Mobilisation of recently absorbed Fe in ex vivo perfused rat duodena and the influence of iron status and subsequently absorbed chelators

To investigate the effect of subsequently absorbed metal chelators on recently absorbed ⁵⁹Fe, duodenal segments from iron-deficient and iron-adequate rats were perfused ex vivo until the ⁵⁹Fe tissue load had reached a steady state. Subsequently, the segments were perfused with 3 model chelators and their iron complexes: nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA) and citrate. Of these, NTA and EDTA bind iron much tighter than citrate, and Fe–NTA complexes exchange iron within seconds while Fe-EDTA complexes need 48 h to reach equilibrium.

Duodenal mucosa-to-serosa transport rates were comparable for all 3 chelators and correlated linearly with luminal concentration. Subsequent perfusion with increasing NTA, Fe–NTA(1:2) and EDTA concentrations mobilised increasing amounts of ⁵⁹Fe from the duodenum. Mobilised ⁵⁹Fe moved preferentially back into the luminal perfusate in iron-adequate segments. In iron-deficient segments, ⁵⁹Fe preferentially continued the absorption process across the basolateral membrane. Fe–EDTA(1:1) hardly mobilised any ⁵⁹Fe back into the lumen, though basolateral transfer increased at high concentrations. Citrate and Fe–citrate(1:1) mobilised ⁵⁹Fe only at very high concentrations.

This behaviour is in accordance with the rules of complex chemistry: strong, fast reacting ligands like NTA show most impact. Slowly reacting complexes like Fe–EDTA(1:1) have little mobilising impact in spite of strong affinity between EDTA and iron. The low affinity between iron and citrate can be compensated by large concentration. Moreover, iron-deficient segments show stronger re-uptake of mobilised ⁵⁹Fe from the lumen and a stronger transfer of ⁵⁹Fe from the tissue across the basolateral membrane. Both are compatible with the more marked expression of divalent metal transporter 1 (DMT-1) and IREG-1 at the brushborder and basolateral membrane of iron-deficient enterocytes. The data suggest that iron ions interact with food ligands during their passage from the apical to the basolateral side of duodenal enterocytes.     

Effect of complexing agents on reduction of Cr(VI) by Desulfovibrio vulgaris ATCC 29579

The reduction of Cr(VI) at the expense of molecular hydrogen was studied using resting cells of Desulfovibrio vulgaris ATCC 29579 in anaerobic resting cell suspensions in MOPS buffer. Bioreduction occurred only in the presence of ligands or chelating agents (CO32-, citrate, NTA, EDTA, DTPA). The stimulatory effect of these ligands on the rate of Cr(VI) reduction was correlated (r = 0.988) with the strength of the ligand/chelate complex of Cr(III). The data are examined with respect to likely solution and redox equilibria in the ionic matrix of the carrier solution, and with respect to the potential for bioremediation of Cr(VI).     

Hemoglobin: A mechanism for the generation of hydroxyl radicals

Oxyhemoglobin (HbO₂) reduces Fe(III) NTA aerobically to become methemoglobin (metHb) and Fe(II)NTA. These conditions are favorable for the generation via Fenton chemistry of the hydroxyl radical that was measured by HPLC using salicylate as a probe. The levels of hydroxyl radicals generated are a function of both the percent metHb formed and the chemical nature of the buffer. The rates of formation of both metHb and hydroxyl radicals were dependent upon the concentration of Fe(III)NTA. Of the buffers tested, HEPES was the most effective scavenger of hydroxyl radicals while the other buffers scavenged in the order: HEPES > Tris > MOPS > NaCl ≈ unbuffered. The addition of catalase to remove H₂0₂ or bathophenanthroline to chelate Fe(II) inhibited virtually all hydroxyl radical formation. Carbonyl formation from free radical oxidation of amino acids was found to be 0.1 mol/mol of hemoglobin. These experiments demonstrate the ability of hemoglobin to participate directly in the generation of hydroxyl radicals mediated by redox metals, and provide insight into potential oxidative damage from metals released into the blood during some pathologic disorders including iron overload.     

Synthesis,structure analysis,solution chemistry,and in vitro insulinomimetic activity of novel oxovanadium(IV) complexes with tripodal ligands containing an imidazole group derived from amino acids

Structures, chemical properties, and in vitro insulinomimetic activities of new vanadyl [oxovanadium(IV), VO(2+)] complexes with five tripodal ligands containing an imidazole functionality were examined. The ligands, N-(carboxymethyl)- N-(4-imidazolylmethyl)amino acids, contain glycine, ( S)- and ( R)-alanine, and ( S)- and ( R)-leucine residues. The molecular structures of the latter four alanine- and leucine-containing complexes were determined by X-ray analysis. The coordination geometry around each vanadium center was octahedral, where an imino nitrogen occupied the apical site and two carboxylate oxygens, an imidazole nitrogen, and a water molecule coordinated in the equatorial plane. The spectroscopic properties of the complexes were characterized by means of IR, electronic absorption, and CD spectra. Acid dissociation constants (p K(a)) and protonation sites of the ligands were determined by a combination of potentiometric titrations and (1)H NMR spectra. The potentiometric study demonstrated that stability constants (log beta) were not so different among the present complexes (14.0-14.9) and a species of molecular complex with a 1:1 metal:ligand ratio existed predominantly at physiological pH 7.4. EPR parameters indicated that the species at pH 7.4 had an octahedral structure similar to the complex in the solid state. On the other hand, an EPR study in phosphate buffer (pH 7.4) suggested that inorganic phosphate coordinated to the vanadium center instead of the imidazole group in the presence of excess phosphate ion. Cyclic voltammograms in the phosphate buffer showed chemically reversible oxidation waves, whereas irreversible oxidation waves were observed in non-coordinating HEPES buffer. Moreover, the oxidation potential of each complex in phosphate buffer was more positive than that in HEPES buffer. Partition coefficients of the present complexes in a n-octanol/saline system were very low, probably due to hydrophilicity of the imidazole group. The in vitro insulinomimetic activities were estimated on the basis of the ability of the complexes to inhibit epinephrine-stimulated free fatty acid release from isolated rat adipocytes. The achiral glycine-derivative complex exhibited the highest insulinomimetic activity, which was higher than that of VOSO(4) as a positive control. Putting our previous observations together, it was found that the vanadyl complexes with tetradentate amino acid derivatives having no alkyl side chain tend to have high in vitro insulinomimetic activity.     

Nuclear magnetic resonance studies of the solution chemistry of metal complexes. 26. Mixed ligand complexes of cadmium, nitrilotriacetic acid, glutathione, and related ligands

The complexation of glutathione and related ligands by the nitrilotriacetic acid complex of Cd2+ (Cd(NTA)-) has been investigated by 1H NMR as a model for the coordination chemistry of Cd2+ and GSH in biological systems. Related ligands included glycine, glutamic acid, cysteine, N-acetylcysteine, penicillamine, N-acetylpenicillamine, mercaptosuccinic acid, and the S-methyl derivative of glutathione. The nature of the complexes formed was deduced from 1H NMR spectra of Cd(NTA)- and the ligands. Mixed ligand complexes (Cd(NTA)L) and single ligand complexes (CdLx) are formed with the thiol ligands, whereas only mixed ligand complexes form with glycine, glutamic acid and S-methylglutathione. Formation constants of the mixed and the single ligand complexes were determined from NMR data. The results indicate that formation constants for binding of a thiolate donor group by Cd2+, either as the free ion or in a coordinately unsaturated complex, are in the range 10(5)-10(6).     

Synthetic analogue approach to metallobleomycins: syntheses, structure and properties of mononuclear and tetranuclear gallium(III) complexes of a ligand that resembles the metal-binding site of bleomycin

As part of our interest into the bioinorganic chemistry of gallium, gallium(III) complexes of the peptide ligand N-(2-(4-imidazolyl)ethyl)pyridine-2-carboxamide (pypepH2) resembling a fragment of the metal-binding domain of bleomycins (BLMs), have been isolated. Reaction of pypepH2 with (Et4N)[GaCl4] and Ga(acac)3 [acac- is the acetylacetonate(-1) ion] affords the mononuclear complex [Ga(pypepH)2]Cl.2H2O (1) and the tetranuclear complex [Ga4(acac)4(pypep)4].4.4H2O (2), respectively. Both complexes were characterized by single-crystal X-ray crystallography, IR spectroscopy and thermal decomposition data. The pypepH- ion in 1 behaves as a N(pyridyl), N(deprotonated amide), N(pyridine-type imidazole) chelating ligand. The doubly deprotonated pypep2- ion in 2 behaves as a N(pyridyl), N(deprotonated amide), N(imidazolate), N'(imidazolate) mu2 ligand and binds to one Ga(III) atom at its pyridyl, amide and one of the imidazolate nitrogens, and to a second metal ion at the other imidazolate nitrogen; a chelating acac- ligand completes six coordination at each Ga(III) centre. The IR data are discussed in terms of the nature of bonding and known structures. The 1H NMR spectrum of 1 suggests that the cation of the complex maintains its integrity in dimethylsulfoxide (DMSO) solution. Complexes 1 and 2 are the first synthetic analogues of metallobleomycins with gallium(III).     

Zinc enzyme modelling with zinc complexes of polar pyrazolylborate ligands

The Zn-OH₂ and Zn-OH complexes of the new tris(pyrazolyl)borate ligands with pyridyl and carboxamido substituents were investigated for their reactivity towards hydrolyzeable substrates. Tp^4−Py,MeZn-OH inserted CO₂ and CS₂ in methanol forming the Zn-OCOOMe and Zn-SCSOMe products. In non-aqueous media, both types of complexes with both types of substituents on the Tp ligands effected stoichiometric cleavage of tris(p-nitrophenyl)phosphate and p-nitrophenyl acetate. In solutions containing water and the MOPS buffer, up to eight p-nitrophenyl groups per equivalent of zinc complex could be cleaved from the esters, and the resulting bis(p-nitrophenyl)phosphate was also degraded to mono(p-nitrophenyl)phosphate. This is the first time that pyrazolylborate-zinc complexes have shown catalytic activity in hydrolytic reactions.     

Superoxide-Dependent Formation of Hydroxyl Radicals from Ferric-Complexes and Hydrogen Peroxide: An Evaluation of Fourteen Iron Chelators

When a variety of ferric chelates are reacted with hydrogen peroxide in phosphate buffer deoxyribose is damaged and this damage is protected against by formate, thiourea and mannitol. Damage done by ferric complexes of citrate, EDTA, NTA, EGTA and HEDA is substantially inhibited by superoxide dismutase (SOD) whereas complexes of PLA. ADP and CDTA are moderately inhibited by SOD. The effects of SOD argue against hydrogen peroxide acting as a reductant in Fenton chemistry driven by ferric complexes and hydrogen peroxide. EDTA has proved to be a useful model for Fenton chemistry that is inhibited by SOD although, it is not unique in this respect.     

Synthesis,structure, spectroscopic and in vitro antitumour studies of a novel gallium(III) complex with 2-acetylpyridine (4)N-dimethylthiosemicarbazone

The reaction of 2-acetylpyridine 4N-dimethylthiosemicarbazone (HL) with GaCl(3) in absolute ethanol in 1:1 molar ratio yielded the complex [GaL(2)][GaCl(4)]. The crystal structure of the gallium(III) complex has been determined by X-ray diffraction methods. Infrared, electronic, ESI mass and (1)H, (13)C, (15)N and (71)Ga NMR spectra, as well as the thermal behaviour are reported. The cytotoxicity assay in several human cancer cell lines (SW480, SK-BR-3 and 41M) suggests that the gallium(III) complex might be endowed with promising antitumour properties. In vitro cytotoxic activity exceeds that of all other tested gallium(III) complexes and is slightly higher than that of HL.     

A gallium complex with a new tripodal tris-hydroxypyridinone for potential nuclear diagnostic imaging: solution and in vivo studies of 67Ga-labeled species

The gallium(III) complex of a new tripodal 3-hydroxy-4-pyridinone (3,4-HP) chelator has been studied in terms of its physico-chemical and in vivo properties aimed at potential application as probe for nuclear imaging. In particular, based on spectrophotometric titrations, the hexa-coordinated (1:1) gallium complex appeared as the major species in a wide physiological acid-neutral pH range and its high stability (pGa = 27.5) should avoid drug-induced toxicity resulting from Ga(III) accumulation in tissues due to processes of transmetallation with endogenenous ligands or demetallation. A multinuclear (¹H and ⁷¹Ga) NMR study gave some insights into the structure and dynamics of the gallium(III) chelate in solution, which are consistent with the tris-(3,4-HP) coordination and an eventual pseudo-octahedral geometry. Biodistribution and scintigraphic studies of the ⁶⁷Ga(III) labelled chelate, performed in Wistar rats, confirmed the in vivo stability of the radiolabelled complex, its non interaction with blood proteins and its quick renal clearance. These results indicate good perspectives for potential application of extrafunctionalized analogues in radiodiagnostic techniques.     

Stable adducts of nerve agents sarin,soman and cyclosarin with TRIS,TES and related buffer compounds—Characterization by LC-ESI-MS/MS and NMR and implications for analytical chemistry

Buffering compounds like TRIS are frequently used in chemical, biochemical and biomedical applications to control pH in solution. One of the prerequisites of a buffer compound, in addition to sufficient buffering capacity and pH stability over time, is its non-reactivity with other constituents of the solution. This is especially important in the field of analytical chemistry where analytes are to be determined quantitatively. Investigating the enzymatic hydrolysis of G-type nerve agents sarin, soman and cyclosarin in buffered solution we have identified stable buffer adducts of TRIS, TES and other buffer compounds with the nerve agents. We identified the molecular structure of these adducts as phosphonic diesters using 1D ¹H-³¹P HSQC NMR and LC-ESI-MS/MS techniques. Reaction rates with TRIS and TES are fast enough to compete with spontaneous hydrolysis in aqueous solution and to yield substantial amounts (up to 20–40%) of buffer adduct over the course of several hours. A reaction mechanism is proposed in which the amino function of the buffer serves as an intramolecular proton acceptor rendering the buffer hydroxyl groups nucleophilic enough for attack on the phosphorus atom of the agents. Results show that similar buffer adducts are formed with a range of hydroxyl and amino function containing buffers including TES, BES, TRIS, BIS-TRIS, BIS-TRIS propane, Tricine, Bicine, HEPES and triethanol amine. It is recommended to use alternative buffers like MOPS, MES and CHES when working with G-type nerve agents especially at higher concentrations and over prolonged times.     

Re-evaluating the effects of organic ligands on copper toxicity to barley root elongation in culture solution

Abstract

In the presence of weak ligands, both free ion activity and organic complexes of Cu should b considered when predicting Cu toxicity in aquatic and soil-plant systems. However, there is littl information about the quantitative contribution of Cu that is organically complexed to Cu toxicity. In thi study, a bioassay using barley root elongation in culture solution was used to investigate the effects o organic ligands with different conditional stability constants on Cu toxicity and the quantitativ contribution of the organically complexed Cu to the Cu toxicity. The results indicated that a significan decrease (p<0.05) in Cu toxicity, assessed by barley root elongation, was observed in response to th addition of organic ligands. The decrease differed, to some extent, with different organic ligands o disodium ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), oxalate and malate at low and constant free Cu²⁺ activity. Addition of EDTA or NTA resulted in strong reduction of Cu toxicity while modest reduction of Cu toxicity was observed for the addition of malate as the relatively wea ligand. Furthermore, the results of the present study revealed that the CuNTA^? and CuEDTA^2? complexes were not toxic, while the Cu–malate complexes were mildly toxic to barley root elongation More importantly, it was found that the toxicity of Cu–malate complexes were nearly 0.5-fold less than that of free Cu²⁺ ions.     

Blockage of chloride channels by HEPES buffer

Chloride channels of neurons of Drosophila are blocked when the cytoplasmic side of the membrane is exposed to the commonly used buffering agents 4-(2-hydroxyethyl)-1-piperazine ethanesulphonic acid (HEPES) and 4-morpholinepropanesulphonic acid (MOPS). In the presence of these compounds, chloride channels appear to function as a complex of multiple protochannels.     

Zwitterionic buffers preserve ram semen quality more efficiently than TRIS during storage at 15 °C

This study was designed to evaluate the effect of various buffers on the storage of ram semen at 15 °C. Second ejaculates from six adult males were collected using an artificial vagina and diluted in either MOPS, TRIS, TES, HEPES, citrate, or phosphate-based extenders. Semen samples were stored at 15 °C and the sperm motility and viability (membrane integrity) variables assessed after 0, 24 and 48 h intervals. Significantly higher progressive sperm motility rates were recorded at 0 h of storage, and higher motile and progressive sperm motility at 24 and 48 h, when zwitterionic-based extenders (MOPS, TES and HEPES) were used, compared to citrate, TRIS, and phosphate-based extenders—with the last group showing a drastic reduction in sperm motility during storage. The zwitterionic groups were also superior to the other treatments in terms of sperm velocity (straight line velocity, VSL; curvilinear velocity, VCL; average path velocity, VAP) at 0 h of storage, although at 24 and 48 h the differences were minimal in the CITRATE group—regarding all velocity variables, and in the TRIS group, regarding the VCL parameter. Sperm diluted in the TRIS-based extender showed a marked increase in the proportion of circular sperm trajectories (lower sperm linearity, LIN, and straightness, STR), and a decrease in the VAP. The reduction in the vigour of the sperm in the TRIS extender (measured by VCL) was less pronounced than in the other groups. At the same time, VSL was reduced, as more sperm moved in circles, and the amplitude of lateral head displacement (ALH) was also dramatically increased. The CITRATE diluent recorded intermediate results—between that of TRIS and the other treatment groups, regarding the variables related to the quality of sperm movement at 0 h storage. However, following CITRATE dilution, semen underwent a clear improvement after a period of 24 and 48 h, so that differences with the zwitterionic groups were attenuated or disappeared. Similarly, the CITRATE group obtained similar or higher viable sperm values, compared to zwitterionic buffers during storage. The TRIS, and particularly the PHOSPHATE diluents, recorded poorer sperm viability after 24 and 48 h of storage. It was concluded that zwitterion-based buffers may be an acceptable alternative for inclusion in the composition of diluents for chilled ram semen storage. On the other hand, TRIS may be seen as having caused drastic modifications of certain sperm kinematic parameters during storage at 15 °C.     

Low-dimensional compounds containing bioactive ligands. Part XIV: High selective antiproliferative activity of tris(5-chloro-8-quinolinolato)gallium(III) complex against human cancer cell lines

Four gallium(III) complexes, [Ga(ClQ)₃]⋅MeOH (1 – MeOH), [Ga(ClQ)₃] (1), [Ga(BrQ)₃] (2), [Ga(dIQ)₃] (3) and [Ga(CQ)₃] (4), were prepared (H-ClQ = 5-chloro-8-quinolinol, H-BrQ = 7-bromo-8-quinolinol, H-dIQ = 5,7-diiodo-8-quinolinol, H-CQ = 5-chloro-7-iodo-8-quinolinol) and characterised by elemental analysis, IR and NMR spectroscopy. Single crystal structure analysis of 1 – MeOH confirmed that the complex has a molecular structure with gallium(III) metal ion coordinated in mer-fashion by N- and O-donor atoms of three ClQ ligands. Stability of all complexes in DMSO was proved by ¹H NMR spectroscopy. The in vitro antiproliferative activity of 1 was evaluated against the A2780, MBA-MB-231 and HCT116 cell lines. Complex 1 displays higher antiproliferative activity (IC₅₀ values in the range 2.1–6 μm) compared to the ClQ ligand and cisplatin; and a significant selective antiproliferative potency (IC₅₀ = 136 μm, for normal MRC5pd30 cell line). Radical scavenging experiments revealed that complex 1 exhibits the highest antioxidant activity of the prepared complexes as well as the ligands.     

Effect of Pressure-Induced Changes in the Ionization Equilibria of Buffers on Inactivation of Escherichia coli and Staphylococcus aureus by High Hydrostatic Pressure

Survival rates of Escherichia coli and Staphylococcus aureus after high-pressure treatment in buffers that had large or small reaction volumes (ΔV°), and which therefore underwent large or small changes in pH under pressure, were compared. At a low buffer concentration of 0.005 M, survival was, as expected, better in MOPS (morpholinepropanesulfonic acid), HEPES, and Tris, whose ΔV° values are approximately 5.0 to 7.0 cm³ mol⁻¹, than in phosphate or dimethyl glutarate (DMG), whose ΔV° values are about −25 cm³ mol⁻¹. However, at a concentration of 0.1 M, survival was unexpectedly better in phosphate and DMG than in MOPS, HEPES, or Tris. This was because the baroprotective effect of phosphate and DMG increased much more rapidly with increasing concentration than it did with MOPS, HEPES, or Tris. Further comparisons of survival in solutions of salts expected to cause large electrostriction effects (Na₂SO₄ and CaCl₂) and those causing lower electrostriction (NaCl and KCl) were made. The salts with divalent ions were protective at much lower concentrations than salts with monovalent ions. Buffers and salts both protected against transient membrane disruption in E. coli, but the molar concentrations necessary for membrane protection were much lower for phosphate and Na₂SO₄ than for HEPES and NaCl. Possible protective mechanisms discussed include effects of electrolytes on water compressibility and kosmotropic and specific ion effects. The results of this systematic study will be of considerable practical significance in studies of pressure inactivation of microbes under defined conditions but also raise important fundamental questions regarding the mechanisms of baroprotection by ionic solutes.     

Effects of TRIS and HEPES on Function of Rabbit Muscle Light Sarcoplasmic Reticulum

ATP hydrolysis activity and calcium transport activity were determined on light sarcoplasmic reticulum from rabbit skeletal muscle. The effects of two buffers, TRIS and HEPES, were compared. Titration of TRIS into sarcoplasmic reticulum preparations in HEPES provided evidence for TRIS inhibition of ATPase activity and TRIS stimulation of calcium transport activity.     

Environmental fate and microbial degradation of aminopolycarboxylic acids

Aminopolycarboxylic acids (APCAs) have the ability to form stable, water-soluble complexes with di- and trivalent metal ions. For that reason, synthetic APCAs are used in a broad range of domestic products and industrial applications to control solubility and precipitation of metal ions. Because most of these applications are water-based, APCAs are disposed of in wastewater and reach thus sewage treatment plants and the environment, where they undergo abiotic and/or biotic degradation processes. Recently, also natural APCAs have been described which are produced by plants or micro-organisms and are involved in the metal uptake by these organisms. For the two most widely used APCAs, nitrilotriacetate (NTA) and ethylenediaminetetraacetate (EDTA), transformation and mineralisation processes have been studied rather well, while for other xenobiotic APCAs and for the naturally occurring APCAs little is known on their fate in the environment. Whereas NTA is mainly degraded by bacteria under both oxic and anoxic conditions, biodegradation is apparently of minor importance for the environmental fate of EDTA. Photodegradation of iron(III)-complexed EDTA is supposed to be mostly responsible for its elimination. Isolation of a number of NTA- and EDTA-utilising bacterial strains has been reported and the spectrum of APCAs utilised by the different isolates indicates that some of them are able to utilise a range of different APCAs whereas others seem to be restricted to one compound. The two best characterised obligately aerobic NTA-utilising genera (Chelatobacter and Chelatococcus) are members of the alpha-subgroup of Proteobacteria. There is good evidence that they are present in fairly high numbers in surface waters, soils and sewage treatment plants. The key enzymes involved in NTA degradation in Chelatobacter and Chelatococcus have been isolated and characterised. The two first catabolic steps are catalysed by a monooxygenase (NTA MO) and a membrane-bound iminodiacetate dehydrogenase. NTA MO has been cloned and sequenced and its regulation as a function of growth conditions has been studied. Under denitrifying conditions, NTA catabolism is catalysed by a NTA dehydrogenase. EDTA breakdown was found to be initiated by a MO also which shares many characteristics with NTA MO from strictly aerobic NTA-degrading bacteria. In contrast, degradation of [S,S]-ethylenediaminedisuccinate ([S,S]-EDDS), a structural isomer of EDTA, was shown to be catalysed by an EDDS lyase in both an EDTA degrader and in a NTA-utilising Chelatococcus strain. So far, transport of APCAs into cells has only been studied for EDTA and the results obtained give strong evidence for an energy-dependent carrier system and Ca(2+) seems to be co-transported with EDTA. Due to their metal-complexing capacities, APCAs occur in the environment mostly in the metal-complexed form. Hence, the influence of metal speciation on various degradation processes is of utmost importance to understand the environmental behaviour of these compounds. In case of biodegradation, the effect of metal speciation is rather difficult to assess at the whole cell level and therefore only limited good data are available. In contrast, the influence of metal speciation on the intracellular enzymatic breakdown of APCAs is rather well documented but no generalising pattern applicable to all enzymes was found.     

Spectroscopic studies of aqueous gallium(III) and aluminum(III) citrate complexes

Aqueous gallium(III) citrate complexes have been studied in the 10(-2) M concentration range with extended X-ray absorption fine structure (EXAFS) and FTIR techniques. From EXAFS data, one mononuclear and one oligomeric species were identified at different Ga(III) to citrate ratios. The first shell of the mononuclear complex was found to be distorted, with average Ga-O bond lengths of 1.95 and 2.06 A, in agreement with the solid-state structure of Ga(Cit)2(3-) (Cit=citrate). Also the oligomeric species was found to have a distorted first shell, with average Ga-O bond lengths of 1.95 and 2.04 A. This complex was found to contain two Ga-Ga distances at 3.03 and 3.56 A, typical for edge and corner sharing GaO6 octahedra, respectively. The gallium(III) and aluminum(III) citrate systems were compared by means of FTIR, and were found to be analogous. The IR results suggest that the bond lengths derived from EXAFS for the 1:2 gallium(III) citrate complex also provide a good estimate of the corresponding distances in the mononuclear 1:1 complex. Direct coordination of citrate to the metal ions in the oligomeric gallium(III) citrate complex was indicated from both EXAFS and IR results, and this complex is stoichiometrically analogous to the Al3(H-1Cit)3(OH)(H2O)4- complex, which has been structurally determined. However, while the formation of the aluminum trimer has been shown to be slow, the gallium trimer was significantly more labile with a rate of formation indicated to be in the order of seconds or faster.     

N-Carboxyalkyl derivatives of 3-hydroxy-4-pyridinones: synthesis,complexation with Fe(III), Al(III) and Ga(III) and in vivo evaluation

A set of three N-carboxyalkyl 3-hydroxy-4-pyridinones has been studied as bidentate M(III) chelators (M=Fe, Al, Ga), with potential for oral administration. After preparation of the ligands, their protonation constants (log K(i)) and the stability constants of their metal complexes have been determined. The distribution coefficients of these compounds, between 1-octanol and Tris buffer pH 7.4, were measured. The effect of these compounds on the biodistribution of 67Ga-citrate loaded rats was investigated and compared with that of the administered 67Ga-complexes. Results indicated that, among these chelating agents, the N-carboxyethyl derivative has the highest affinity towards this set of metal ions, irrespective of the metal, and that it could even compete with transferrin, the main Fe-plasma protein. The binding affinity and the hydrophilic character decrease with the increase in the size of the alkylic chain. The biological assays indicate that the complex formation in vivo is characterized by a high kinetics and thermodynamic stability, suggesting a competition with the transferrin. All the ligands were found to enhance the excretion of the gallium. Noteworthy is the observed Ga bone fixation, mostly with the ethyl derivative, thus suggesting the potential use of the complex as a bone seeking agent.