Uncovering the Role of Methylmercury on DNA Lesions at Cytotoxic Concentrations in Glutathione-Depleted Cells: Insights from Experimental and Computational Studies.

Organomercurials (RHg+), especially methylmercury (MeHg+) and ethylmercury (EtHg+), are considered to be more neurotoxic than the inorganic counterpart (Hg2+). They cause massive DNA damage in cells, especially in neurons, where cellular glutathione (GSH) levels are significantly low. However, the mechanism by which RHg+ exerts massive DNA damage at cytotoxic concentrations in brain cells remains obscure. In this study, we investigated the effect of RHg+ on the structural and electronic properties of nucleosides and its effects on DNA damage. The direct interaction of RHg+ with the nucleoside significantly weakens N-glycosidic bonds, decreases the C-H bond energy of sugar moieties, and increases the electrophilicity of the C8-center of purine bases. As a consequence, RHg+-conjugated DNA molecules are extremely labile and highly sensitive to any nucleophiles/radicals present in GSH-depleted cells and, thus, undergo enhanced oxidative and unusual alkylative DNA damage. We also report a functional model of organomercurial lyase, which showed excellent cytoprotective effect against RHg+-induced cytotoxicity; this reverses the activity of glutathione reductase inhibited by MeHgCl and ceases oxidative and alkylating DNA damage. This intriguing finding provides new mechanistic insight into the mode of action of organomercurials in GSH-depleted cells and their adverse effects on individuals with neurodegenerative disorders associated with oxidative stress.

Regulation of Tyrosinase Enzyme Activity by Glutathione Peroxidase Mimics.

Hydrogen peroxide plays a crucial role in the melanogenesis process by regulating the activity of the key melanin-forming enzyme tyrosinase, responsible for the browning of fruits, vegetables, and seafood. Therefore, a molecule with dual activities, both efficient tyrosinase inhibition and strong hydrogen peroxide degrading ability, may act as a promising antibrowning agent. Herein, we report highly efficient selone-based mushroom tyrosinase inhibitors 2 and 3 with remarkable glutathione peroxidase (GPx) enzyme-like activity. The presence of benzimidazole moiety enhances the tyrosinase inhibition efficiency of selone 2 (IC50 = 0.4 µM) by almost 600 times higher than imidazole-based selone 1 (IC50 = 238 µM). Interestingly, the addition of another aromatic ring to the benzimidazole moiety has led to the development of an efficient lipid-soluble tyrosinase inhibitor 3 (IC50 = 2.4 µM). The selenium center and the -NH group of 2 and 3 are extremely crucial to exhibit high GPx-like activity and tyrosinase inhibition potency. The hydrophobic moiety of the inhibitors (2 and 3) further assists them in tightly binding at the active site of the enzyme and facilitates the C═Se group to strongly coordinate with the copper ions. Inhibitor 2 exhibited excellent antibrowning and polyphenol oxidase inhibition properties in banana and apple juice extracts.

Hg-C bond protonolysis by a functional model of bacterial enzyme organomercurial lyase MerB.

Herein, we report a novel synthetic compound 1, having a highly nucleophilic selenolate (Se-) moiety and a thiol (-SH) functional group, which showed efficient Hg-C bond protonolysis of various R-Hg-X molecules including neurotoxic methylmercury and thimerosal, via direct -SH proton transfer to the highly activated C-atom of a departed R group with low activation energy barrier at room temperature (21 °C), in the absence of any external proton source and, thus, acts as a functional model of MerB.

Role of Hydrogen Bonding by Thiones in Protecting Biomolecules from Copper(I)-Mediated Oxidative Damage.

The sulfur-containing antioxidant molecule ergothioneine with an ability to protect metalloenzymes from reactive oxygen species (ROS) has attracted significant interest in both chemistry and biology. Herein, we demonstrated the importance of hydrogen bonding in S-oxygenation reactions between various thiones and H2O2 and its significance in protecting the metal ion from H2O2-mediated oxidation. Among all imidazole- and benzimidazole-based thiones (1-10), ImMeSH (2) showed the highest reactivity toward H2O2-almost 10 and 75 times more reactive than N, N'-disubstituted ImMeSMe (5) and BzMeSMe (10), respectively. Moreover, metal-bound ImMeSH (2) of [TpmCu(2)]+ (13) was found to be 51 and 1571 times more reactive toward H2O2 than the metal-bound ImMeSMe (5) of [TpmCu(5)]+ (16), and BzMeSMe (10) of [TpmCu(10)]+ (21), respectively. The electron-donating N-Me substituent and the free N-H group at the imidazole ring played a very crucial role in the high reactivity of ImMeSH toward H2O2. The initial adduct formation between ImMeSH and H2O2 (ImMeSH·H2O2) was highly facilitated (-23.28 kcal mol-1) due to the presence of a free N-H group, which leads to its faster oxygenation than N, N'-disubstituted ImMeSMe (5) or BzMeSMe (10). As a result, ImMeSH (2) showed a promising effect in protecting the metal ion from H2O2-mediated oxidation. It protected biomolecules from Cu(I)-mediated oxidative damage of through coordination to the Cu(I) center of [TpmCu(CH3CN)]+ (11), whereas metal-bound ImMeSMe or BzMeSMe failed to protect biomolecules under identical reaction conditions.

Copper-Driven Deselenization: A Strategy for Selective Conversion of Copper Ion to Nanozyme and Its Implication for Copper-Related Disorders.

Synthetic organic molecules, which can selectively convert excess intracellular copper (Cu) ions to nanozymes with an ability to protect cells from oxidative stress, are highly significant in developing therapeutic agents against Cu-related disorder like Wilson's disease. Here, we report 1,3-bis(2-hydroxyethyl)-1 H-benzoimidazole-2-selenone (1), which shows a remarkable ability to remove Cu ion from glutathione, a major cytosolic Cu-binding ligand, and thereafter converts it into copper selenide (CuSe) nanozyme that exhibits remarkable glutathione peroxidase-like activity, at cellular level of H2O2 concentration, with excellent cytoprotective effect against oxidative stress in hepatocyte. Cu-driven deselenization of 1, under physiologically relevant conditions, occurred in two steps. The activation of C═Se bond by metal ion is the crucial first step, followed by cleavage of the metal-activated C═Se bond, initiated by the OH group of N-(CH2)2OH substituent through neighboring group participation (deselenization step), resulted in the controlled synthesis of various types of Cu2-xSe nanocrystals (NCs) (nanodisks, nanocubes, and nanosheets) and tetragonal Cu3Se2 NCs, depending upon the oxidation state of the Cu ion used to activate the C═Se bond. Deselenization of 1 is highly metal-selective. Except Cu, other essential metal ions, including Mn2+, Fe2+, Co2+, Ni2+, or Zn2+, failed to produce metal selenide under identical reaction conditions. Moreover, no significant change in the expression level of Cu-metabolism-related genes, including metallothioneines MT1A, is observed in liver cells co-treated with Cu and 1, as opposed to the large increase in the concentrations of these genes observed in cells treated with Cu alone, suggesting the participation of 1 in Cu homeostasis in hepatocyte.

Cytoprotective effects of imidazole-based [S1] and [S2]-donor ligands against mercury toxicity: a bioinorganic approach.

Here we report the coordination behaviour of an imidazole-based [S1]-donor ligand, 1,3-dimethyl-imidazole-2(3H)-thione (L1), and [S2]-donor ligand, 3,3'-methylenebis(1-methyl-imidazole-2(3H)-thione) (L2) or 4,4'-(3,3'-methylenebis-(2-thioxo-2,3-dihydro-imidazole-3,1-diyl))dibutanoic acid (L3), with HgX2 (X = Cl, Br or I) in solution and the solid state. NMR, UV-Vis spectroscopic, and single crystal X-ray studies demonstrated that L1 or L2 coordinated rapidly and reversibly to the mercury center of HgX2 through the thione moiety. Treatment of L2 with HgCl2 or HgBr2 afforded 16-membered metallacycle k1-(L2)2Hg2Cl4 or k1-(L2)2Hg2Br4 where two Cl or Br atoms are located inside the ring. In contrast, treatment of L2 with HgI2 afforded a chain-like structure of k1-[L2Hgl2]n, possibly due to the large size of the iodine atom. Interestingly, [S1] and [S2]-donor ligands (L1, L2, and L3) showed an excellent efficacy to protect liver cells against HgCl2 induced toxicity and the strength of their efficacy is in the order of L3 > L2 > L1. 30% decrease of ROS production was observed when liver cells were co-treated with HgCl2 and L1 in comparison to those cells treated with HgCl2 only. In contrast, 45% and 60% decrease of ROS production was observed in the case of cells co-treated with HgCl2 and thiones L2 and L3, respectively, indicating that [S2]-donor ligands L2 and L3 have better cytoprotective effects against oxidative stress induced by HgCl2 than [S1]-donor ligand L1. Water-soluble ligand L3 with N-(CH2)3CO2H substituents showed a better cytoprotective effect against HgCl2 toxicity than L2 in liver cells.

Metal-free C(sp2)-H functionalization of azoles: K2CO3/I2-mediated oxidation, imination, and amination.

The direct C2-H oxidation and imination of a wide variety of azoles was achieved by using a commercially available simple K2CO3/I2 reagent combination. The iodinated azole adduct, produced via the in situ generation of N-heterocyclic carbene, is the key intermediate for C2-H oxidation, imination, and amination of azoles. Significantly, these reactions proceed under mild conditions with high to excellent yields, are scalable to large quantity and exhibit a broad substrate scope. Interestingly, this direct C2-H imination method allowed us to access various pharmacologically active N6-alkyl or N6-aryl substituted benzimidazoquinazolinone scaffolds through intramolecular C-H imination in a sequential one-pot reaction.

Protection of Endogenous Thiols against Methylmercury with Benzimidazole-Based Thione by Unusual Ligand-Exchange Reactions.

Organomercurials, such as methylmercury (MeHg+ ), are among the most toxic materials to humans. Apart from inhibiting proteins, MeHg+ exerts its cytotoxicity through strong binding with endogenous thiols cysteine (CysH) and glutathione (GSH) to form MeHgCys and MeHgSG complexes. Herein, it is reported that the N,N-disubstituted benzimidazole-based thione 1 containing a N-CH2 CH2 OH substituent converts MeHgCys and MeHgSG complexes to less toxic water-soluble HgS nanoparticles (NPs) and releases the corresponding free thiols CysH and GSH from MeHgCys and MeHgSG, respectively, in solution by unusual ligand-exchange reactions in phosphate buffer at 37 °C. However, the corresponding N-substituted benzimidazole-based thione 7 and N,N-disubstituted imidazole-based thione 3, in spite of containing a N-CH2 CH2 OH substituent, failed to convert MeHgX (X=Cys, and SG) to HgS NPs under identical reaction conditions, which suggests that not only the N-CH2 CH2 OH moiety but the benzimidazole ring and N,N-disubstitution in 1, which leads to the generation of a partial positive charge at the C2 atom of the benzimidazole ring in 1:1 MeHg-conjugated complex of 1, are crucial to convert MeHgX to HgS NPs under physiologically relevant conditions.