Abstract
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.
Highlights
Aromatic and heteroaromatic thiosemicarbazones (TSCs) and their transition metal complexes have gained considerable attention due to their coordination chemistry and broad range of biological activities [1–9]
For complexes 1–3, axially symmetric electron paramagnetic resonance (EPR) spectra with unresolved hyperfine couplings were measured in both frequency domains, while better g-tensor components resolution was provided by the Q-band (Figure 1b)
The values of gk and g⊥, refined by computer simulations, of the experimental EPR spectra of complexes 1–3 (Table 1) matched well with the relation gk > g⊥ > 2.0023, which is consistent with the dx2−y2 ground electronic state of the
Summary
Aromatic and heteroaromatic thiosemicarbazones (TSCs) and their transition metal complexes have gained considerable attention due to their coordination chemistry and broad range of biological activities [1–9]. The position and type of substituent on the TSC backbone, along with the character of the metal ion, have a strong impact on the overall properties and stability of the coordination compound, as well as its biological activity. When salicylaldehyde thiosemicarbazone is coupled with prolines a significant increase in water solubility is observed [22–24]. When coordinated to Cu(II) ions, these TSC-derivatives exhibit cytotoxic activity against ovarian carcinoma CH1 cells [25–27]. A new series of sodium 5-sulfonate-salicylaldehyde thiosemicarbazones containing different substituents at the terminal nitrogen atom (L1–L4 in Scheme 1) was synthesized recently, with the aim to obtain water-soluble TSC derivatives with potential biological activity.
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