Differential Pulse Polarography of the Zn2+ Complexation by Glutathione Fragments Cys‐Gly and gamma‐Glu‐Cys

The metal binding properties of glutathione (GSH) and their fragments γ‐Glu‐Cys and Cys‐Gly are of biological and environmental interest. In this work a differential pulse polarographic study of the Zn²⁺/γ‐Glu‐Cys and Zn²⁺/Cys‐Gly systems was carried out for a better understanding of the results obtained in previous studies on the Zn²⁺‐GSH system. In the case of γ‐Glu‐Cys, complexation with Zn²⁺ was not detected. In the case of Cys‐Gly, the parallel analysis, by multivariate curve resolution with alternating least squares, of data from the titration of peptide with metal and of metal with peptide suggested the presence of two types of bound Zn²⁺. This could be attributed to Zn²⁺ strongly bound to two sulfur atoms of two peptides, to form a complex of 1 : 2 stoichiometry, and to Zn²⁺ weakly bound to carboxylate and/or amino groups.     

Differential pulse voltammetric study of the complexation of Cd(II) by the phytochelatin (γ-Glu---Cys)2Gly assisted by multivariate curve resolution

A multivariate curve resolution method by alternating least-squares (MCR-ALS) is applied to differential pulse voltammograms measured on the Cd(II)+(γ-Glu---Cys)₂Gly system as a model of metal–phytochelatin interactions at concentrations of both components in the range 10⁻⁷–10⁻⁵ mol l⁻¹. The course of complexation is different when peptide is titrated with metal from that when metal is titrated with peptide. The combined analysis of both matrices from titrations of peptide with metal and of metal with peptide allowed the resolution of the system. The analysis of the resulting pure voltammograms and concentration profiles of the resolved components suggested the presence of four different types of bound Cd(II) and made possible the formulation of a complexation model.     

Enhancement of hydrolysis through the formation of mixed hetero-metal species: dioxouranium(VI)-cadmium(II) mixtures

In order to continue the investigation on the formation of hetero-metal polynuclear hydrolytic species, in this paper we report some results (at I = 0.16 mol L(-1) in NaNO3, at t = 25 degrees C by potentiometry, ISE-H+, glass electrode) on the hydrolysis of several mixtures (in different ratios) of the dioxouranium(VI) and cadmium(II) cations. The same experimental and calculation procedure of previously investigated systems was followed, and all measurements were performed by two different operators, using completely independent instruments and reagents. Many different speciation models were considered in the calculations, and a simple statistical analysis of obtained results was proposed too. UO2(2+) and Cd2+ form two hetero-metal polynuclear hydrolytic species, namely UO2Cd(OH)3+ and (UO2)2Cd(OH)4(2+), with logbeta(pqr) = -3.25 +/- 0.25 and -13.75 +/- 0.10, respectively. The formation of hetero-metal hydrolytic species is thermodynamically favored with respect to the homo-metal ones, and causes an enhancement of the percentage of hydrolyzed metal cations; comparisons with previously studied systems reveal that the hydrolytic behavior of UO2(2+)/Cd2+ mixtures is more similar to that observed for UO2(2+)/Cu2+ than for UO2(2+)/(C2H5)2Sn2+, and the tendency to form hetero polynuclear hydrolytic species with dioxouranium(VI) by other cations follows the trend (C2H5)2Sn2+ > Cu2+ > or = Cd2+.     

Cadmium removal from single- and multi-metal (Cd + Pb + Zn + Cu) solutions by sorption on hydroxyapatite

Heavy metal contamination of waters and soils is particularly dangerous to the living organisms. Different studies have demonstrated that hydroxyapatite has a high removal capacity for divalent heavy metal ions in contaminated waters and soils. The removal of Cd from aqueous solutions by hydroxyapatite was investigated in batch conditions at 25+/-2 degrees C. Cadmium was applied both as single- or multi-metal (Cd + Pb + Zn + Cu) systems with initial concentrations from 0 to 8 mmol L(-1). The adsorption capacity of hydroxyapatite in single-metal system ranged from 0.058 to 1.681 mmol of Cd/g of hydroxyapatite. In the multi-metal system competitive metal sorption reduced the removal capacity by 63-83% compared to the single-metal system. The sorption of Cd by hydroxyapatite follows the Langmuir model. Cadmium immobilization occurs through a two-step mechanism: rapid surface complexation followed by partial dissolution of hydroxyapatite and ion exchange with Ca resulting in the formation of a cadmium-containing hydroxyapatite.     

Interaction of molybdocene dichloride with cysteine-containing peptides: coordination, regioselective hydrolysis, and intramolecular aminolysis

Reactions of the organometallic compound molybdocene dichloride (Cp2MoCl2, Cp = eta5-cyclopentadienyl) with the cysteine-containing peptides L-cysteinylglycine (Cys-Gly), N-acetyl-L-cysteine (AcCys), glycyl-L-cysteine (Gly-Cys), glycyl-L-cysteinylglycine (Gly-Cys-Gly), and gamma-L-glutamyl-L-cysteinylglycine (glutathione, GSH) have been studied in aqueous solution in the pH range 2-9. The dipeptides Cys-Gly and Gly-Cys and the acetylated amino acid AcCys form 1:1 and 2:1 complexes of composition [Cp2Mo(peptide-S)(OH(2))]n+/- and [Cp2Mo(peptide-S)2]n+/- as well as the chelates [Cp2Mo(AcCys-S,O)], [Cp2Mo(Gly-Cys-S,O)]+, and [Cp2Mo(Cys-Gly-S,N)] with the Cp2Mo2+ unit binding to the deprotonated thiolate group and the free amino or carboxylate group of the cysteine residue. Upon treatment of Gly-Cys-Gly and the naturally occurring tripeptide GSH with Cp2MoCl2 at elevated temperature, release of free glycine was observed. The Cp2Mo2+ entity coordinates to the thiolate group of GSH and mediates regioselective hydrolysis of the Cys-Gly peptide bond by intramolecular metal hydroxide activation. Cp2Mo2+-promoted hydrolysis of GSH was followed at pD 7.4 and 5.2 and 40 and 60 degrees C. By contrast, the Cys-Gly bond in [Cp2Mo(Gly-Cys-Gly-S,N)] is cleaved by intramolecular aminolysis at pD > or = 7.4 and 60 degrees C leading to glycine and the Cp2Mo2+ complex of the 2,5-diketopiperazine derivative cyclo-(Gly-Cys). Chelating coordination of the Cp2Mo2+ moiety to the thiolate group and to the deprotonated amide nitrogen of the tripeptide changes the configuration of the peptide bond from (preferred) trans to cis, thus enabling nucleophilic attack of the primary amino group at the Cys-Gly bond. The reaction product [Cp2Mo{cyclo-(Gly-Cys)}] x 2H2O has been characterized by X-ray crystallography.     

Electrochemical behavior of phytochelatins and related peptides at the hanging mercury drop electrode in the presence of cobalt(II) ions

Direct current voltammetry and differential pulse voltammetry have been used to investigate the electrochemical behaviour of two phytochelatins: heptapeptide (gamma-Glu-Cys)3-Gly and pentapeptide (gamma-Glu-Cys)2-Gly, tripeptide glutathione gamma-Glu-Cys-Gly and its fragments: dipeptides Cys-Gly and gamma-Glu-Cys at the hanging mercury drop electrode in the presence of cobalt(II) ions. Most interesting results were obtained with direct current voltammetry in the potential region of -0.80 V up to -1.80 V. Differential pulse voltammetry of the same solutions of Co(II) with peptides gives more complicated voltammograms with overlapping peaks, probably in connection with the influence of adsorption at slow scan rates necessarily used in this method. However, in using Brdicka catalytic currents for analytical purposes, differential pulse voltammograms seem to be more helpful. Presented investigations have shown that particularly the prewave of cobalt(II) allows distinguishing among phytochelatins, glutathione, and its fragments.     

Comparison of the binding of cadmium(II), mercury(II), and arsenic(III) to the de novo designed peptides TRI L12C and TRI L16C

Designed alpha-helical peptides of the TRI family with a general sequence Ac-G(LKALEEK)(4)G-CONH(2) were used as model systems for the study of metal-protein interactions. Variants containing cysteine residues in positions 12 (TRI L12C) and 16 (TRI L16C) were used for the metal binding studies. Cd(II) binding was investigated, and the results were compared with previous and current work on Hg(II) and As(III) binding. The metal peptide assemblies were studied with the use of UV, CD, EXAFS, (113)Cd NMR, and (111m)Cd perturbed angular correlation spectroscopy. The metalated peptide aggregates exhibited pH-dependent behavior. At high pH values, Cd(II) was bound to the three sulfurs of the three-stranded alpha-helical coiled coils. A mixture of two species was observed, including Cd(II) in a trigonal planar geometry. The complexes have UV bands at 231 nm (20 600 M(-1) cm(-1)) for TRI L12C and 232 nm (22 600 M(-1) cm(-1)) for TRI L16C, an average Cd-S bond length of 2.49 A for both cases, and a (113)Cd NMR chemical shift at 619 ppm (Cd(II)(TRI L12C)(3)(-)) or 625 ppm (Cd(II)(TRI-L16C)(3)(-)). Nuclear quadrupole interactions show that two different Cd species are present for both peptides. One species with omega(0) = 0.45 rad/ns and low eta is attributed to a trigonal planar Cd-(Cys)(3) site. The other, with a smaller omega(0), is attributed to a four-coordinate Cd(Cys)(3)(H(2)O) species. At low pH, no metal binding was observed. Hg(II) binding to TRI L12C was also found to be pH dependent, and a 3:1 sulfur-to-mercury(II) species was observed at pH 9.4. These metal peptide complexes provide insight into heavy metal binding and metalloregulatory proteins such as MerR or CadC.     

Competitive binding of cadmium by plant thiols: an electrochemical study assisted by multivariate curve resolution

Multivariate curve resolution with alternating least squares (MCR-ALS) has been applied to voltammetric data obtained from analysis of the competitive binding of cysteine (Cys) and cysteine–glycine (Cys-Gly) by Cd(II) as a first approach towards mixtures of phytochelatins and related compounds in natural media. From different starting points, the possibilities of formation of mixed complexes and/or displacements between ligands are investigated. Analysis of the resulting unitary voltammograms and concentration profiles of the resolved components by MCR-ALS suggests that the strongest ligand (Cys-Gly) is able to displace the weakest (Cys) from its metal complexes, whereas this does not happen in the opposite direction. On the other hand, no evidence of Cd mixed-ligand complexes was found. Figure Differential pulse polarograms measured in the independent titrations of 1 × 10^-5 mol L^-1 Cys, 1 × 10^-5 mol L^-1 Cys-Gly, and a mixture of Cys-Gly (0.5 × 10^-5 mol L^-1) and Cys (1 × 10^-5 mol L^-1) with Cd²⁺, at TRIS-HNO₃ buffer (0.1 mol L^-1 and PH 7.5) in the presence of 0.1 mol L^-1 KNO₃     

Amino acids and peptides. XXVII. Synthesis of phytochelatin-related peptides and examination of their heavy metal-binding properties

Phytochelatin (PC)-related peptides were prepared by a conventional solution method and their heavy metal-binding properties were examined. Different from the Cu2+ and Cu+ -binding properties of metallothionein (MT)-related peptides, the Cu2+ and Cu+ -binding properties of PC-related peptides were fairly dependent on structure. It is of interest that gamma-Glu-Cys-Gly (glutathione) exhibited quite different Cu2+ and Cu+ -binding properties from those of other PC-related peptides and its binding abilities were comparable to those of MT-related peptides. The Cd2+ -binding properties of glutathione were similar to those of Cys, and the Cd2+ -binding abilities of PC-related peptides increased in proportion to the increase of gamma-Glu-Cys peptide unit.     

Cd2+-induced conformational change of a synthetic metallopeptide: slow metal binding followed by a slower conformational change

A two-stranded alpha-helical coiled coil was prepared having a Cys 4 metal-binding site within its hydrophobic interior. The addition of Cd2+ results in the incorporation of 2 equiv of metal ion, which is accompanied by a conformational change of the peptide, as observed by circular dichroism (CD) spectroscopy. Isothermal titration calorimetry (ITC) shows that the addition of Cd2+ is accompanied by two thermodynamic events. A comparison of the time dependence of the ITC behavior with those of the UV absorption and CD behavior allows the assignment of these events to a preliminary endothermic metal-binding step followed by a slower exothermic conformational change.     

Controlling and fine tuning the physical properties of two identical metal coordination sites in de novo designed three stranded coiled coil peptides

Herein we report how de novo designed peptides can be used to investigate whether the position of a metal site along a linear sequence that folds into a three-stranded α-helical coiled coil defines the physical properties of Cd(II) ions in either CdS(3) or CdS(3)O (O-being an exogenous water molecule) coordination environments. Peptides are presented that bind Cd(II) into two identical coordination sites that are located at different topological positions at the interior of these constructs. The peptide GRANDL16PenL19IL23PenL26I binds two Cd(II) as trigonal planar 3-coordinate CdS(3) structures whereas GRANDL12AL16CL26AL30C sequesters two Cd(II) as pseudotetrahedral 4-coordinate CdS(3)O structures. We demonstrate how for the first peptide, having a more rigid structure, the location of the identical binding sites along the linear sequence does not affect the physical properties of the two bound Cd(II). However, the sites are not completely independent as Cd(II) bound to one of the sites ((113)Cd NMR chemical shift of 681 ppm) is perturbed by the metalation state (apo or [Cd(pep)(Hpep)(2)](+) or [Cd(pep)(3)](-)) of the second center ((113)Cd NMR chemical shift of 686 ppm). GRANDL12AL16CL26AL30C shows a completely different behavior. The physical properties of the two bound Cd(II) ions indeed depend on the position of the metal center, having pK(a2) values for the equilibrium [Cd(pep)(Hpep)(2)](+) → [Cd(pep)(3)](-) + 2H(+) (corresponding to deprotonation and coordination of cysteine thiols) that range from 9.9 to 13.9. In addition, the L26AL30C site shows dynamic behavior, which is not observed for the L12AL16C site. These results indicate that for these systems one cannot simply assign a "4-coordinate structure" and assume certain physical properties for that site since important factors such as packing of the adjacent Leu, size of the intended cavity (endo vs exo) and location of the metal site play crucial roles in determining the final properties of the bound Cd(II).     

Dicynthaurin (ala) monomer interaction with phospholipid bilayers studied by fluorescence leakage and isothermal titration calorimetry

The interaction of the antimicrobial peptide dicynthaurin (ala) monomer with model membranes of zwitterionic and negatively charged lipids and mixtures thereof was studied by means of isothermal titration calorimetry (ITC), fluorescent leakage, and dynamic light scattering (DLS) measurements. For the ITC analysis, we have applied the surface partitioning equilibrium model which shows that the interaction is predominately driven by hydrophobic effects (Kb between 2 x 10(4) and 1 x 10(5) M(-1)). Under low salt conditions, the enhanced electrostatic interaction leads to larger peptide concentrations immediately above the vesicle surface, which initiates the insertion of the peptide into the bilayer more effectively. Fluorescent leakage measurements have shown a fast leakage of the fluorescent dye within seconds after peptide addition. The analysis of the leakage kinetics was performed in terms of an initial pore formation model (up to t = 1000 s) that takes the reversible surface aggregation of bound peptide monomers into account. From this analysis, a minimum aggregation number of n = 7 +/- 2 per pore is obtained.     

Multivariate curve resolution applied to the simultaneous analysis of electrochemical and spectroscopic data: study of the Cd(II)/glutathione-fragment system by voltammetry and circular dichroism spectroscopy

Multivariate curve resolution with alternating least squares (MCR-ALS) is applied for the first time to the simultaneous analysis of electrochemical and spectroscopic data. Then, a data analysis is done with augmented matrices constituted by Differential Pulse Polarography and Circular Dichroism data submatrices. The use of proper, and different for each submatrix, constrains in the iterative ALS optimization allows to obtain chemically meaningful results constituted by a common matrix containing the concentration profiles, and two matrices with the pure electrochemical and spectroscopic signals. MCR-ALS is applied to the study of the complexation of Cd by Cys-Gly, a glutathione-fragment of great interest for understanding metal-phytochelatins complexation.     

Metal-binding thermodynamics of the histidine-rich sequence from the metal-transport protein IRT1 of Arabidopsis thaliana

The widespread ZIP family of transmembrane metal-transporting proteins is characterized by a large intracellular loop that contains a histidine-rich sequence whose biological role is unknown. To provide a chemical basis for this role, we prepared and studied a peptide corresponding to this sequence from the first iron-regulated transporter (IRT1) of Arabidopsis thaliana, which transports Fe2+ as well as Mn2+, Co2+, Zn2+, and Cd2+. Isothermal titration calorimetry (ITC) measurements, which required novel experiments and data analysis, and supporting spectroscopic methods were used to quantify IRT1's metal-binding affinity and associated thermodynamics. The peptide, PHGHGHGHGP, binds metal ions with 1:1 stoichiometry and stabilities that are consistent with the Irving-Williams series. Comparison of the metal-binding thermodynamics of the peptide with those of trien provides new insight about enthalpic and entropic contributions to the stability of the metal-peptide complex. Although Fe2+ and other IRT1-transported metal ions do not bind very tightly, this His-rich sequence has a very high entropy-driven affinity for Fe3+, which may have biological significance.     

Precise chelatometric titrations of zinc, cadmium, and lead with molecular spectroscopy.

Spectral data during a chelatometric titration were utilized for determining an equivalence point of the titration. Three-dimensional titration data were analyzed by fitting to a theoretical equation that was derived from equilibrium equations among a metal ion, an indicator and EDTA. The equation with optimized parameters successfully explained the three-dimensional titration data. This method was applied to EDTA titration systems of zinc, cadmium and lead with xylenol orange (XO) as an indicator. These systems were analyzed while taking the formation of 1:1 and 2:1 metal-indicator complexes into account. By this method, an accurate equivalence point was determined as well as stability constants and molar absorption spectra of metal-indicator complexes in each titration system. Moreover, the comparability among the concentrations of the metal standard solutions prepared from pure metals (Zn, Pb, and Cd) was also confirmed by the proposed evaluation method.     

Investigation of Cd(II), Pb(II) and Cu(I) complexation by glutathione and its component amino acids by ESI-MS and size exclusion chromatography coupled to ICP-MS and ESI-MS

The elucidation of structures of glutathione (GSH) complexes play an important role in the fundamental understanding of biochemical pathways of metal ion deactivation in plants. This article attempts to feature key studies for stoichiometry of metal complexes with glutathione and its constituent amino acids to obtain a better understanding of the different metal affinities of the complexation sites of glutathione. The SEC-ICP-MS experiments have indicated that oxidation process of glutathione was accelerated by metal ion presence in following order Cu⁺, Pb²⁺ and Cd²⁺. The redox activity of metal ions was confirmed by ESI-MS experiments, which allowed to observe formation of glutathione disulphide (GSSG) in time. The stoichiometry of Cd²⁺, Cu⁺ and Pb²⁺ complexes with GSH was defined by observing the isotope pattern of investigated metals and hydrogen loss or transfer during binding. The complexes with metal bound to sulphur of 1:1 and 1:2 stoichiometry were found in case of cadmium and lead. The number of hydrogen atoms lost during metal binding and the SEC-ESI-MS results allowed to elucidate that copper is bound by GSSG in ratio 1:1 and 1:2. Additionally, size exclusion chromatography coupled to electrospray MS allowed to differentiate more stable complexes from weak ones that could be created in the gas phase.     

Monitoring of the metal displacement from the recombinant mouse liver metallothionein Zn(7)-complex by capillary zone electrophoresis with electrospray MS detection

A solution of a Zn-complex of recombinant mouse MT-1 isoform (Zn(7)-MT-1) was prepared and titrated with Cd(2+) ions. A method based on the coupling of capillary zone electrophoresis (CZE) with electrospray MS detection was developed for the analysis of the stoichiometry of the species formed during the titration. The method offered the possibility of the on-line removal of up to 100 mM Tris or phosphate buffer solutions that would otherwise suppress the electrospray signal. By allowing the determination of the metal stoichiometry of the complex species present in solution the method was shown to be complementary to circular dichroism and UV-VIS spectrophotometry conventionally used for similar studies. The titration of the Zn(7)-MT complex with Cd(II) showed the sequential displacement of the Zn by Cd. The unusually high stability of the Cd(6)Zn(1)-MT species was observed which suggests a structural role of the remaining Zn(II) ion.     

Concave Reagents 36: Transition Metal Complexes With Concave 1,10-phenanthrolines

Complexes between concave 1,10-phenanthrolines 1 and transition metal ions (Mn 2+ , Fe 2+ , Fe 3+ , Co 2+ , Ni 2+ , Cu + , Cu 2+ , Zn 2+ , Ag + , Cd 2+ ) have been synthesized and characterized by various methods including UV titration, NMR spectroscopy, EPR spectroscopy, cyclovoltammetry, microanalysis and X-ray analysis. The concave 1,10-phenanthrolines 1 form 1:1 complexes with the metal ions while the complexes of analogous, but non-bimacrocyclic 1,10-phenanthrolines such as 2 and 2,9-dimethyl-1,10-phenanthroline exist in varying stoichiometries.     

新型镉(Ⅱ)配位聚合物的合成与性能

以1,3,5-三(咪唑基)苯(tib),4,4-二苯醚二甲酸(H2L)和Cd(NO3)2·6H2O为原料,在水热条件下合成了一个新的Cd(Ⅱ)配合物[Cd2(L)4(tib)2DMF]n·DMF(Cd-2 MOF)(1,CCDC:1901161),其结构经IR,元素分析和X-射线单晶衍射表征。结果表明:配合物1属三斜晶系,P 21/n空间群,晶胞参数a=10.0363(9)A,b=14.1842(13)A,c=20.2842(18)A,α=97.754(3)°,β=98.410(3)°,γ=104.960(3)°。室温固态荧光测试结果显示配合物1具有很强的发射峰,并研究其变温荧光性质,通过后功能化修饰掺杂稀土元素实现了Ln^3+封装的MOF(Ln=Eu,Tb,Eu/Tb)材料发射的细调。     

Metal-ion-coordinating properties of the dinucleotide 2'-deoxyguanylyl(5'-->3')-2'-deoxy-5'-guanylate (d(pGpG)3-): isomeric equilibria including macrochelated complexes relevant for nucleic acids

The interaction between divalent metal ions and nucleic acids is well known, yet knowledge about the strength of binding of labile metal ions at the various sites is very scarce. We have therefore studied the stabilities of complexes formed between the nucleic acid model d(pGpG) and the essential metal ions Mg2+ and Zn2+ as well as with the generally toxic ions Cd2+ and Pb2+ by potentiometric pH titrations; all four ions are of relevance in ribozyme chemistry. A comparison of the present results with earlier data obtained for M(pUpU)- complexes allows the conclusion that phosphate-bound Mg2+ and Cd2+ form macrochelates by interaction with N7, whereas the also phosphate-coordinated Pb2+ forms a 10-membered chelate with the neighboring phosphate diester bridge. Zn2+ forms both types of chelates with formation degrees of about 91% and 2.4% for Zn[d(pGpG)]cl/N7 and Zn[d(pGpG)]-cl/PO, respectively; the open form with Zn2+ bound only to the terminal phosphate group, Zn[d(pGpG)]-op, amounts to about 6.8 %. The various intramolecular equilibria have also been quantified for the other metal ions. Zn2+, Cu2+, and Cd2+ also form macrochelates in the monoprotonated M[H;d(pGpG)] species (the proton being at the terminal phosphate group), that is, the metal ion at N7 interacts to some extent with the P(O)2(OH)- group. Thus, this study demonstrates that the coordinating properties of the various metal ions toward a pGpG unit in a nucleic acid differ: Mg2+, Zn2+, and Cd2+ have a significant tendency to bridge the distance between N7 and the phosphate group of a (d)GMP unit, although to various extents, whereas Pb2+ (and possibly Ca2+) prefer a pure phosphate coordination.