Effect of biodegradable chelating ligands on Fe uptake in and growth of marine microalgae

Iron (Fe) is an important nutrient for phytoplankton. The low solubility of Fe in oxic waters can be a growth-limiting factor for phytoplankton. Synthetic aminopolycarboxylates (APCs) such as ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) are widely used as Fe complexing agents for microalgae culture. However, the presence of these non-ready biodegradable APC_S in aquatic systems may have serious environmental consequences. In the present study, the effects of biodegradable chelating ligands (hydroxyiminodisuccinic acid (HIDS), methylglycinediacetic acid (MGDA), and iminodisuccinate (IDS)) on Fe uptake in and growth of three coastal microalgae (Heterosigma akashiwo, Prymnesium parvum, and Skeletonema marinoi-dohrnii complex) were investigated, and the results were compared with those of non-ready biodegradable APCs (EDTA, ethylenediamine tetra-methylene phosphonic acid (EDTMP), and DTPA). The biodegradable chelating ligands did not have significant growth inhibition effect on the phytoplankton. Although the growth of the algae (except S. marinoi-dohrnii complex) was not affected substantially by 1.5 and 7.5 μM of DTPA, growth inhibition occurred by 7.5 μM of EDTMP and 150 μM of EDTA, DTPA, and EDTMP. The effect of chelating ligands on microalgal growth was likely to be associated with the intracellular Fe uptake influenced by the chelating ligands. On average, intracellular Fe concentrations for biodegradable chelating ligands were substantially higher than those for non-ready biodegradable APCs. Except H. akashiwo, the ratio of intra/extracellular Fe concentrations was highest for MGDA followed by IDS and HIDS. The results indicate that biodegradable chelating ligands are more efficient than non-ready biodegradable APCs in intracellular Fe uptake and algal growth.     

Biospeciation, by potentiometry and computer simulation, of Sm-EDTMP, a bone tumor palliative agent

¹⁵³Sm-EDTMP (ethylenediaminetetra(methylenephosphonic) acid) is of considerable interest as a bone therapeutic radiopharmaceutical but its properties in solution are not yet well characterized. The protonation constants of EDTMP and the formation constants of the complexes of Sm-EDTMP have accordingly been measured potentiometrically by glass electrode titrations at 25°C in 0.15 M NaCl. Six protonation constants (log ₀₁₁ = 9.638, log ₀₁₂ = 17.330, log ₀₁₃ = 23.597, log ₀₁₄ = 28.636, log ₀₁₅ = 31.501, log ₀₁₆ = 32.624) and the formation constants of the [Sm(EDTMP)H_-1]^6- (log _11-1 = 4.865), [SmEDTMP]^5- (log ₁₁₀ = 12.018), [Sm(EDTMP)H]^4- (log ₁₁₁ = 17.892) and [Sm(EDTMP)H₂]^3- (log ₁₁₂ = 23.437) complexes were determined. Computer simulations indicate that the [SmEDTMP]^5- and the hydroxy [Sm(EDTMP)H_-1]^6- species are the major Sm(III) complexes formed in blood plasma, which explains the high degree of localization in the kidney and urine observed in biodistribution studies. Calcium ions are probably the maior competitor for EDTMP in blood plasma. As the presence of secondary skeletal metastases results in a high rate of bone turnover, it is possible that the high concentration of calcium at these sites encourages localization of ¹⁵³Sm-EDTMP.     

A rapid non-equilibrium critical precipitation assay to assess aluminium-ligand interactions

Abstract

In many natural situations (e.g. environmental and biological) aqueous metal-ligand interactions occur in complex, dynamic solutions and do not adhere to true equilibrium. Nonetheless, equilibrium-based assays in simple solutions are generally used to model metal-ligand interactions of natural systems. Moreover, these are time consuming and not easily applied or understood by many applied scientists. Here, a ‘critical precipitation assay’ was used to investigate the interaction of common ligands with aluminium at pH 7.0, under non-equilibrium conditions. Results obtained were correlated with literature-derived stability constants for the aluminium-ligand interactions, while high-resolution ¹H nuclear magnetic resonance spectroscopy (¹H NMR) was used to confirm the nature of observed interactions. Weak interaction with aluminium was confirmed for traditional weak ligands (e.g. bicarbonate) as these were unable to compete with the hydroxide ion for aluminium at pH 7.0. Two types of interaction were seen for the ‘stronger’ ligands that could compete with hydroxy-polymerisation. Firstly, distinct aluminium:ligand stoichiometric ratios were observed for ligands such as ethylenediaminetetra-acetic acid (1:1) or 1,3,5-trideoxy-1,3,5-tris( dimethylamino)-cis-inositol (1:2). Secondly, most ligands, including citrate and maltol, did not prevent hydroxy-polymerisation but did maintain more aluminium ‘in solution’ (approximately 2.5:1 aluminium:ligand) than permitted by acceptable aluminium:ligand stoichiometric ratios, suggesting the formation of dynamic metastable hydroxy-bridged aluminium-ligand complexes. ¹H NMR with aluminium and maltol or citrate, supported this idea as complex spectral patterns were observed prior to precipitation. Aluminium maintained in solution at pH 7.0 correlated, with literature-derived stability constants suggesting that non-equilibrium aluminium-ligand interactions approximate to equilibrium and that this assay could be used as a quick screening method for investigation of aluminium-ligand interactions.     

Liver Restores Immune Homeostasis after Local Inflammation despite the Presence of Autoreactive T Cells

The liver must keep equilibrium between immune tolerance and immunity in order to protect itself from pathogens while maintaining tolerance to food antigens. An imbalance between these two states could result in an inflammatory liver disease. The aims of this study were to identify factors responsible for a break of tolerance and characterize the subsequent restoration of liver immune homeostasis. A pro-inflammatory environment was created in the liver by the co-administration of TLR ligands CpG and Poly(I:C) in presence or absence of activated liver-specific autoreactive CD8⁺ T cells. Regardless of autoreactive CD8⁺ T cells, mice injected with CpG and Poly(I:C) showed elevated serum ALT levels and a transient liver inflammation. Both CpG/Poly(I:C) and autoreactive CD8⁺T cells induced expression of TLR9 and INF-γ by the liver, and an up-regulation of homing and adhesion molecules CXCL9, CXCL10, CXCL16, ICAM-1 and VCAM-1. Transferred CFSE-labeled autoreactive CD8⁺ T cells, in presence of TLR3 and 9 ligands, were recruited by the liver and spleen and proliferated. This population then contracted by apoptosis through intrinsic and extrinsic pathways. Up-regulation of FasL and PD-L1 in the liver was observed. In conclusion, TLR-mediated activation of the innate immune system results in a pro-inflammatory environment that promotes the recruitment of lymphocytes resulting in bystander hepatitis. Despite this pro-inflammatory environment, the presence of autoreactive CD8⁺ T cells is not sufficient to sustain an autoimmune response against the liver and immune homeostasis is rapidly restored through the apoptosis of T cells.     

Negative and positive ion chemical ionization mass spectrometry of dioximes and their nickel complexes

The negative ion mass spectra of Ni(LH)₂ (where LH₂ is glyoxime, methylglyoxime, dimethylglyoxime and diphenylglyoxime), in the presence of ammonia or methane at 0.5 torr, are reported and compared with the spectra of the free ligands. In each case, the base peak of the complex is the molecular negative ion and the extent of fragmentation was found to decrease gradually going from the glyoximato to the diphenylglyoximato derivative. In the chemical ionization mass spectra of the free ligands, the [M]⁻ ion is absent in all cases. The base peak is [M  H]⁻ for methylglyoximine, dimethylglyoxime and diphenylglyoxime and [M  H  H₂]⁻ for glyoxime. The fragmentation occurs largely by loss of H, OH, H₂O and NO species. The positive ion chemical ionization mass spectra of the same complexes show very abundant [M + H]⁺ and [M]⁺ ions and weak fragments, whilst a rather high fragmentation is observed for the corresponding free ligands.     

Preparation,radiolabeling and biodistribution of a new class of bisaminothiol (BAT) ligands as possible imaging agents

In developing new ligands as potential brain and heart perfusion imaging agents two ligands based upon N₂S₂ donor atoms with the biphenyl backbone were synthesized. Biphenyl-2,2′-bis(N-1-amino-2-methyl-propane-2-thiol) (BP-BAT-TM) and biphenyl-2,2′-bis(N-1-amino-2-ethyl-butane-2-thiol) (BP-BAT-TE) form stable, neutral and lipid soluble complexes with [^99mTc]pertechnetate in the presence of tin(II) tartarate as a reducing agent. The [^99mTc]BP-BAT-TM complex penetrates the blood-brain barrier following i.v. injection into rats. Washout from the brain is fast, indicating no retention. The biodistribution of [^99mTc]BP-BAT-TE in rats showed an intitial heart uptake (0.8% /organ, at 2 min) and a slow washout (0.74% at 15 min). No brain uptake was found (0.05%). Significant uptake and retention in liver was observed. An imaging study of [^99mTc]BP-BAT-TE in a monkey showed no brain uptake and a clear indication of liver uptake and gall bladder clearance. These results indicate that this ligand system may be suitable as the basic core structure for the development of new imaging agents. Further studies with structural variations in the biphenyl backbone are warranted to develop new ^99mTc imaging agents for clinical applications.     

Dinitrosyl Iron Complexes with Thiol-Containing Ligands Exist in Living Organisms Mainly in the Binuclear Form

The levels of the mononitrosyl iron complex with diethyldithiocarbamate that form in the liver of mice in vivo and in vitro after intraperitoneal injection of binuclear dinitrosyl iron complexes with N-acetyl-L-cysteine or glutathione, S-nitrosoglutathione, sodium nitrite, or the vasodilating drug isosorbide dinitrate (Isoket®) have been assessed by electron paramagnetic resonance (EPR). The levels of the complex in mice that received binuclear dinitrosyl iron complexes with thiol-containing ligands or S-nitrosoglutathione do not change after the treatment of liver preparations with the strong reducing agent dithionite, in contrast to those formed after nitrite or isosorbide dinitrate administration, whose levels sharply increase after the same treatment. It is inferred that in the latter case an EPR-active mononitrosyl iron complex with diethyldithiocarbamate is produced with the absence or presence of dithionite in the reaction of NO formed from nitrite with Fe²⁺-diethyldithiocarbamate and Fe³⁺-diethyldithiocarbamate complexes, respectively. In the former case, the mononitrosyl iron complex with diethyldithiocarbamate is produced by transition of iron-mononitrosyl fragments from already present iron-dinitrosyl groups of binuclear dinitrosyl complexes, whose content is three to four times higher than the content of the mononuclear form of these complexes in the tissue. The results we obtained indicate that when dinitrosyl iron complexes with thiol-containing ligands, either introduced into the body or produced with the participation of endogenous NO, appear in animal tissues in vivo, these complexes are presented in these tissues mainly in their diamagnetic, EPR-silent binuclear form.

     

Exploring the binding interactions of structurally diverse dichalcogenoimidodiphosphinate ligands with α-amylase: Spectroscopic approach coupled with molecular docking

Postprandial hyperglycemia has orchestrated untimely death among diabetic patients over the decades and regulation of α-amylase activity is now becoming a promising management option for type 2 diabetes. The present study investigated the binding interactions of three structurally diverse dichalcogenoimidodiphosphinate ligands with α-amylase to ascertain the affinity of the ligands for α-amylase using spectroscopic and molecular docking methods. The ligands were characterized using ¹H and ³¹P NMR spectroscopy and CHN analysis. Diselenoimidodiphosphinate ligand (DY300), dithioimidodiphosphinate ligand (DY301), and thioselenoimidodiphosphinate ligand (DY302) quenched the intrinsic fluorescence intensity of α-amylase via a static quenching mechanism with bimolecular quenching constant (Kq) values in the order of x10¹¹ M^-1s^-1, indicating formation of enzyme-ligand complexes. A binding stoichiometry of n≈1 was observed for α-amylase, with high binding constants (Ka). α-Amylase inhibition was as follow: Acarbose > DY301>DY300>DY302. Values of thermodynamic parameters obtained at temperatures investigated (298, 304 and 310 K) revealed spontaneous complex formation (ΔG<0) between the ligands and α-amylase; the main driving forces were hydrophobic interactions (with DY300, DY301, except DY302). UV–visible spectroscopy and Förster resonance energy transfer (FRET) affirmed change in enzyme conformation and binding occurrence. Molecular docking revealed ligands interaction with α-amylase via some key catalytic site amino acid residues (Asp197, Glu233 and Asp300). DY301 perhaps showed highest α-amylase inhibition (IC₅₀, 268.11 ± 0.74 μM) due to its moderately high affinity and composition of two sulphide bonds unlike the others. This study might provide theoretical basis for development of novel α-amylase inhibitors from dichalcogenoimidodiphosphinate ligands for management of postprandial hyperglycemia.     

Syntheses and characterization of neutral complexes of Zn(II) with mono- and dianionic pyrrole-2-imine ligands [(pyrrole-2-CHN)nR]n− (n = 1, 2)

The reactions of the dianionic [(pyrrole-2-CHN)₂R]^2? ligands [(N′₂N₂)^2?] (R = (R)(S)-1,2-cyclohexane or 1,2-ethane) with Zn(II) yield neutral dimeric [Zn₂(N′₂N₂)₂] complexes. The dimeric nature of the complexes was established by field-desorption mass spectrometry. ¹H NMR studies show that these complexes have dimeric structures in solution in which the (N′₂N₂)^2? ligands act as di-bidentates.The metal centres have tetrahedral geometries and bot have Δ or Λ configurations. The complex with the (R)(S)-1,2-cyclohexanediyl bridges has a rigid structure in solution. Neither intermolecular nor intramolecular exchange processes are observed The ¹H NMR spectrum of the complex with the 1,2-ethanediyl bridging groups shows that at 213 K in CDCl₃ a fast conformational movement is already taking place between two identical structures of the complex. It is not possible to determine whether in this complex intermolecular exchange processes are also taking place.The reactions of the anionic [pyrrole-2-CHNR′]^? ligands [(N′N)^?] (R′ = t-Bu, i-Pr, (S)-CHMePh or 2,6-xylyl) with Zn(II) yield the neutral Zn(N′N)₂ complexes. These complexes were synthesized to study the coordination properties of the [pyrrole-2-CHNR′]^? moieties with Zn(II). A ¹H NMR study established that the zinc centres in the complexes containing the prochiral i-Pr or chiral (S)-CHMePh substituents have tetrahedral geometries with Δ or Λ configurations in CDCl₃ at 213 K. These complexes undergo an intramolecular exchange process at higher temperatures (above 260 K when R′ = i-Pr) which involves inversion of the configuration of the zinc centre. A mechanism for this exchange process is proposed.     

A new ternary complex between horse liver alcohol dehydrogenase, NAD+, and acetone

Acetone was found to form a dead-end ternary complex with horse liver alcohol dehydrogenase and oxidized nicotinamide adenine dinucleotide (NAD⁺) when the reactants were incubated for a long time at relatively high concentrations. The complex formation was demonstrated by measuring the increase in absorbance at 320 nm, the quenching of protein fluorescence, and the loss of enzyme activity. Since acetone is a substrate of liver alcohol dehydrogenase, and the presence of acetaldehyde or pyrazole prevents acetone from forming the dead-end complex with liver alcohol dehydrogenase and NAD⁺, the acetone molecule in the complex may be bound to the substrate binding site of liver alcohol dehydrogenase. The dissociation of the complex was demonstrated by prolonged dialysis or by addition of reduced nicotinamide adenine dinucleotide (NADH) and iso-butyramide. A modified nicotinamide adenine dinucleotide was obtained as a main product from the dead-end complex after dissociation of the complex or denaturation of the apoenzyme. The modified nicotinamide adenine dinucleotide was found to exhibit an absorption spectrum similar to that of NADH; however, it was not oxidizable by liver alcohol dehydrogenase in the presence of acetaldehyde and exhibited no fluorescence.     

Complexes with the fac-{M(CO)3} (M = Tc, Re) moiety and long alkyl chain ligands as Lipiodol surrogates

Accumulation of radiopharmaceuticals in the liver is frequently observed and represents in general a limiting factor when developing novel labeled compounds for any purpose in nuclear medicine. Aiming at the treatment of liver cancer with radiopharmaceuticals, such accumulation is desired but the compounds have to remain in the liver over an extended time period rather than being washed out or redistributed over time in the whole body. Lipiodol is known to remain in the liver and we present here a study for the preparation of ¹⁸⁶Re and ^99mTc labeled Lipiodol surrogates expected to behave similarly. We have synthesized two bidentate and two tridentate ligands conjugated to a pendant C18 chain as well as their corresponding fac-[Re(CO)₃]⁺ and fac-[Tc(CO)₃]⁺ complexes. Three of the rhenium complexes have been structurally characterized. Labelling with [¹⁸⁶Re(OH₂)₃(CO)₃]⁺ and [^99mTc(OH₂)₃(CO)₃]⁺, respectively, gave yields in the range of 90%. The complexes could be extracted into Lipiodol due to their high lipophilicity and close structural relationship with the major components of Lipiodol. The complexes are stable in water and in Lipiodol for more than 24 h. These Lipiodol surrogates present new low-valent technetium and rhenium complexes for applications in liver cancer imaging and therapy.     

Uranyl(VI) complexes of ethylene-1,2-dioxydiacetic acid and ethylene-1,2-diaminodiacetic acid in aqueous solution: a potentiometric and calorimetric study

The stability constants and the changes in enthalpy and entropy for the formation of uranyl(VI) complexes with dicarboxylate ligands of the type (-CH₂RCH₂COO)₂²⁻, where RO or NH, have been determined by potentiometric and calorimetric titrations at 25.0 °C in 1.0 mol dm⁻³ aqueous solution of sodium perchlorate.The ethylene-l,2-dioxydiacetate ligand forms either 1:1 and 1:2 chelated complexes or unchelated protonated complexes, owing to the low stability of the chelate ring. Because of precipitation of solid compounds only one complex of 1:1 stoichiometry was observed in the uranyl(VI)—ethylene-l,2-diaminodiacetate system.Factors influencing the stability constants and the enthalpy and entropy changes in the uranyl(VI) complexes with these potentially tetradentate ligands are discussed in comparison with analogous complexes involving bi- and tridentate dicarboxylate ligands.     

Fluorine-substituted corticosteroids: Synthesis and evaluation as potential receptor-based imaging agents for positron emission tomography of the brain

We have prepared eight fluorine-substituted corticosteroids representing ligands selective for Type I and Type II corticosteroid receptor subtypes as potential imaging agents for corticosteroid receptor-containing regions of the brain. Receptor binding affinity assays show that fluorine substitution for hydroxyl or hydrogen in these steroids generally results in some reduction in affinity, with the result that the absolute affinity of these fluorine-substituted ligands for receptor is less than that typical for steroid hormones that show receptor-based, target selective uptake in vivo. Five of these compounds were prepared in fluorine-18 labeled form by a simple sulfonate ester displacement reaction, and their tissue distribution was studied in the adrenalectomized rat. There is no selective accumulation nor selective retention of the Type I selective corticosteroids (¹⁸F-RU 26752, 21-[¹⁸F]fluoroprogesterone, 21-[¹⁸F]fluoro-11β-hydroxyprogesterone) in either the brain, or other target tissues (pituitary, kidney, liver). The Type II selective corticosteroids (¹⁸F-RU 28362, ¹⁸F-triamcinolone acetonide) show uptake into the hippocampus which can be partially blocked by a competing ligand; in target tissues outside the brain, the blocking is more complete. All of the ¹⁸F-labeled compounds show considerable defluorination, evident as high bone activity levels. These results, coupled with earlier findings in the literature, suggest that radiolabeled corticosteroid receptor ligands with both greater metabolic stability and higher receptor binding affinity and selectivity are needed for imaging corticosteroid receptors in the hippocampus.     

Theoretical investigations of the experimentally observed selectivity of a cobalt imprinted polymer

A cobalt imprinted polymer synthesised, for reducing the volume of radioactive waste generated during nuclear reactor decontaminations, using vinylbenzyl iminodiacetate (VbIDA) as the functional ligand, has been found to be selective for cobaltous ions over excess ferrous ions. The selectivity of the polymer has been investigated through theoretical calculation of the formation energies of complexes involved by using the ab-initio density functional theory (DFT) code SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms). The formation energies of complexes of Fe²⁺, Co²⁺, Cu²⁺ and Ni²⁺ with the free functional ligands as well as with ligands attached to the crosslinkers have been calculated. The calculations revealed that the ferrous forms an unstable complex with the ligands attached to the crosslinkers. The formation energy calculation results were found to corroborate the experimentally observed selectivity order.     

Uptake and subcellular cleavage of organomercury compounds by rat liver and kidney

The uptake and transformation of phenylmercuric acetate (PMA), ethylmercury chloride (EMC), and methylmercury chloride (MMC) by rat liver, kidney, and brain slices were investigated. A slightly greater enzymatic activity for the cleavage of PMA and EMC was observed in the kidney than in the liver; brain tissue had the least activity.The average rates of C-Hg cleavage were 2.4 to 3.0 nmole g^?1 h^?1 for PMA and 0.9 to 1.0 nmole g^?1 h^?1 for EMC. No cleavage of C-Hg bond from MMC was observed by the kidney and liver.The cleavage of PMA was an enzymatic process and the enzyme was located in the nuclear fraction of both liver and kidney. The apparent Michaelis constant and the maximal velocity value for PMA conversion by rat liver slices were 625 μM and 19.2 nmole per g tissue per 2 h, respectively.It is postulated that the PMA molecule is first split at the C-Hg bond to yield benzene and mercuric ion.The cleavage of the EMC molecule appears to be more complex.     

Metal complexes of N-hydroxy-imino-di-α-propionic acid and related ligands

N-hydroxy-imino-di-α-propionic acid, the ligand present in the natural oxovanadium(IV) complex ‘amavadin’ which occurs in the toadstool Amanita muscaria, has been synthesised, as well as two related ligands—N-hydroxy-iminodiacetic acid and imino-di-α-propionic acid—useful for comparison purposes. The formation of complexes of these ligands with VO²⁺, Ni²⁺ has been studied and their stability constants have been determined.The two N-hydroxy-substituted ligands, of low basicity, form ML₂ complexes with VO²⁺, unlike the more basic derivatives of iminodiacetic acid. Since substitution of ligands bonded to the apical site trans to the oxo ligand is very fast and the formation of ML₂ complexes of VO²⁺ exposes that apical site to the reaction media, this may be the reason why oxovanadium(IV) and the unusual derivative of iminodiacetic acid present in ‘amavadin’ were selected for the biological role that this complex plays in the toadstool.     

Antileukemic activity and cellular effects of rhodium(III) crown thiaether complexes

The cytostatic properties of novel rhodium(III) thiacrown ether complexes [RhCl(LL)([9]aneS₃)]ⁿ⁺ with either aromatic κ² N ligands (n = 2) or anionic chelate ligands (n = 1) have been investigated for the human cancer cell lines HT-29 and MCF-7 and for immortalized HEK-293 cells. Taken together with literature IC₅₀ values for analogous complexes with polypyridyl ligands or 1,4-dithiane, the in vitro assays indicate that dicationic complexes with soft κ² N (imino) or κ² S (thiaether) ligands exhibit significantly higher antiproliferative effects than those with hard κ² N (amino) ligands. Dicationic complexes are more active than monocationic complexes with similar ligands. Pronounced apoptosis-inducing properties towards Jurkat cells were established for complexes with LL = bpm, dpq, and 1,4-dithiane. The order of activity (bpm > 1,4-dithiane > dpq > bpy) contrasts to that observed for adhesive cancer cells (bpm > bpy, 1,4-dithiane > dpq). Necrosis is insignificant in all cases. The percentage of Jurkat cells exhibiting apoptosis after 24 or 48 h incubation periods is directly correlated to the percentage of cells exhibiting high levels of reactive oxygen species. As established by online monitoring with a sensor chip system, treatment of MCF-7 cells with the bpm and 1,4-dithiane complexes leads to a significant and permanent concentration-dependent decrease in oxygen consumption and cellular adhesion.     

Interaction between alkaline earth cations and oxo-ligands. DFT study of the affinity of the Ca<Superscript>2+</Superscript> cation for carbonyl ligands

The affinity of the Ca²⁺ ion for a set of substituted carbonyl ligands was analyzed with both the DFT (B3LYP/6-31+G(d)) and semi-empirical (PM6) methods. Two types of ligands were studied: a set of monosubstituted [O=CH(R)] and a set of disubstituted ligands [O=C(R)₂] (R=H, F, Cl, Br, OH, OCH₃, CH₃, CN, NH₂ and NO₂), with R either directly bound to the carbonyl carbon atom or to the para position of a phenyl ring. The interaction energy was calculated to quantify the affinity of the Ca²⁺ cation for the ligands. Geometric and electronic parameters were correlated with the intensity of the metal-ligand interaction. The electronic nature of the substituent is the main parameter that determines the interaction energy. Donor groups make the interaction energy more negative (stabilizing the complex formed), while acceptor groups make the interaction energy less negative (destabilizing the complex formed).     

Coordination chemical studies on metalloenzymes. IX. Properties of the ternary complex between cobalt(II)-bovine carbonic anhydrase and bidentate ligands

The spectrum, thermodynamic parameters, and proton longitudinal relaxation time of the ternary complex between various bidentate ligands (2-pyridinecarboxylate, 2-quinolincarboxylate, 8-quinolinecarboxylate, and 2-pyridylacetate) and cobalt(II)-bovine carbonic anhydrase were measured to clarify the nature of the ternary complex. The formation constants of the ternary complexes of bidentate ligands are in the order of (2-pyridinecarboxylate ? 8-quinolinecarboxylate ? 2-quinolinecarboxylate ≈2-pyridylacetate). The degree of the shift of the band characteristic of five-coordinate species at 13-15 kcm^-1 (cm^-1 × 10^-3) and that of the higher energy band at 21–22 kcm^-1 decrease almost in the same order. These results are explained on the basis of the contribution of the bond formation between the nitrogen atom of the heterocyclic ring of ligands and cobalt ion. The formation constants of the ternary complex of bidentate ligands were compared to the stability constants of various ligands with a cobalt ion but there is no correlation in these values. The rate constant of break-up of the ternary complex was discussed on the coordination geometry of the ternary complex on the basis of the degree of the distortion.     

Speciation of copper(II) in natural waters in the presence of ligands of high and intermediate strength

Abstract

A new procedure is presented for the determination of the ligands of copper(II) in natural waters, based on titration with the metal ion, monitored by measuring the concentration of copper(II) sorbed on the carboxylic resin Amberlite CG 50. The data are treated by the Ruzic linearization method to obtain the concentration of the ligands and the conditional stability constant of the complexes. Ligands with reaction coefficient α_M higher than 0.1 K*w/V are detected, where K* is the ratio of the concentration of sorbed metal to the concentration of free metal in solution, which can be evaluated from the sorption equilibria of copper(II) on Amberlite CG 50, w is the amount of water in the resin phase, and V the volume of the solution phase. Some natural waters at high and low salinity were examined. The ligand concentration determined in these samples ranged from around 50 to 2000 nM, while the original copper concentrations from 11 to 130 nM. The ligand concentration was always much higher than that of copper(II). The conditional stability constants were very high, particularly in low salinity waters, where values as high as K’= 10^15.7 were obtained. In high salinity waters values around 10⁹ were found for the complex formation constant of the ligands titrated with copper(II). The investigation was also extended to a model solution, containing EDTA, obtaining K’ = 10^15.5, in acceptable agreement with that evaluated from the literature values.