Thiolic Ligands Research Articles

The Free-Radical Nature of Nitric Oxide Molecules as a Determinant of their Conversion to Nitrosonium Cations in Living Systems

This paper presents new results that confirm our previous inference that the binuclear form of biologically active dinitrosyl iron complexes (B-DNICs) with thiol-containing ligands (glutathione or N-acetyl-L-cysteine) may act as a donor of nitrosonium cations, which are responsible for S-nitrosothiol formation during B-DNIC decomposition in acid solutions under both aerobic and anaerobic conditions. The presence of nitrosonium cations within B-DNICs is determined by the dispropoportionation reaction of free-radical nitric oxide (NO) molecules while binding to Fe2+ cations (two molecules per one ion) during B-DNIC synthesis. When thiolic ligands are oxidized in DNICs or inactivated by thiol-specific reagents, the nitrosonium cations released during decomposition of these DNICs at neutral pH values are hydrolyzed and transformed to nitrite anions. A similar transformation occurs when mononuclear DNICs (M-DNICs) with nonthiolic ligands are decomposed at neutral pH values. It has been found that S-nitrosothiol formation in the decomposition of B-DNICs with thiolic ligands at acidic pH values can be inhibited by the presence of a two to threefold excess of free thiol molecules (outside the B-DNIC) with regard to the B-DNIC level. This inhibition is due to the reduction of nitrosonium cations induced by free thiol molecules and catalyzed by iron ions. The NO molecules that result from the reduction are released from the DNICs. Thus, both forms of DNICs, M and B, that form in living systems can act not only as donors of NO, which is now recognized as one of the universal regulators of metabolic processes, but also as donors of nitrosonium cations, which initiate S-nitrosation of low- and high-molecular-weight (protein-bound) thiols.

Selenate mitigates arsenite toxicity in rice (Oryza sativa L.) by reducing arsenic uptake and ameliorates amino acid content and thiol metabolism

Arsenic (As) is a toxic element with the potential to cause health effects in humans. Besides rice is a source of both amino acids (AAs) and mineral nutrients, it is undesired source of As for billions of people consuming rice as the staple food. Selenium (Se) is an essential metalloid, which can regulate As toxicity by strengthening antioxidant potential. The present study was designed to investigate AsIII stress mitigating effect of SeVI in rice. The level of As, thiolic ligands and AAs was analyzed in rice seedlings after exposure to AsIII/SeVI alone and AsIII+SeVI treatments. Selenate supplementation (AsIII 25μM+SeVI 25μM) decreased total As accumulation in both root and shoot (179 & 144%) as compared to AsIII alone treatment. The AsIII+SeVI treatment also induced the levels of non-protein thiols (NPTs), glutathione (GSH) and phytochelatins (PCs) as compared to AsIII alone treatment and also modulated the activity of enzymes of thiol metabolism. The content of amino acids (AAs) was significantly altered with SeVI supplementation. Importantly, essential amino acids (EAAs) were enhanced in AsIII+SeVI treatment as compared to AsIII alone treatment. In contrast, stress related non-essential amino acids (NEAAs) like GABA, Glu, Gly, Pro and Cys showed enhanced levels in AsIII alone treatment. In conclusion, rice supplemented with SeVI tolerated As toxicity with reduced As accumulation and increased the nutrition quality by increasing EAAs.

Sulfur mediated reduction of arsenic toxicity involves efficient thiol metabolism and the antioxidant defense system in rice

Arsenic (As) contamination is a global issue, with South Asia and South East Asia being worst affected. Rice is major crop in these regions and can potentially pose serious health risks due to its known As accumulation potential. Sulfur (S) is an essential macronutrient and a vital element to combat As toxicity. The aim of this study was to investigate the role of S with regards to As toxicity in rice under different S regimes. To achieve this aim, plants were stressed with AsIII and AsV under three different S conditions (low sulfur (0.5mM), normal sulfur (3.5mM) and high sulfur (5.0mM)). High S treatment resulted in increased root As accumulation, likely due to As complexation through enhanced synthesis of thiolic ligands, such as non-protein thiols and phytochelatins, which restricted As translocation to the shoots. Enzymes of S assimilatory pathways and downstream thiolic metabolites were up-regulated with increased S supplementation; however, to maintain optimum concentrations of S, transcript levels of sulfate transporters were down-regulated at high S concentration. Oxidative stress generated due to As was counterbalanced in the high S treatment by reducing hydrogen peroxide concentration and enhancing antioxidant enzyme activities. The high S concentration resulted in reduced transcript levels of Lsi2 (a known transporter of As). This reduction in Lsi2 expression level is a probable reason for low shoot As accumulation, which has potential implications in reducing the risk of As in the food chain.

Selenium ameliorates arsenic induced oxidative stress through modulation of antioxidant enzymes and thiols in rice (Oryza sativa L.).

Arsenic (As) contamination of rice is a major problem for South-East Asia. In the present study, the effect of selenium (Se) on rice (Oryza sativa L.) plants exposed to As was studied in hydroponic culture. Arsenic accumulation, plant growth, thiolic ligands and antioxidative enzyme activities were assayed after single (As and Se) and simultaneous supplementations (As + Se). The results indicated that the presence of Se (25 µM) decreased As accumulation by threefold in roots and twofold in shoots as compared to single As (25 µM) exposed plants. Arsenic induced oxidative stress in roots and shoots was significantly ameliorated by Se supplementation. The observed positive response was found associated with the increased activities of ascorbate peroxidase (APX; EC 1.11.1.11), catalase (CAT; EC 1.11.1.6) and glutathione peroxidase (GPx; EC 1.11.1.9) and induced levels of non-protein thiols (NPTs), glutathione (GSH) and phytochelatins (PCs) in As + Se exposed plants as compared to single As treatment. Selenium supplementation modulated the thiol metabolism enzymes viz., γ-glutamylcysteine synthetase (γ-ECS; EC 6.3.2.2), glutathione-S-transferase (GST; EC 2.5.1.18) and phytochelatin synthase (PCS; EC 2.3.2.15). Gene expression analysis of several metalloid responsive genes (LOX, SOD and MATE) showed upregulation during As stress, however, significant downregulation during As + Se exposure as compared to single As treatment. Gene expressions of enzymes of antioxidant and GSH and PC biosynthetic systems, such as APX, CAT, GPx, γ-ECS and PCS were found to be significantly positively correlated with their enzyme activities. The findings suggested that Se supplementation could be an effective strategy to reduce As accumulation and toxicity in rice plants.

Reversible dissociation and ligand-glutathione exchange reaction in binuclear cationic tetranitrosyl iron complex with penicillamine.

This paper describes a comparative study of the decomposition of two nitrosyl iron complexes (NICs) with penicillamine thiolic ligands [Fe2(SC5H11NO2)2(NO)4]SO4 ·5H2O (I) and glutathione- (GSH-) ligands [Fe2(SC10H17N3O6)2(NO)4]SO4 ·2H2O (II), which spontaneously evolve to NO in aqueous medium. NO formation was measured by a sensor electrode and by spectrophotometric methods by measuring the formation of a hemoglobin- (Hb-) NO complex. The NO evolution reaction rate from (I) k 1 = (4.6 ± 0.1)·10−3 s−1 and the elimination rate constant of the penicillamine ligand k 2 = (1.8 ± 0.2)·10−3 s−1 at 25°C in 0.05 M phosphate buffer, pH 7.0, was calculated using kinetic modeling based on the experimental data. Both reactions are reversible. Spectrophotometry and mass-spectrometry methods have firmly shown that the penicillamine ligand is exchanged for GS− during decomposition of 1.5·10−4 M (I) in the presence of 10−3 M GSH, with 76% yield in 24 h. As has been established, such behaviour is caused by the resistance of (II) to decomposition due to the higher affinity of iron to GSH in the complex. The discovered reaction may impede S-glutathionylation of the essential enzyme systems in the presence of (I) and is important for metabolism of NIC, connected with its antitumor activity.

Arsenite tolerance in rice (Oryza sativa L.) involves coordinated role of metabolic pathways of thiols and amino acids

Thiolic ligands and several amino acids (AAs) are known to build up in plants against heavy metal stress. In the present study, alteration of various AAs in rice and its synchronized role with thiolic ligand was explored for arsenic (As) tolerance and detoxification. To understand the mechanism of As tolerance and stress response, rice seedlings of one tolerant (Triguna) and one sensitive (IET-4786) cultivar were exposed to arsenite (0-25 μM) for 7 days for various biochemical analyses using spectrophotometer, HPLC and ICPMS. Tolerant and sensitive cultivars respond differentially in terms of thiol metabolism, essential amino acids (EEAs) and nonessential amino acids (NEEAs) vis-á-vis As accumulation. Thiol biosynthesis-related enzymes were positively correlated to As accumulation in Triguna. Conversely, these enzymes, cysteine content and GSH/GSSG ratio declined significantly in IET-4786 upon As exposure. The level of identified phytochelatin (PC) species (PC(2), PC(3) and PC(4)) and phytochelatin synthase activity were also more pronounced in Triguna than IET-4786. Nearly all EAAs were negatively affected by As-induced oxidative stress (except phenylalanine in Triguna), but more significantly in IET-4786 than Triguna. However, most of the stress-responsive NEAAs like glutamic acid, histidine, alanine, glycine, tyrosine, cysteine and proline were enhanced more prominently in Triguna than IET-4786 upon As exposure. The study suggests that IET-4786 appears sensitive to As due to reduction of AAs and thiol metabolic pathway. However, a coordinated response of thiolic ligands and stress-responsive AAs seems to play role for As tolerance in Triguna to achieve the effective complexation of As by PCs.

Synthesis and characterization of thiolato-bridged cadmium(II) complexes with NNS-chelating thiolic ligands

Cadmium(II) complexes with 2-[(2-aminoethyl)amino]ethanethiol (HL 1), 2-[(3-aminopropyl)amino]ethanethiol (HL 2), 2-[(2-pyridylmethyl)amino]ethanethiol (HL 3), and 2-[[2-(2-pyridyl)ethyl]amino]ethanethiol (HL 4), [Cd(L 1)](ClO 4) ( 1), [Cd(L 2)](ClO 4)·1/2CH 3OH ( 2), [Cd{Cd(L 2) 2} 2](ClO 4) 2·CH 3CON(CH 3) 2 ( 3a·CH 3CON(CH 3) 2), [Cd{Cd(L 2) 2} 2]Cl 2·2CH 3OH ( 3b·2CH 3OH), [Cd{Cd(L 3) 2} 2](ClO 4) 2 ( 4), [Cd(L 4)](ClO 4) ( 5), have been synthesized and characterized by measurements of the infrared and electronic spectra. The X-ray crystal structures show that 3a and 3b have a thiolato-bridged trinuclear core with a linear arrangement of three metal atoms.

Synthesis and Structural Characterization of Thiolato-Bridged Zinc(II) Complexes with NNS-Tridentate Thiolic Ligands

Abstract Zinc(II) complexes with 2-[(2-aminoethyl)amino]ethanethiol (HL1), 2-[(3-aminopropyl)amino]ethanethiol (HL2), 2-[(2-pyridylmethyl)amino]ethanethiol (HL3), and 2-{[2-(2-pyridyl)ethyl]amino}ethanethiol (HL4), [Zn(L1)](ClO4) (1), [Zn{Zn(L1)2}2](ClO4)2 (2), [Zn(L2)](ClO4) (3), [Zn{Zn(L3)2}2](ClO4)2 (4), and [Zn3(L4)3](ClO4)3 (5), have been synthesized and characterized by measurements of the infrared and electronic spectra. The molecular structures of these complexes were determined by the single-crystal X-ray structure analysis. Complex 1 has a thiolato-bridged polymeric chain structure, in which each metal atom is essentially four-coordinated in a distorted tetrahedron. Complexes 2 and 4 have a thiolato-bridged trinuclear complex with a linear arrangement of three metal atoms. Complex 5 is a thiolato-bridged trinuclear complex displaying a six-membered ring of Zn3S3.

Syntheses and Structural Characterization of Dinuclear and Trinuclear Iron(II) Complexes of Tridentate Thiolic Ligands with an NNS Donor Set

Abstract Thiolate-bridged complexes of iron(II) with 2-[(2-aminoethyl)amino]ethanethiol (HLa), 2-[(3-aminopropyl)amino]ethanethiol (HLb), and 2-[[2-(2-pyridyl)ethyl]amino]ethanethiol (HLc) — [Fe{Fe(La)2}2](ClO4)2·CH3OH (1), [Fe{Fe(Lb)2}2]Cl2·2CH3OH (2), and [Fe2(Lc)2(NO3)2] (3), respectively — have been synthesized and characterized by elemental analyses, and infrared and electronic spectroscopies as well as magnetic susceptibilities (80—300 K). The crystal structures of all of these complexes have been measured by single-crystal X-ray diffraction. Each of complexes 1 and 2 has a thiolate-bridged trinuclear iron(II) structure showing a linear arrangement of the three iron atoms with a geometry that may be described as Oh-Td-Oh. Complex 3 is a thiolate-bridged dinuclear iron(II) molecule, in which each iron atom is coordinated in a distorted octahedron. The electronic spectroscopic and magnetic properties are discussed in relation to the crystal structures.

Syntheses and Characterization of Thiolate-Bridged Manganese(II) Complexes with NNS-Tridentate Thiolic Ligands

Abstract Manganese(II) complexes with 2-[(2-aminoethyl)amino]ethanethiol (HLa), 2-[(3-aminopropyl)amino]ethanethiol (HLb), 2-[(2-pyridylmethyl)amino]ethanethiol (HLc), and 2-[[2-(2-pyridyl)ethyl]amino]ethanethiol (HLd)–[Mn(La)Cl]n (1), [Mn{Mn(Lb)2}2]Cl2·2CH3OH (2), [Mn(Lc)Cl]n (3), and [Mn(Ld)Cl(CH3OH)]n (4) have been synthesized and characterized by elemental analyses, infrared and electronic spectroscopies, and magnetic susceptibilities (80—300 K). The crystal structures of all these complexes were determined by the single-crystal X-ray diffraction method. Complexes 1 and 3 have a thiolate-bridged polymeric chain structure in which each manganese atom is five-coodinated in a trigonal bipyramid. Complex 2 is a thiolate-bridged trinuclear manganese(II) complex displaying a linear arrangement of the three manganese atoms with the two Mn–Mn interactions being equidistant [3.420(1) Å]. Complex 4 is a thiolate-bridged polymeric infinite chain showing a distortion in the octahedral geometry around each manganese atom. The magnetic properties are discussed in relation to the crystal structures.

Redox properties of thiolate-bridged binuclear nickel(II) complexes and correlation of their electronic spectra

The redox properties of a series of thiolate-bridged binuclear nickel(II) complexes with NNS-tridentate and SNNS-tetradentate thiolic ligands have been studied by cyclic voltammetry. The first redox potential (E1/2=−0.63 to −0.95 Vversus Ag/AgCl) has a linear correlation with the absorption maxima of the\(\tilde v_{d - d}\) transition.

Synthesis, Properties, and Crystal Structure of Binuclear Nickel(II) and Palladium(II) Complexes of Tridentate Thiolic Ligands with NNS-Donor Set

Abstract Nickel(II) and palladium(II) complexes with tridentate thiolic ligands of the NNS-donor set have been prepared and characterized on the basis of elemental analyses, IR and electronic spectra, molar conductances, and magnetic susceptibilities. The crystal structure of the nickel(II) perchlorate of 3-[2-(2-pyridyl)ethylamino]-1-propanethiol was determined by single-crystal X-ray diffraction method. The structure consists of thiolatebridged binuclear units. Each metal ion is coordinated by the amino nitrogen, pyridine nitrogen, and two bridging sulfur atoms, forming an essentially planar coordination unit, and the two N2S2 planes are connected at the S1–S2 edge with a dihedral angle of 136.1°.

Dinuclear Cobalt(II), Iron(II), and Manganese(II), and Trinuclear Manganese(II) Complexes with Tridentate Ligands of NNS-Donor Set Derived from 2-Aminoethanethiol

Abstract Thiolate-bridged dinuclear cobalt(II), iron(II), and Manganese(II) complexes and trinuclear manganese(II) complexes of thiolic ligands, 2-[(2-aminoethyl)amino]ethanethiol, 2-[(3-aminopropyl)amino]ethanethiol, 2-[(2-pyridylmethyl)amino]ethanethiol, and 2-[[(2-pyridylethyl]amino]ethanethiol, were prepared and their magnetic properties were examined in the temperature range 80–300 K. The results indicated a significant antiferromagnetic spin-exchange interactions through the bridging sulfur atom for all the complexes.