Hydrogen Atom Transfer from HOO. to ortho-Quinones Explains the Antioxidant Activity of Polydopamine.

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.

Antioxidant activity of two edible isothiocyanates: Sulforaphane and erucin is due to their thermal decomposition to sulfenic acids and methylsulfinyl radicals.

Sulforaphane(SFN) and erucin(ERN) are isothiocyanates (ITCs) bearing, respectively, methylsulfinyl and methylsulfanyl groups. Their chemopreventive and anticancer activity is attributed to ability to modulate cellular redox status due to induction of Phase 2 cytoprotective enzymes (indirect antioxidant action) but many attempts to connect the bioactivity of ITCs with their radical trapping activity failed. Both ITCs are evolved from their glucosinolates during food processing of Cruciferous vegetables, therefore, we studied antioxidant behaviour of SFN/ERN at elevated temperature in two lipid systems. Neither ERN nor SFN inhibit the oxidation of bulk linolenic acid (below 100  °C) but both ITCs increase oxidative stability of soy lecithin (above 150 °C). On the basis of GC-MS analysis we verified our preliminary hypothesis (Antioxidants2020, 9, 1090) about participation of sulfenic acids and methylsulfinyl radicals as radical trapping agents responsible for the antioxidant effect of edible ITCs during thermal oxidation of lipids at elevated temperatures (above 140 °C).

A Lesson Learnt from Food Chemistry-Elevated Temperature Triggers the Antioxidant Action of Two Edible Isothiocyanates: Erucin and Sulforaphane.

In this communication we demonstrate that two natural isothiocyanates, sulforaphane (SFN) and erucin (ERN), inhibit autoxidation of lipids at 140 °C but not below 100 °C. This effect is due to thermal decomposition of ERN and SFN to sulfenic acids and methylsulfinyl radicals, species able to trap lipidperoxyl radicals. Our observations shed new light on thermal processing of vegetables containing these two isothiocyanates.

Synthesis, DFT Calculations, and In Vitro Antioxidant Study on Novel Carba-Analogs of Vitamin E.

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 (BDEOH) 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.

Hydroperoxyl Radicals (HOO. ): Vitamin†E Regeneration and H-Bond Effects on the Hydrogen Atom Transfer.

Hydroperoxyl (HOO. ) and alkylperoxyl (ROO. ) radicals show a different behavior in H-atom-transfer processes. Both radicals react with an analogue of α-tocopherol (TOH), but HOO. , unlike ROO. , is able to regenerate TOH by a fast H-atom transfer: TO. +HOO. →TOH+O2 . The kinetic solvent effect on the H-atom transfer from TOH to HOO. is much stronger than that observed for ROO. because noncovalent interactions with polar solvents (Solv⋅⋅⋅HOO. ) destabilize the transition state.

Biological activity of Pd(II) complexes with mono- and disubstituted pyridines--experimental and theoretical studies.

Due to the similarity between Pd and Pt, the complexes of palladium(II) can be considered as potential anticancer agents. Activity of six PdCl2(X2Py)2 complexes (Py=pyridine, and X=CH3 or Cl) was measured by MTT test using MCF7, CCRF-SB, PC3 and human B-lymphoblastoid cell lines. We found that the effect of PdCl2(XnPy)2 was cell-specific and time-dependent. Obtained results were discussed and compared with the activation parameters calculated for the hydrolysis of PdCl2(XnPy)2, indicating a correlation between viability of MCF7 and CCRF-SB cells and rates of hydrolysis of the Pd(II) complexes.

Crystal structure, electronic properties and cytotoxic activity of palladium chloride complexes with monosubstituted pyridines.

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 PdCl(2)(XPy)(2) complexes (where: Py = pyridine; X = H, CH(3) 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 PdCl(2)(3-MePy)(2), PdCl(2)(4-MePy)(2) and PdCl(2)(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 PdCl(2)(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 PdCl(2)(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.