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A computational protocol aimed to design new antioxidants with versatile behavior is presented. It is called Computer-Assisted Design of Multifunctional Antioxidants and is based on chemical properties (CADMA-Chem). The desired multi-functionality consists of in different methods of antioxidant protection combined with neuroprotection, although the protocol can also be used to pursue other health benefits. The dM38 melatonin derivative is used as a study case to illustrate the protocol in detail. This was found to be a highly promising candidate for the treatment of neurodegeneration, in particular Parkinson's and Alzheimer's diseases. This also has the desired properties of an oral-drug, which is significantly better than Trolox for scavenging free radicals, and has chelates redox metals, prevents the ●OH production, via Fenton-like reactions, repairs oxidative damage in biomolecules (lipids, proteins, and DNA), and acts as a polygenic neuroprotector by inhibiting catechol-O-methyl transferase (COMT), acetylcholinesterase (AChE) and monoamine oxidase B (MAOB). To the best of our best knowledge, CADMA-Chem is currently the only protocol that simultaneously involves the analyses of drug-like behavior, toxicity, manufacturability, versatile antioxidant protection, and receptor-ligand binding affinities. It is expected to provide a starting point that helps to accelerate the discovery of oral drugs with the potential to prevent, or slow down, multifactorial human health disorders.

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Nopal (Opuntia ficus indica) belonging to the Cactacea family has many nutritional benefits attributed to a wide variety of phenolic and flavonoid compounds. Coumaric acid (COA), ferulic acid (FLA), protocatechuic acid (PRA), and gallic acid (GAA) are the phenolic acids (PhAs) present in nopal. In this study, the role of these PhAs in copper-induced oxidative stress was investigated using the density functional theory (DFT). The PhAs form 5 thermodynamically favorable complexes with Cu(II), their conditional Gibbs free energies of reaction (ΔG', at pH = 7.4, in kcal/mol) are from -23 kcal/mol to -18 kcal/mol. All of them are bi-dentate complexes. The complexes of PRA and GAA are capable of inhibiting the Cu(II) reduction by both O2•- and Asc-, their reactions with the chelated metal are endergonic having rate constants about ~10-5-102 M-1 s-1, PhAs can prevent the formation of hydroxyl free radicals by chelating the copper ions. Once the hydroxyl radicals are formed by Fenton reactions, the complexes of PhAs with Cu(II) can immediately react with them, thus inhibiting the damage that they can cause to molecules of biological interest. The reactions between PhAs-Cu(II) complexes and hydroxyl free radical were estimated to be diffusion-limited (~108 M-1s-1). Thus, these chelates can reduce the harmful effects caused by the most reactive free radical existent immediately after it is formed by Fenton reactions.

RESUMO

Oxidative conditions are frequently enhanced by the presence of redox metal ions. In this study, the role of capsaicin (8-methyl-N-vanillyl-6-nonenamide, CAP) in copper-induced oxidative stress was investigated using density functional theory simulations. It was found that CAP has the capability to chelate Cu(II), leading to complexes that are harder to reduce than free Cu(II). CAP fully turns off the Cu(II) reduction by Asc-, and slows down the reduction in this cation by O2•-. Therefore, CAP is proposed as an •OH-inactivating ligand by impeding the reduction in metal ions (OIL-1), hindering the production of •OH via Fenton-like reactions, at physiological pH. CAP is also predicted to be an excellent antioxidant as a scavenger of •OH, yielded through Fenton-like reactions (OIL-2). The reactions between CAP-Cu(II) chelates and •OH were estimated to be diffusion-limited. Thus, these chelates are capable of deactivating this dangerous radical immediately after being formed by Fenton-like reactions.

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BACKGROUND: Melatonin is well known for its antioxidant capacity, which has been attributed to the combined protective effects of the parent molecule and its metabolites. However, the potential role of 2-hydroxymelatonin (2OHM) and 4-hydroxymelatonin (4OHM) in such protection has not been previously investigated. METHODS: The calculations were performed using the Density Functional Theory, with the M05-2X and M05 functionals, the 6-311+G(d,p) basis set and the solvation model based on density (SMD). RESULTS: 4OHM shows excellent antioxidant activity via radical-trapping, reacting with peroxyl radicals faster than Trolox and melatonin. 4OHM can be moderately efficient as a preventing antioxidant by inhibiting Cu(II). This effect would lower the Cu(I) availability, which is the redox state required for the OH to be formed, via Fenton-like reactions. 4OHM turns off the oxidant effects of copper-ascorbate mixtures. The presence of a phenolic group was identified as the key structural feature in the antioxidant activity of 4OHM. On the other hand, 2OHM does not present a phenolic group, despite its formal name. Its keto tautomer was identified as the most abundant one (~100%). This may explain the relative low antioxidant protection of 2OHM. CONCLUSIONS: 4OHM significantly contributes to the overall antioxidant activity exhibited by melatonin, while the effects of 2OHM in this context are predicted to be only minor. This low reactivity might justify the relatively large abundance of 2OHM in biological systems. GENERAL SIGNIFICANCE: Hydroxylated melatonin metabolites, such as 4OHM, may play an important role in the protective effects of melatonin against oxidative stress.

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There is currently no doubt about the serious threat that oxidative stress (OS) poses to human health. Therefore, a crucial strategy to maintain a good health status is to identify molecules capable of offering protection against OS through chemical routes. Based on the known efficiency of the phenolic and melatonin (MLT) families of compounds as antioxidants, it is logical to assume that phenolic MLT-related compounds should be (at least) equally efficient. Unfortunately, they have been less investigated than phenols, MLT and its non-phenolic metabolites in this context. The evidence reviewed here strongly suggests that MLT phenolic derivatives can act as both primary and secondary antioxidants, exerting their protection through diverse chemical routes. They all seem to be better free radical scavengers than MLT and Trolox, while some of them also surpass ascorbic acid and resveratrol. However, there are still many aspects that deserve further investigations for this kind of compounds.

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Free-radical scavenging by tryptophan and eight of its metabolites through electron transfer was investigated in aqueous solution at physiological pH, using density functional theory and the Marcus theory. A test set of 30 free radicals was employed. Thermochemical and kinetic data on the corresponding reactions are provided here for the first time. Two different pathways were found to be the most important: sequential proton loss electron transfer (SPLET) and sequential double proton loss electron transfer (SdPLET). Based on kinetic analyses, it is predicted that the tryptophan metabolites kynurenic acid and xanthurenic acid are the best free-radical scavengers among the tested compounds; they were estimated to be at least 24 and 12 times more efficient than Trolox for scavenging (•)OOH. These findings are in line with previous reports suggesting that the antioxidant activity that has been attributed to tryptophan is actually due to its metabolites, and they demonstrate the particular importance of phenolic metabolites to such activity. Graphical Abstract Kynurenic acid (KNA) and xanthurenic acid (XNA) are the major contributors to the free-radical scavenging activity of tryptophan.

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