The g-C₃N₄/TiO₂ nanopowders prepared by the annealing of melamine and TiO₂ P25 at 550 °C were investigated under dark and upon UV or visible-light photoactivation using X- and Q-band electron paramagnetic resonance (EPR) spectroscopy. The EPR spectra of powders monitored at room temperature and 100 K showed the impact of the initial loading ratio of melamine/TiO₂ on the character of paramagnetic centers observed. For the photocatalysts synthesized using a lower titania content, the paramagnetic signals characteristic for the g-C₃N₄/TiO₂ nanocomposites were already found before exposure. The samples annealed using the higher TiO₂ loading revealed the photoinduced generation of paramagnetic nitrogen bulk centers (g-tensor components g₁ = 2.005, g₂ = 2.004, g₃ = 2.003 and hyperfine couplings from the nitrogen A₁ = 0.23 mT, A₂ = 0.44 mT, A₃ = 3.23 mT) typical for N-doped TiO₂. The ability of photocatalysts to generate reactive oxygen species (ROS) upon in situ UV or visible-light photoexcitation was tested in water or dimethyl sulfoxide by EPR spin trapping using 5,5-dimethyl 1-pyrroline N-oxide. The results obtained reflect the differences in photocatalyst nanostructures caused by the differing initial ratio of melamine/TiO₂; the photocatalyst prepared by the high-temperature treatment of melamine/TiO₂ wt. ratio of 1:3 revealed an adequate photoactivity in both spectral regions.

Redox active transition metal ions (e.g., iron and copper) have been implicated in the etiology of many oxidative stress-related diseases including also neurodegenerative disorders. Unbound copper can catalyze formation of reactive oxygen species (hydroxyl radicals) via Fenton reaction/Haber–Weiss chemistry and therefore, under physiological conditions, free copper is potentially toxic and very rarely exists inside cells. Copper(II) bound to the aminoacid L-histidine represents a species discovered in blood in the mid 60s and since then extensive research on this complex was carried out. Copper bound to L-histidine represents an exchangeable pool of copper(II) in equilibrium with the most abundant blood plasma protein, human serum albumin. The structure of this complex, in aqueous solution, has been a subject of many studies and reviews, however without convincing success. The significance of the (1:2) copper(II)-L-histidine complex at physiological pH documents its therapeutic applications in the treatment of Menkes disease and more recently in the treatment of infantile hypertrophic cardioencephalomyopathy. While recently the (1:2) Cu(II)-L-His complex has been successfully crystallized and the crystal structure was solved by X-ray diffraction, the structure of the complex in fluid solution at physiological pH is not satisfactorily known. The aim of this paper is to study the (1:2) Cu(II)-L-histidine complex at low temperatures by X-band and S-band EPR spectroscopy and at physiological pH at room temperature by Fourier transform CW-EPR spectroscopy.

X- and Q-band electron paramagnetic resonance (EPR) spectroscopy was used to characterize polycrystalline Cu(II) complexes that contained sodium 5-sulfonate salicylaldehyde thiosemicarbazones possessing a hydrogen, methyl, ethyl, or phenyl substituent at the terminal nitrogen. The ability of thiosemicarbazone proligands to generate superoxide radical anions and hydroxyl radicals upon their exposure to UVA irradiation in aerated aqueous solutions was evidenced by the EPR spin trapping technique. The UVA irradiation of proligands in neutral or alkaline solutions and dimethylsulfoxide (DMSO) caused a significant decrease in the absorption bands of aldimine and phenolic chromophores. Mixing of proligand solutions with the equimolar amount of copper(II) ions resulted in the formation of 1:1 Cu(II)-to-ligand complex, with the EPR and UV-Vis spectra fully compatible with those obtained for the dissolved Cu(II) thiosemicarbazone complexes. The formation of the complexes fully inhibited the photoinduced generation of reactive oxygen species, and only subtle changes were found in the electronic absorption spectra of the complexes in aqueous and DMSO solutions upon UVA steady-state irradiation. The dark redox activity of copper(II) complexes and proligand/Cu(II) aqueous solutions towards hydrogen peroxide which resulted in the generation of hydroxyl radicals, was confirmed by spin trapping experiments.