The role of sulfur and heavier chalcogens in the chemistry of antioxidants

Role of Sulphur and Heavier Chalcogens on the Antioxidant Power and Bioactivity of Natural Phenolic Compounds

The activity of natural phenols is primarily associated to their antioxidant potential, but is ultimately expressed in a variety of biological effects. Molecular scaffold manipulation of this large variety of compounds is a currently pursued approach to boost or modulate their properties. Insertion of S/Se/Te containing substituents on phenols may increase/decrease their H-donor/acceptor ability by electronic and stereo-electronic effects related to the site of substitution and geometrical constrains. Oxygen to sulphur/selenium isosteric replacement in resveratrol or ferulic acid leads to an increase in the radical scavenging activity with respect to the parent phenol. Several chalcogen-substituted phenols inspired by Vitamin E and flavonoids have been prepared, which in some cases prove to be chain-breaking antioxidants, far better than the natural counterparts. Conjugation of catechols with biological thiols (cysteine, glutathione, dihydrolipoic acid) is easily achieved by addition to the corresponding ortho-quinones. Noticeable examples of compounds with potentiated antioxidant activities are the human metabolite 5-S-cysteinyldopa, with high iron-induced lipid peroxidation inhibitory activity, due to strong iron (III) binding, 5-S-glutathionylpiceatannol a most effective inhibitor of nitrosation processes, and 5-S-lipoylhydroxytyrosol, and its polysulfides that proved valuable oxidative-stress protective agents in various cellular models. Different methodologies have been used for evaluation of the antioxidant power of these compounds against the parent compounds. These include kinetics of inhibition of lipid peroxidation alkylperoxyl radicals, common chemical assays of radical scavenging, inhibition of the OH• mediated hydroxylation/oxidation of model systems, ferric- or copper-reducing power, scavenging of nitrosating species. In addition, computational methods allowed researchers to determine the Bond Dissociation Enthalpy values of the OH groups of chalcogen modified phenolics and predict the best performing derivative. Finally, the activity of Se and Te containing compounds as mimic of glutathione peroxidase has been evaluated, together with other biological activities including anticancer action and (neuro)protective effects in various cellular models. These and other achievements are discussed and rationalized to guide future development in the field.

Janus -faced oxidant and antioxidant profiles of organo diselenides

To date, organoseleniums are pre-eminent for peroxide decomposition and radical quenching antioxidant activities. On the contrary, here, a series of Janus-faced aminophenolic diselenides have been prepared from substituted 2-iodoaniline and selenium powder using copper-catalyzed methodology. Subsequently, condensation with substituted salicylaldehyde afforded the Schiff base, which on reduction, yielded the desired substituted aminophenolic diselenides in 72%-88% yields. The generation of reactive oxygen species (ROS) from oxygen gas by the synthesized aminophenolic diselenides was studied by analyzing the oxidation of dichlorofluorescein diacetate (DCFDA) dye and para-nitro-thiophenol by fluorescence and UV-Visible spectroscopic methods. Furthermore, density functional theory calculations and crystal structure analysis revealed the role of functional amine and hydroxyl sites present in the Janus-faced organoselenium catalyst for the activation of molecular oxygen, where NH and phenolic groups bring the oxygen molecule close to the catalyst by N-H⋯O and O-H⋯O intermolecular interactions. Additionally, these functionalities stabilize the selenium-centered radical in the formed transition states. Antioxidant activities of the synthesized diselenides have been explored as the catalyst for the decomposition of hydrogen peroxide using benzenethiol sacrificial co-reductant by a well-established thiol assay. Radical quenching antioxidant activity was studied by the quenching of DPPH radicals at 516 nm by UV-Visible spectroscopy. The structure activity correlation suggests that the electron-rich phenol and electron-rich and sterically hindered selenium center enhance the oxidizing property of the aminophenolic diselenides. Janus-faced diselenides were also evaluated for their cytotoxic effect on HeLa cancer cells via MTT assay, which suggests that the compounds are effective at 15-18 μM concentration against cancer cells. Moreover, the combination with therapeutic anticancer drugs Erlotinib and Doxorubicin showed promising cytotoxicity at the nanomolar concentration (8-28 nM), which is sufficient to suppress the growth of the cancer cells.