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PubMed2004

EDTA and mixed-ligand complexes of tetravalent and trivalent plutonium.

EDTA forms stable complexes with plutonium that are integral to nuclear material processing, radionuclide decontamination, and the potentially enhanced transport of environmental contamination. To characterize the aqueous Pu(4+/3+)EDTA species formed under the wide range of conditions of these processes, potentiometry, spectrophotometry, and cyclic voltammetry were used to measure solution equilibria. The results reveal new EDTA and mixed-ligand complexes and provide more accurate stability constants for previously identified species. In acidic solution (pH < 4) and at 1:1 ligand to metal ratio, PuY (where Y4- is the tetra-anion of EDTA) is the predominant species, with an overall formation constant of log beta110 = 26.44. At higher pH, the hydrolysis species, PuY(OH)- and PuY(OH)(2)2-, form with the corresponding overall stability constants log beta(11 - 1) = 21.95 and log beta(11 - 2) = 15.29. The redox potential of the complex PuY at pH = 2.3 was determined to be E(1/2) = 342 mV. The correlation between redox potential, pH, and the protonation state of PuY- was derived to estimate the redox potential of the Pu(4+/3+)Y complex as a function of pH. Under conditions of neutral pH and excess EDTA relative to Pu4+, PuY(2)4- forms with an overall formation constant of log beta120 = 35.39. In the presence of ancillary ligands, mixed-ligand complexes form, as exemplified by the citrate and carbonate complexes PuY(citrate)3- (log beta1101 = 33.45) and PuY(carbonate)2- (log beta1101 = 35.51). Cyclic voltammetry shows irreversible electrochemical behavior for these coordinatively saturated Pu4+ complexes: The reduction wave is shifted approximately -400 mV from the reduction wave of the complex PuY, while the oxidation wave is invariant.

Journal Article↗
PubMed2004

Imidazole and imidazolate iron complexes: on the way for tuning 3D-structural characteristics and reactivity. Redox interconversions controlled by protonation state.

X-ray structures for six Fe(II) and Fe(III) complexes from two closely heptadentate N-tripodal ligands, L1H(3) = tris[(imidazol-4-yl)-3-aza-3-butenyl]amine and L2H(3) = tris[(imidazol-2-yl)-3-aza-3-butenyl]amine, are described: three complexes in the L1 series (namely, [Fe(II)(L1H(3))](2+) and [Fe(III)(L1H(3))](3+) at low pH and [Fe(III)(L1)](0) at high pH) and three complexes in the L2 series (namely, [Fe(II)(L2H(3))](2+) at low pH and [Fe(II)(L2H)](0) and [Fe(III)(L2)](0) at high pH). Most of these complexes are stable in both Fe(II) and Fe(III) redox states and with the ligand in various protonation states. In the solid state, hydrogen bonds networks were obtained. Structural differences induced by 2- or 4-imidazole substitution are described and discussed. In solution, interconversions between different forms, with regard to oxidation and protonation states, were investigated by UV-visible spectroscopy, cyclic voltammetry, and potentiometry. The deprotonation pattern of these polyimidazole iron(II) and iron(III) complexes is described in detail. pK(a)s of the imidazolate/imidazole moieties in MeOH/H(2)O are reported. Two new species, namely, [Fe(II)(L1)](-) and [Fe(II)(L2)](-), were shown to be obtained in DMSO upon strong base addition and characterized by UV-vis spectroscopy and cyclic voltammetry. Half-wave potentials of Fe(III)/Fe(II) complexes with ligand moieties in several protonation states are reported, both in DMSO and in MeOH/H(2)O. Because of the presence of free imidazole groups coordinated to the iron, the potential of the iron(III)/iron(II) couples can be tuned by pH. A shift of DeltaE = E(deprot) - E(prot) ranging from -270 to -320 mV per exchanged proton in DMSO was measured. This study shows moreover that interconversions (with regard to both redox and protonation states) can be reversed several times. As the complexes have been isolated in order to be tested as superoxide dismutase mimics, preliminary reactions with dioxygen and with superoxide, considered as oxidant and reducer of biological importance, are reported. In these two series, O(2)(-) behaves either as a base or as a reducer and no adducts have been observed.

Journal Article↗
PubMed2004

Ionic liquids based upon metal halide/substituted quaternary ammonium salt mixtures.

The synthesis of ionic liquids based upon functionalized quaternary ammonium salts and metal salts of zinc, tin, or iron is demonstrated. The freezing point of these ionic liquids was studied as a function of the quaternary ammonium cation. The complex anions were identified and quantified using mass spectrometry and potentiometry. It is shown that the primary zinc anion is Zn(2)Cl(5)(-) with Zn(3)Cl(7)(-) becoming more abundant in more Lewis basic solutions. Similar results were observed for ionic liquids containing SnCl(2). The surface tension was also measured and was used to explain the high viscosity of the ionic liquids in terms of the large ion:hole size ratio and the small probability of finding a hole of suitable dimensions adjacent to a given ion to permit movement. The phase behavior of a variety of quaternary ammonium halides/ZnCl(2) mixtures is characterized and it is shown that the depression of freezing point is related to the increase in size of the component ions.

Journal Article↗
PubMed2006

CO2 Fixation by Cu2+ and Zn2+ complexes of a terpyridinophane aza receptor. Crystal structures of Cu2+ complexes, pH-metric, spectroscopic, and electrochemical studies.

The synthesis of the terpyridinophane-type polyamine 2,6,9,12,16-pentaaza[17]-(5,5' ')-cyclo-(2,2':6',2' ')-terpyridinophane heptahydrobromide tetrahydrate (L.7HBr.4H2O) is described. L presents six protonation constants with values in the range 9.21-3.27 logarithmic units. L interacts with Cu2+ and Zn2+ forming in both cases, neutral, protonated, and hydroxylated mono- and binuclear complexes whose constants have been calculated by potentiometry in 0.15 M NaClO4 at 298.1 K. The crystal structures of the compounds [Cu(HL-carb)(H2O)](ClO4)3.2H2O (1) and [Cu2(H2L)(CO3)]2(ClO4)8.9H2O (2) have been solved by X-ray diffraction. In 1, the metal center presents square pyramidal geometry. The base of the pyramid is formed by the three nitrogen atoms of pyridine and one oxygen atom of a CO2 group which is forming a carbamate bond with the central nitrogen atom of the polyamine chain. The axial position is occupied by a water molecule. In 2, one Cu2+ is bound by the three pyridine nitrogens and the other one by the three central nitrogen atoms of the polyamine chain. The square planar coordination geometry is completed by a carbonate group taken up from the atmosphere that behaves as a bridging mu,mu'-ligand between the two centers. The pH-metric titrations on the ternary Cu2+-L-carbonate and Zn2+-L-carbonate systems show the extensive formation of adduct species which above pH 6 are formed quantitatively in solution. The stoichiometries of the main species formed in solution at pH = 6.8 agree with those found in the crystalline compounds. CO2 uptake by the Zn2+ and Cu2+ 1:1 complexes in aqueous solution has also been followed by recording the variations in the band at ca. 300 nm. The formation of the Zn2+ carbamate moiety has been evidenced by 13C NMR and ESI spectroscopy.

Carbon Dioxide↗
PubMed2007

Dinuclear complexes formed with the triazacyclononane derivative ENOTA4-: high-pressure 17O NMR evidence of an associative water exchange on [MnII2(ENOTA)(H2O)2].

Mn2+ has five unpaired d-electrons, a long electronic relaxation time, and labile water exchange, all of which make it an attractive candidate for contrast agent application in medical magnetic resonance imaging. In the quest for stable and nonlabile Mn2+ complexes, we explored a novel dimeric triazacyclononane-based ligand bearing carboxylate functional groups, H4ENOTA. The protonation constants of the ligand and the stability constants of the complexes formed with some endogenously important metals (Ca2+, Cu2+, Zn2+), as well as with Mn2+ and Ce3+, have been assessed by NMR methods, potentiometry, and UV-vis spectrophotometry. Overall, the thermodynamic stability of the complexes is lower as compared to that of the corresponding NOTA analogues (H3NOTA, 1,4,7-triaazacyclononane-1,4,7-triacetic acid). The crystal structure of Mn2(ENOTA)(H2O) x 5H2O contains two six-coordinated Mn2+, in addition to the three amine nitrogens and the two oxygens from the pendent monodentate carboxylate groups, and one water (Mn2) or one bridging carboxylate oxygen (Mn1) completes the coordination sphere of the metal ion. In an aqueous solution, this bridging carboxylate is replaced by a water molecule, as evidenced by the 17O chemical shifts and proton relaxivity data that point to monohydration for both metal ions in the dinuclear complex. A variable-temperature and -pressure 17O NMR study has been performed on [Mn2(ENOTA)(H2O)2] to assess the rate and, for the first time on a Mn2+ chelate, also the mechanism of the water exchange. The inner sphere water is slightly more labile in [Mn2(ENOTA)(H2O)2] (k298ex = 5.5 x 107 s-1) than in the aqua ion (2.1 x 107 s-1, Merbach, A. E.; et al. Inorg. Chem. 1980, 19, 3696). The water exchange proceeds via an almost limiting associative mechanism, as evidenced by the large negative activation volume (deltaV = -10.7 cm3 mol-1). The proton relaxivities measured on [Mn2(ENOTA)(H2O)2] show a low-field dispersion at approximately 0.1 MHz arising from a contact interaction between the MnII electron spin and the water proton nuclear spins.

Aza Compounds↗
PubMed1997

Alkaline Earth Metal and Lanthanide(III) Complexes of Ligands Based upon 1,4,7,10-Tetraazacyclododecane-1,7-bis(acetic acid).

The macrocyclic ligand DO2A (1,4,7,10-tetraazacyclododecane-1,7-bis(acetic acid)) was prepared and used as a building block for four new macrocyclic ligands having mixed side-chain chelating groups. These ligands and their complexes with Mg(II), Ca(II), and Ln(III) were studied extensively by potentiometry, high-resolution NMR, and water proton relaxivity measurements. The protonation constants of all compounds compared well with those of other cyclen-based macrocyclic ligands. All Ca(II) complexes were found to be more stable than the corresponding Mg(II) complexes. Trends for the stabilities of the Ln(III) complexes are discussed and compared with literature data, incorporating the effects of water coordination numbers, Ln(III) contraction, and the nature of the side chains and the steric hindrance between them. (1)H NMR titrations of DO2A revealed that the first and second protonations take place preferentially at the secondary ring nitrogens, while the third and fourth involved protonation of the acetates. (17)O NMR shifts showed that the DyDO2A(+) complex had two inner-sphere water molecules. Water proton spin-lattice relaxation rates for the GdDO2A(+) complex were also consistent with water exchange between bulk water and two inner-sphere Gd(III) coordination positions. Upon formation of the diamagnetic complexes of DO2A (Ca(II), Mg(II), La(III), and Lu(III)), all of the macrocyclic ring protons became nonequivalent due to slow conformational rearrangements, while the signals for the acetate CH(2) protons remained a singlet.

Journal Article↗
PubMed1997

Copper(II) Complexes of Bis(1,4,7-triazacyclononane) Ligands with Polymethylene Bridging Groups: An Equilibrium and Structural Study.

Copper(II) complexation by a series of ligands containing two 1,4,7-triazacyclononane, [9]aneN(3), groups conjoined by polymethylene chains two to six carbons in length is described. Equilibrium modeling studies in aqueous solution using pH-potentiometry indicate that the smallest homologue of the series, EM2, forms only Cu(EM2)(2+) in dilute aqueous solutions. All other ligands of the series form stable 1:1 (protonated and nonprotonated) and 2:1 dicopper(II) (hydroxo and non-hydroxo) complexes. Those ligands that contain bridging chains of four or more carbon atoms likely form dimeric or oligomeric complex species in solution. The EM ligands with the shortest polymethylene bridging groups form the most stable 1:1 species. There is little difference among the ligands (n = 3-6) in complex stability of the protonated, CuH(2)(EMn)(4+), and dicopper(II), Cu(2)(EMn)(4+), species. UV-vis spectroscopic continuous variation studies at pH 4.0 and 7.5 are interpreted on the basis of the principal equilibrium species obtained from the equilibrium models. Single-crystal X-ray diffraction studies on four complexes ([Cu(EM2)]SO(4).6H(2)O (1), [Cu(2)(EM2)Cl(4)].2H(2)O (2), [Cu(2)(EM6)Cl(4) ] (3), and [Cu(EM3)][ZnBr(4)].H(2)O (4)) characterize structural features of several 1:1 monomeric and dicopper(II) complexes in the crystalline solid. The monomeric compounds contain CuN(6) chromophores while the dicopper(II) compounds contain square pyramidal CuN(3)Cl(2) coordination geometry. Compound 1 crystallizes in space group P&onemacr; with a = 7.849(2) Å, b = 9.783(2) Å, c = 16.919(5) Å, alpha = 78.42(3) degrees, beta = 85.76(3) degrees, gamma = 73.06(3) degrees, and Z = 2. 2: P2(1)/n with a = 9.689(3) Å, b = 11.733(3) Å, c = 10.124(3) Å, beta = 98.20(2) degrees, and Z = 2. 3: P2(1)/n with a = 7.278(2) Å, b = 12.416(3) Å, c = 13.781(2) Å, beta = 90.15(2) degrees, and Z = 2. 4: P2(1)/c with a = 9.295(3) Å, b = 16.233(4) Å, c = 16.544(5) Å, beta = 92.62(2) degrees, and Z = 4. Cyclic voltammograms of aqueous solutions prepared by dissolving [Cu(2)(EM2)Cl(4)].2H(2)O confirm its dissociation to Cu(EM2)(2+). Aqueous solutions containing 1:1 molar ratios of Cu(II) and EM2 in 0.1 mol dm(-)(3) KCl at 25 degrees C show a one-electron chemically reversible reduction at scan rates of 500 mV s(-)(1) with E(1/2) (Cu(II)-Cu(I)) = -868 mV relative to SCE. EPR (X- and Q- band) spectra of frozen solutions (1:1 DMSO/H(2)O and glycerol/H(2)O) of Cu(EM2)(2+) at 100 K are typical of axial copper(II) features (X-band parameters: g( parallel) =2.225 (A( parallel) = 164 x 10(-)(4)) and g( perpendicular) = 2.045).

Journal Article↗
PubMed1998

Trinuclear Lanthanoid Complexes of 1,3,5-Triamino-1,3,5-trideoxy-cis-inositol with a Unique, Sandwich-Type Cage Structure(1).

A variety of trinuclear complexes [M(3)(H(-)(3)L)(2)](3+) [M = Y, La, Eu, Gd, Dy; L = 1,3,5-triamino-1,3,5-trideoxy-cis-inositol (taci) and 1,3,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inositol (tdci)] was prepared as solid materials of the composition M(3)(H(-)(3)L)(2)X(3).pH(2)O.qEtOH (X = Cl, NO(3); 2.5 </= p </= 9; q = 0, 0.33) and characterized by elemental analyses, NMR spectroscopy, and FAB(+) mass spectrometry. The crystal structures of [La(3)(H(-)(3)taci)(2)(H(2)O)(4)Cl]Cl(2).3H(2)O and [Gd(3)(H(-)(3)taci)(2)(H(2)O)(6)]Cl(3).3H(2)O were elucidated by single-crystal X-ray diffraction studies. The La complex crystallizes in the orthorhombic space group Pbca, a = 17.10(2) Å, b = 16.20(4) Å, c = 20.25(4) Å, Z = 8 for C(12)Cl(3)H(38)La(3)N(6)O(13). The Gd complex crystallizes in the monoclinic space group P2(1)/n, a = 10.294(3) Å, b = 15.494(5) Å, c = 19.994(6) Å, beta = 95.36(2) degrees, Z = 4 for C(12)Cl(3)Gd(3)H(42)N(6)O(15). The two complexes exhibited a unique, sandwich-type cage structure, where the two triply deprotonated taci ligands encapsulate an equilateral triangle of the three metal centers. The metal cations are coordinated to the equatorial, terminal amino groups and are bridged by the axial &mgr;(2)-alkoxo groups. The coordination spheres are completed by additional peripheral ligands such as H(2)O or Cl(-) counterions. The coordination number of the metal cations is 8. Magnetic susceptibility measurements of the Gd complex revealed very weak antiferromagnetic coupling interactions between the three Gd centers. Complex formation and species distribution in aqueous solution was investigated by potentiometry and pD-dependent NMR spectroscopy. An exclusive formation of the [Eu(3)(H(-)(3)taci)(2)](3+) unit in solution was found in the range 7 </= pH </= 10. The formation constants were determined for the Y, Eu, Gd, Dy, and Lu complexes with taci. The stability of the lanthanoid complexes increased monotonically with decreasing ionic radius of the metal center.

Journal Article↗
PubMed1999

Solution Characterization of Vanadium(V) and -(IV) N-(Phosphonomethyl)iminodiacetate Complexes: Direct Observation of One Enantiomer Converting to the Other in an Equilibrium Mixture(1).

The vanadium(V) and -(IV) complexes formed with N-(phosphonomethyl)iminodiacetate (pida(4)(-)) complexes in water and in aqueous mixed-solvent systems were characterized by employing potentiometry, multinuclear NMR, EPR, and visible spectroscopy. The vanadium(V) complexes in water had a 1:1 metal to ligand stoichiometry. The diprotonated vanadium(V)-pida complex with a charge of -1 existed below pH 2. After the first deprotonation, the dianionic species was stable from pH 1 to 6. The trianionic species was stable at neutral pH. Observations are most consistent with the protonation sites of these complexes on the organic ligand. Intra- and intermolecular exchange processes were identified using (13)C homonuclear EXSY spectroscopy. Given the asymmetric vanadium atom in the VO(2)pida(3)(-) complex, one of the exchange processes observed was the conversion of one enantiomer to the other in an equilibrium mixture. Studies of the corresponding vanadium(IV)-pida system in dioxane-water mixtures showed the presence of not only mononuclear species but also a dinuclear species (which was recently characterized by X-ray crystallography). In 80:20 (v/v) dioxane-water mixtures, the dinuclear species became the major species in solution, as evidenced by absorption spectroscopy and EPR spectroscopy. Potentiometric measurements in mixed solvents failed to conclusively favor any stoichiometry. In conclusion, the mononuclear vanadium(IV) and -(V) complexes with the H(4)pida ligand appear to be structurally related and could readily be distinguished from the dinuclear vanadium(IV)-pida complex that was characterized both in the solid state and in mixed-solvent solutions.

Journal Article↗
PubMed1999

Molecular Recognition of Activated and Unactivated Phosphate Diesters.

The molecular recognition of two simple phosphate diesters by a terpyridine-copper (TCu) complex was studied by X-ray crystallography and potentiometry. Molecular recognition of the substrate is the first step in a hydrolysis reaction. The two phosphate diesters bis(p-nitrophenyl) phosphate (BNP) and diphenyl phosphate (DPP) coordinate axially to an approximate square pyramidal copper complex where a chloride and three terpyridine nitrogens form the corners of the base. The coordination geometries are almost identical. The molecular recognition constants for the formation of TCu-BNP and TCu-DPP were measured potentiometrically and have log values of 1.2 and 2.5, respectively. By a significant margin, the TCu complex favors DPP over BNP in solution. A pH versus rate constant profile shows that TCu-OH can hydrolyze BNP but not DPP. Activated and unactivated phosphate diesters are typically hydrolyzed by similar mechanisms. In this study the difference in reactivity lies outside the process of molecular recognition. Activating (electron-withdrawing) effects of the p-nitro groups on the nature of the leaving groups must play an important role in the susceptibility of phosphate diesters to hydrolysis.

Journal Article↗
PubMed2000

Synthesis and characterization of racemic mixture and meso isomers of bis(trans-2-aminocyclohexyl)aminepentaacetic acid and the stabilities of their Gd(III) complexes.

The synthesis and characterization of two multidentate ligands, the racemic mixture and meso isomers of bis-(trans-2-aminocyclohexyl)aminepentaacetic acid, are described. Equilibrium constants for their Gd(III) complexes were determined by direct potentiometry. The formation constants (KML = [ML]/[M][L]) of Gd(III)-Cycyracemic in 0.10 M KCl at 25.0 degrees C is 10(20.71; that for the meso isomer is 10(20.42). The crystal structure of (1S,2S,1'S,2'S)-bis(trans-2-aminocyclohexyl)amine-N,N,N',N",N"-pentaacetic acid, penta-tert-butyl ester (C42H75N3O10), is reported. This compound crystallizes in the triclinic system with space group P1 with cell parameters a = 10.805(2) A, b = 11.382(2) A, c = 20.999(4) A, alpha = 91.41(3) degrees, beta = 98.23(3) degrees, gamma = 113.88(3) degrees, V = 2327.8(8) A3, Z = 2, Dx = 1.116 g mL-1. The results are compared to the crystal structures of the gadolinium complexes and the predictions derived from molecular mechanics.

Algorithms↗
PubMed2003

Direct electrochemistry of a bacterial sulfite dehydrogenase.

Sulfite dehydrogenase from Starkeya novella is an alphabeta heterodimer comprising a 40.6 kDa subunit (containing the Mo cofactor) and a smaller 8.8 kDa heme c subunit. The enzyme catalyses the oxidation of sulfite to sulfate with the natural electron acceptor being cytochrome c550. Its catalytic mechanism is thought to resemble that found in eukaryotic sulfite oxidases. Using protein film voltammetry and redox potentiometry, we have identified both Mo- and heme-centered redox responses from the enzyme immobilized on a pyrolytic graphite working electrode: E m,8 (Fe III/II) +177 mV; E m,8 (Mo VI/V) +211 mV and E m,8 (Mo V/IV) -118 mV vs NHE; Upon addition of sulfite to the electrochemical cell a steady-state voltammogram is observed and an apparent Michaelis constant (Km) of 26(1) microM was determined for the enzyme immobilized on the working electrode surface, which is comparable with the value obtained from solution assays.

Bacterial Proteins↗
PubMed2004

Hydrolysis of uranium(VI) at variable temperatures (10-85 degrees C).

The hydrolysis of uranium(VI) in tetraethylammonium perchlorate (0.10 mol dm(-3) at 25 degrees C) was studied at variable temperatures (10-85 degrees C). The hydrolysis constants (*beta(n,m)) and enthalpy of hydrolysis (Delta H(n,m)) for the reaction mUO(2)(2+) + nH(2)O = (UO(2))(m)(OH)(n)((2m-n))+) + nH(+) were determined by titration potentiometry and calorimetry. The hydrolysis constants, *beta(1,1), *beta(2,2), and *beta(5,3), increased by 2-5 orders of magnitude as the temperature was increased from 10 to 85 degrees C. The enthalpies of hydrolysis, Delta H(2,2) and Delta H(5,3), also varied: Delta H(2,2) became more endothermic while Delta H(5,3) became less endothermic as the temperature was increased. The heat capacities of hydrolysis, Delta C(p(2,2)) and Delta C(p(5,3)), were calculated to be (152 +/- 43) J K(-1) mol(-1) and -(229 +/- 34) J K(-1) mol(-1), respectively. UV/Vis absorption spectra supported the trend that hydrolysis of U(VI) was enhanced at elevated temperatures. Time-resolved laser-induced fluorescence spectroscopy provided additional information on the hydrolyzed species at different temperatures. Approximation approaches to predict the effect of temperature were tested with the data from this study.

Journal Article↗
PubMed2003

ED-XRF as a tool for rapid minerals control in milk-based products.

An ED-XRF method for the rapid determination of a series of analytes (phosphorus, sulfur, chlorine, potassium, calcium, iron, zinc) in milk-based products has been developed and validated. The investigated samples were commercial products obtained from various parts of the world. Reference values measured by inductively-coupled plasma-optical emission spectroscopy and by potentiometry for chloride were used to calibrate the ED-XRF. Calibrations were established with 30 samples, and validation was made using a second set of 30 samples. An evaluation of this alternative method was done by comparison with data from the reference methods. Pellets of 4 g were prepared under 2 tons of pressure. For each sample, 3 pellets were prepared and analyzed. Limits of quantification and repeatabilities were evaluated for the described analytes.

Animals↗
PubMed2004

Metal complexation by the peptide-bound maillard reaction products N(epsilon)-fructoselysine and N(epsilon)-carboxymethyllysine.

Although the Maillard reaction between proteins and carbohydrates is of central importance for food processing and in vivo processes, only little is known about changes of the metal-binding properties induced by protein glycation. The purpose of this study was to examine the complex formation of the quantitatively important peptide-bound Maillard reaction products (MRPs) N(epsilon)-fructoselysine and N(epsilon)-carboxymethyllysine with the biologically relevant metal ions copper(II) and zinc(II). The MRPs were synthesized as the N(alpha)-hippuryllysine derivatives in order to block the coordination function of the alpha-amino group. Stability constant measurements were performed in aqueous solution using pH potentiometry. N(alpha)-Hippuryl-N(epsilon)-fructoselysine forms moderate Cu(II) complexes (Log(10) K(1) = 5.8; Log(10) K(2) = 4.0) but fails to form any complexes with Zn(II). N(alpha)-Hippuryl-N(epsilon)-carboxymethyllysine gives slightly stronger complexes with Cu(II) (Log(10) K(1) = 7.3; Log(10) K(2) = 6.3), but again no complexation with Zn(II) was observed. These results show that post-translational modification of proteins by carbohydrates leads to the formation of new coordination centers for metal ions within a protein chain. Further studies are necessary to clarify the consequences of this phenomenon in terms of protein quality and physiological processes.

Copper↗
PubMed2005

Rapid multimineral determination in infant cereal matrices using wavelength dispersive X-ray fluorescence.

A rapid and simple method for the determination of a series of macroelements (sodium, magnesium, phosphorus, chlorine, potassium, and calcium) and trace elements (manganese, iron, and zinc) by wavelength dispersive X-ray fluorescence has been developed and validated for infant cereal matrices. Reference values were obtained by inductively coupled plasma optical emission spectroscopy and by potentiometry. The 88 investigated samples were commercially available products. Pellets of 4 g were prepared under 10 tonnes of pressure. For each sample, 3 pellets were prepared and analyzed. Limits of quantification and repeatabilities were evaluated. Calibrations were established with 43 samples, and method validation was made using a second set of 45 samples. An evaluation of this alternative method was done by comparison with data obtained from the reference methods. The results show the good performances of the alternative method to routine infant cereals analysis.

Edible Grain↗
PubMed2006

Speciation of phytate ion in aqueous solution. Sequestering ability toward mercury(II) cation in NaClaq at different ionic strengths.

As a contribution to understanding the speciation of mercury in the environment and to the study of the sequestering ability of phytate (Phy) toward heavy metal and organometal cations, this paper describes the results of an investigation (at t = 25 degrees C by potentiometry, ISE-H+ glass electrode) of its interactions with mercury(II) cation in NaCl aqueous solutions at different ionic strengths (I = 0.15 and 1.0 mol L(-1)), in the pH range 2.5 < or = pH < or = 9.5 and considering metal-to-ligand ratios of 1:1 < or = Hg/Phy < or = 4:1. The formation of 11 HgiHjPhy(12-2i-j)(- species with i = 1 and 0 < or = j < or = 7 and i = 2 and 0 < or = j < or = 2 was observed. Their complex formation constant values proved to be fairly dependent on ionic strength. The speciation of phytic acid and mercury(II) is also dependent on the metal-to-ligand ratio; the dependence of the stability of phytate-mercury(II) species on the phytate protonation step was modeled, and an empirical predictive relationship was proposed. From the results obtained, phytate has very good sequestering ability toward Hg2+, even in the presence of considerable excesses of chloride ion, that is, another ligand strongly interacting with mercury; this supports future studies both on the use of plants that naturally synthesize it for phytoremediation purposes and on its direct application in remediation techniques.

Cations↗
PubMed2000

Pb and Cu speciation and bioavailability in port wine.

Information about the speciation of Pb and Cu in different types of Port wines (white, single-year and blended aged red, and young red wines) was gathered to estimate their respective bioavailabilities to man. For this purpose, wines were subjected to in vitro simulated gastrointestinal digestion, and the following properties were studied in the wines and its gastric and intestinal digests: (1) average conditional stability constant (K(av)) of the Cu complexes (by potentiometry), of the strongest Pb complexes (those inert to cathodic voltammetry, K'(av)), and of the respective ligand concentrations (CC or CC(inert)); (2) the distribution of the metal among the different groups of compounds of different molecular weights and/or polarities in the different bands separated by reverse phase high-performance liquid chromatography; (3) the total metal concentration present in the wines and the respective fractions present in the soluble and in the dialyzable fractions of the digest (an estimation of the assimilable fraction). The study showed that the complexing affinity for Pb (expressed by either CC(inert) or K'(av)) of white and very aged red Port wines was lower than for the remainder of the wines. For Cu, the strength of the ligands in the white wines was lower (<log K(av) values) than in the other wines, but their concentrations (CC) were higher. For Pb, CC(inert) was much higher after the digestion than for the untreated wines, whereas the log K'(av) values were approximately 1 order of magnitude lower. These parameters could not be determined for Cu in the gastrointestinal digests. For all of the studied wines the dialyzable fraction of Pb during the intestinal digestion was low (10-22% of the total Pb) and the dialyzable fractions of Cu were close to 50% of the total Cu.

Biological Availability↗