Melanins are stable and non‐toxic biomaterials with a great potential as chemopreventive agents for diseases connected with oxidative stress, but the mechanism of their antioxidant action is unclear. Herein, we show that polydopamine (PDA), a well‐known synthetic melanin, becomes an excellent trap for alkylperoxyl radicals (ROO^., typically formed during autoxidation of lipid substrates) in the presence of hydroperoxyl radicals (HOO^.). The key reaction explaining this peculiar antioxidant activity is the reduction of the ortho‐quinone moieties present in PDA by the reaction with HOO^.. This reaction occurs via a H‐atom transfer mechanism, as demonstrated by the large kinetic solvent effect of the reaction of a model quinone (3,5‐di‐tert‐butyl‐1,2‐benzoquinone) with HOO^. (k=1.5×10⁷ and 1.1×10⁵ M⁻¹ s⁻¹ in PhCl and MeCN). The chemistry disclosed herein is an important step to rationalize the redox‐mediated bioactivity of melanins and of quinones.

Palladium(II) complexes attract great attention due to their remarkable catalytic and biological activity. In the present study X-ray characterization, UV-Vis and Time-Dependent Density Functional Theory (TD-DFT) calculations for six PdCl₂(XPy)₂ complexes (where: Py = pyridine; X = H, CH₃ or Cl) were applied in order to investigate substituent effects on their crystal structures and electronic properties and to combine the results with their catalytic and cytotoxic activity. The structures of complexes PdCl₂(3-MePy)₂, PdCl₂(4-MePy)₂ and PdCl₂(2-ClPy)₂, have been described for the first time and we compared our results with available data for the whole series of six complexes. All compounds exhibit a square planar coordination geometry in which the palladium ion coordinates two nitrogen atoms of pyridine ligands and two chlorine atoms in trans positions. For complexes with ortho substituted XPy ligands a cis disposition of substituents takes place, whereas for other ligands: 3-MePy and 3-ClPy – the substituents are in trans positions. For XPy the energies of π–π* and n–π* transitions depend on the position and nature of the X substituent in the XPy ring. After complex formation a hipsochromic shift (24–34 nm) of π–π* and a bathochromic shift of n–π* bands are observed. The UV-Vis spectra of PdCl₂(XPy)₂ confirm that square planar coordination geometry of complexes I–VI and two dπ–π* transitions are expected. With the help of the TD-DFT calculations we proved that dπ–π* transitions in solutions of PdCl₂(XPy)₂ complexes result from MLCT (metal-to-ligand charge transfer) with contribution from chlorine atoms to palladium. We also studied substituent effects on cytotoxic properties of Pd(II) complexes against the human breast cancer cell line MCF7, the human prostate cancer cell line PC3, and the human T-cell lymphoblast-like cell line CCRF. The studied complexes were the most active against the CCRF cell line and less or even no cytotoxic effect was observed for PC3 cells. Complexes with MePy ligands showed increased cytotoxic activity compared to unsubstituted pyridine ligands.

Vitamin E is the most active natural lipophilic antioxidant with a broad spectrum of biological activity. α-Tocopherol (α-T), the main representative of the vitamin E family, is a strong inhibitor of lipid peroxidation as a chain-breaking antioxidant. Antioxidant and antiradical properties of vitamin E result from the presence of a phenolic hydroxyl group at the C-6 position. Due to stereoelectronic effects in the dihydropyranyl ring, the dissociation enthalpy for phenolic O–H bond (BDE_OH) is reduced. The high chain-breaking reactivity of α-T is mainly attributed to orbital overlapping of the 2p-type lone pair on the oxygen atom (O1) in para position to the phenolic group, and the aromatic π-electron system. The influence of the O1 atom on the antioxidant activity of vitamin E was estimated quantitatively. The all-rac-1-carba-α-tocopherol was synthesized for the first time. Along with model compounds, 1-carba-analog of Trolox and its methyl ester were screened for their in vitro antioxidant activity by inhibition of styrene oxidation, and for the radical-reducing properties by means of 2,2-diphenyl-1-picrylhydrazyl free radical (DPPH) scavenging assay. To study the antioxidant activity, density functional theory (DFT) was also applied. Reaction enthalpies related to HAT (hydrogen atom transfer), SET–PT (sequential electron transfer—proton transfer), and SPLET (sequential proton loss—electron transfer) mechanisms were calculated.

The transformation of aromatic nitrocompounds into amines by CO/H₂O is catalyzed by palladium(II) complexes. Recently, we have proposed that the catalytic cycle includes Pd⁰ as the transient intermediate and herein, for the first time, we describe the application of palladium nanoparticles (PdNPs) stabilized by monodentate N-heterocyclic ligands as nanocatalysts facilitating the reduction of Ar–NO₂ into Ar–NH₂ by CO/H₂O. Among the series—Pd(II) complexes, PdNPs and commercial Pd_black—the highest catalytic activity was observed for PdNPs (3.0 ± 0.5 nm) stabilized by 4-Me-pyridine in the presence of 2-Cl-pyridine. The results may be helpful for mechanistic considerations on the role of metallic nanoparticles as active species in other organic processes.