiments to be carried out in a pH 7 buffer, thus observing UV/vis spectra also in aqueous environment.…”

“…This method is based on the oxidation of the hydrogen at the tetramethylpiperidine moiety of TMP molecule by the photogenerated singlet oxygen, thus generating semi-stable nitroxide radicals 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (Tempol) and/or 4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (Tempone). [31,51,52] When the photoinduced generation of singlet oxygen was monitored in the system E2h/DMSO/TMP/air, an EPR signal was present during the first 45 s of irradiation for all derivatives whereas even shortly prolonged exposure caused its termination (data not shown). Analogous sets of EPR spectra have already been recorded previously by the study of TMP photooxidation in DMSO in the presence of different quinolone derivatives.…”

“…4b). This may be caused by the decomposition of superoxide radical anion, [31] as well as by the decreased stability of the spinadduct…”

Scope and limitations of the TEMPO/EPR method for singlet oxygen detection: the misleading role of electron transfer

“…The EPR signal of the free radical TEMPO (g = 2.0060, a N = 17.3 G) generated by reaction of singlet oxygen with TEMP was recorded [31]. The measurements were performed in a Wildman…”

Mechanistic Studies on the Phototoxicity of Rosuvastatin, Itraconazole and Imatinib

“…The EPR signal of the free radical TEMPO (g = 2.0060, a N = 17.3 G) generated by reaction of singlet oxygen with TEMP was recorded [123]. The measurements were performed in a Wildman Suprasil/aqueous quartz ware flat cell (volume of 150 µL, 60 mm of length) with a Bruker EMX 10/12 EPR spectrometer, using the following parameters: microwave power, 20 mW; modulation amplitude, 1.0 G; and modulation frequency, 100 kHz.…”