Interactions of aluminium(III) with glycerolphosphates and glycerophosphorylcholine

The complexation of aluminium(III) with glycerol-1-phosphate (G1P) and glycerol-2-phosphate (G2P) in aqueous solutions has been studied as a function of pH, by pH-potentiometry, 31P NMR spectroscopy and ESI mass spectrometry. Various mononuclear complexes (MLH(2)(3+), MLH(2+), ML(+), ML(2)H, ML(2)(-)) and polynuclear species (M(3)L(3)H(-1)(2+), M(3)L(2)H(-n)((n-5)-) with n=5, 6, 7, M(2)L(2)H(-1)(+) ) are formed in the system where the full protonated ligands are noted LH(2). NMR experiments clearly show that G1P and G2P already interact with Al(III) at pH 1. The potentiometric results are confirmed by ESI measurements and 31P NMR studies. No metal ion-induced deprotonation and coordination of the alcoholic-OH functions seem to occur during the complexation. The situation is very different for the glycerophosphorylcholine ligand (GPC identical with LH). Only the complex ML(3+) is formed in aqueous solution with a relatively low formation constant (K=5 at 37 degrees C). This species is clearly identified in 31P and 27Al NMR spectra. The complexation study as a function of the temperature allowed us to determine the thermodynamic parameters of the complex formation. The complexation is not governed by the reaction enthalpy that is found to be positive but by the entropy that is largely positive.     

Potentiometric and spectroscopic studies on the dimethyltin(IV) complexes of 2-hydroxyhippuric acid

Equilibrium and spectroscopic (1H, 13C NMR and 119Sn M?ssbauer) studies in aqueous solution are reported for dimethyltin(IV) complexes of 2-hydroxyhippuric acid (Sal-Gly). Below pH 4, oxygen-coordinated complexes MLH and ML are formed. In the pH range 5-8.5, the species MLH(-1), predominates at any metal-to-ligand ratio. The ligand exchange of this species is slow on the NMR time scale, which allows its structural characterization by NMR spectroscopy: the coordination polyhedron around the tin atom is distorted trigonal bipyramidal, with tridentate [O-,N-,COO-] coordination of Sal-Gly, involving two equatorial methyl groups. The NMR results reveal that the main cause of the distortion of the polyhedron is the large CH3-Sn-CH3 angle of 136+/-4 degrees. The presented results supplement the data available on the dimethyltin(IV)-promoted amide deprotonation of peptides, and provide further arguments for the fundamental role of the carboxylate as an anchoring group in this process.     

The systems V<Superscript>IV</Superscript>O<Superscript>2+</Superscript>-glutathione and related ligands: a potentiometric and spectroscopic study

The equilibria in the system V(IV)O(2+)-glutathione in aqueous solution were studied in the pH range 2-11 by a combination of pH-potentiometry and spectroscopy (EPR, visible absorption and circular dichroism). The results of the various methods are consistent and the equilibrium model includes the species MLH(3), MLH(2), MLH, ML(2)H(2), MLH(-1), and MLH(-2) and several hydrolysis products (where H(4)L denotes totally protonated glutathione); individual formation constants and spectra are given. ML(2)H(2) is the predominant species at physiological pH. Plausible structures for each stoichiometry are discussed. The related V(IV)O(2+) systems of S-methylglutathione and gamma- L-glutamyl- L-cysteinyl ethyl ester were studied by means of the same spectroscopic techniques in order to support the established binding modes for the glutathione complexes. The importance of glutathione and oxidized glutathione in binding V(IV)O(2+) in cells is assessed.     

The system VO2+ +oxidized glutathione: a potentiometric and spectroscopic study

The equilibria in the system VO2+ +oxidized glutathione in aqueous solution have been studied in the pH range 2-11 by a combination of pH potentiometry and spectroscopy (EPR, visible absorption and circular dichroism). The results of the various methods are self-consistent and the equilibrium model includes the species MLH4, MLH3, MLH2, MLH, ML, MLH(-1), MLH(-2) and several hydrolysis products (where H4L denotes oxidized glutathione); individual formation constants and spectra are given. Plausible structures for each stoichiometry are discussed.     

A multi-approach study of the interaction of the Cu(II) and Ni(II) ions with alanylglycylhistamine, a mimicking pseudopeptide of the serum albumine N-terminal residue

The protonation equilibria of alanylglycylhistamine (Ala-Gly-Ha) and the complexation of this ligand with Cu(II) and Ni(II) have been studied by pH-potentiometry, 1H and 14N NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), circular dichroism (CD), UV-Vis spectrophotometry and electron paramagnetic resonance (EPR). From pH approximately 2-12, the following complexes: MLH, MLH(-1), MLH(-2) and MLH(-3) are successively formed in aqueous solutions, the ligand under its neutral form being noted L. At physiological pH, the MLH(-2) complex is predominant. The coordination in this complex is assumed by one amino, two deprotonated peptide and one imidazole nitrogen atoms. The ESI-MS study confirmed the formation of the MLH(-1), MLH(-2) and MLH(-3) complexes. The structure of MLH(-2) was determined by single crystal X-ray analysis. CD and UV-Vis techniques allowed us to propose that the imidazole-N3 nitrogen acts as the anchor group for the coordination to the metal(II) ions rather than the amino group. At high pH values, the further deprotonation of the N-H imidazole group, leading to the formation of MLH(-3), occurs, as revealed by 1H NMR spectroscopy.     

Investigation of the ternary D-myo-inositol 1,2,6-tris(phosphate)-spermine-Zn2+ system in solution

Interactions of Ins(1,2,6)P3 (IP), with spermine (Spm) and zinc cations have been studied by potentiometric and 31P NMR titrations. In the 4-11 pH range, two IPSpmZn2H3 and IPSpmZn2H mixed complexes are formed which are largely predominant with respect to the binary species. According to 31P NMR titration it is likely that one of the zinc cations preferably binds phosphates P1 and P6. The adduct formation between Ins(1,2,6)P3 and spermine seems also favourable to the formation of the mixed complexes. The occurrence of ternary complexes involving inositol-phosphates, biogenic amines, and metallic cations may be of relevance in the regulation of biological processes.     

Solution equilibrium studies on metal complexes of 2,3-dihydroxy-phenylalanine-hydroxamic acid (Dopaha) and models: catecholate versus hydroxamate coordination in iron(III)-, aluminium(III)- and molybdenum(VI)-Dopaha complexes

Equilibrium results based on pH potentiometric, spectrophotometric and (1)H NMR measurements for the complexes of Fe(III), Al(III) and Mo(VI) with 2,3-dihydroxy-phenylalanine-hydroxamic acid (Dopaha) as well as for binary model systems Fe(III)-, Al(III)-, Mo(VI)-acetohydroxamic acid (Aha), -alpha-alaninehydroxamic acid (alpha-Alaha) and -1,2-dihydroxy-3,5-benzene-disulphonate (Tiron) and ternary model systems Fe(III)-, Al(III)-, Mo(VI)-Tiron-Aha, are summarized in this paper. The amine-type coordination mode is not detectable with these metal ions at all. Precipitation occurs at pH <5.5 with Fe(III) and Al(III) even at a Dopaha-to-metal ion ratio of 10:1. Hydroxamate-type coordination was demonstrated with both metals below the pH range of precipitation but, after dissolution, catecholate-type coordination was exclusively found. The hydroxamate-type coordination mode occurs only in the very acidic pH range for Mo(VI) complexes and the crossover from hydroxamate to catecholate binding occurs at pH >3. A ligand-bridged dinuclear species, [(MoO(2))(2)(Dopaha)(2)](2+), involving mixed-type (catecholate and hydroxamate) coordination modes is formed in the pH range 2.5-5.5. [MoO(2)A(2)H(2)], with catecholate-type coordination, forms above pH 3. On increasing the pH further, deprotonation of the coordinated Dopaha and hydrolytic processes result in the formation of catecholate-coordinated [MoO(3)AH] and [MoO(3)A]. MoO(4)(2-) and free Dopaha exist above pH 10.     

Al-ATP as an intracellular carrier of Al(III) ion.

1. Using 27Al and 31P NMR spectroscopy in conjunction with an Al lactate aqueous reagent at pH 7.2, Al complexes of ATP and of phospholipids were characterized in synthetic-aqueous and organic-phospholipid chemical systems and in the intact human red blood cell. 2. The observed 31P NMR chemical shifts of the Al-ATP complex in aqueous laboratory preparations or the intact human red blood cell were, respectively, alpha phosphate, -11.53 delta; beta phosphate, -22.65 delta; and gamma phosphate, -10.95 delta. 3. The observed complexed 27Al chemical shift was -2.22 delta. 4. The relative affinities for Al of the phospholipids determined from 31P NMR spectroscopic titrations were PA much greater than Cl much greater than PS greater than PG approximately equal to PI greater than PE plus approximately equal to PE much greater than SPH greater than PC.     

Potentiometric study of vitamin D3 complexes with manganese(II), iron(II), iron(III) and zinc(II) in water-ethanol medium.

Vitamin D3 (LH) complexes with manganese(II), iron(II), iron(III) and zinc(II) were identified in water-ethanol medium (30/70). Their stability constants were determined at 298 K and at a constant ionic strength of 0.100 M using potentiometric methods. The computerisation of the experimental data showed the presence of ML (M = metal, L = deprotonated vitamin D3) and ML2 species in all cases; in addition, the ML3 iron(III) complex was detected. The calculated overall stability constants beta for MnIIL, FeIIL, FeIIIL and ZnIIL are, respectively, in logarithms, 12.4, 16.5, 28.5 and 16.5. Under the experimental conditions, the only protonated species MLH detected was with iron(III).     

nmr and infrared spectroscopic investigations of the Al(III), Ga(III), and Be(II) complexes of ATP

The interaction of Al(III) with ATP has been examined by ³¹P and ¹H nmr and infrared spectroscopy. At pH 6.2, Al(III) forms a long-lived complex with ATP, in which chemical exchange between free and complexed ATP is slow on the nmr time scale. Infrared spectra of the Al(III)-ATP complex exhibit large perturbations in the band corresponding to the -PO₃^2? antisymmetric stretching mode. At higher pH values, equilibria involving Al(III) and OH^? become favored with the result that Al(III) no longer influences the spectroscopic properties of ATP. Similar spectroscopic results are obtained for the Ga(III) and Be(II) complexes of ATP.     

Interaction of Al(III) with the peptides AspAsp and AspAspAsp

The interactions of Al(III) with the dipeptide AspAsp and the tripeptide AspAspAsp in aqueous solutions were studied by pH-potentiometry and multinuclear 1H- and 13C- nuclear magnetic resonance (NMR) spectroscopy. Their numerous negatively charged COO(-) functions allow these ligands to bind Al(III) even in weakly acidic solutions. Various mononuclear 1:1 complexes are formed in different protonation states. 13C-NMR spectroscopy unambiguously proved participation of the COO(-) functions in a monodentate or chelating mode in Al(III) binding, however, the terminal-NH(2) group seems to be excluded from the coordination. Depending on the metal ion to ligand ratio precipitation occurs at pH approximately 5 to 6. This indicates that the COO(-) groups at the low level of preorganization in such small peptides are not sufficient to keep the Al(III) ion in solution and to prevent the precipitation of Al(OH)(3) at physiological pH. To achieve this, a more specific arrangement of the side-chain donors seems necessary.     

Aluminum binding to phosphatidylcholine lipid bilayer membranes: 27Al and 31P NMR spectroscopic studies

27Al and 31P nuclear magnetic resonance (NMR) spectroscopies were used to investigate aluminum interactions at pH 3.4 with model membranes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). A solution state 27Al NMR difference assay was developed to quantify aluminum binding to POPC multilamellar vesicles (MLVs). Corresponding one-dimensional (1D) fast magic angle spinning (MAS) 31P NMR spectra showed that aluminum induced the appearance of two new isotropic resonances for POPC shifted to -6.4 ppm and -9.6 ppm upfield relative to, and in slow exchange with, the control resonance at -0.6 ppm. Correlation of the (27)Al and (31)P NMR binding data revealed a 1:2 aluminum:phospholipid stoichiometry in the aluminum-bound complex at -9.6 ppm and a 1:1 aluminum:phospholipid stoichiometry in that at -6.4 ppm. Slow MAS 31P NMR spectra demonstrated shifts in the anisotropic chemical shift tensor components of the aluminum-bound POPC consistent with a close coordination of aluminum with phosphorus. A model of the aluminum-bis-phospholipid complex is proposed on the basis of these findings.     

Aluminum binding to phosphatidylcholine lipid bilayer membranes: aluminum exchange lifetimes from 31P NMR spectroscopy

Two-dimensional (2D) (31)P magic angle spinning (MAS) nuclear magnetic resonance (NMR) exchange spectroscopy (EXSY) demonstrated that aluminum binds to the phosphate group of phosphatidylcholine (PC) in multilamellar vesicles at pH 3.2, forming preferentially 2/1, in addition to 1/1 (PC/Al) complexes in slow exchange with one another, and with free PC, on the NMR timescale. Exchange rate constants between these three co-existing species were measured as a function of temperature using one-dimensional (1D) selective inversion recovery (SIR) (31)P MAS NMR. Over the temperature range from 5 to 35 degrees C all three exchange rate constants increased by roughly an order of magnitude from k approximately 1-2 to 10-14s(-1), exhibiting Arrhenius behavior with activation energies on the order of 30-45 kJ mol(-1) and correspondingly positive enthalpies of activation. Entropies of activation were uniformly negative, consistent with an ordered transition state. From a biological perspective, the results demonstrate that aluminum binding to PC in biomembranes is transient on a biologically relevant time scale, so that the lipid bilayer portion of biomembranes is unlikely to act as a long term repository for aluminum, but rather should be viewed as a temporary reservoir of biologically available aluminum.     

Aluminum speciation studies in biological fluids. Part 2. Quantitative investigation of aluminum-citrate complexes and appraisal of their potential significance in vivo

Because of the recent implications of aluminum in the pathogenesis of various disease states, its in vivo chemistry has been receiving growing attention from bioinorganic chemists over the last few years. In this context, the elucidation of the main factors that govern aluminum bioavailability constitutes an urgent objective. Clearly, prevention measures require that mechanisms of aluminum absorption be definitely characterized, whereas specific sequestering agents are needed to detoxify patients with high-aluminum-body burdens. In particular, speciation studies are necessary to discriminate among the chemical forms under which aluminum predominates in vivo. Low molecular weight (LMW) species, which are the most active in terms of bioavailability, cannot be assessed by analytical techniques, and so computer simulations must be used. In recent clinical studies as well as in preliminary simulations dealing with aluminum distribution in blood plasma, citrate has been recognized as the most important LMW ligand of aluminum. The present paper thus reports a quantitative investigation of aluminum-citrate equilibria, carried out at 37 degrees C in NaCl 0.15 mol dm-3 in accordance with the experimental protocol defined in our previous study on aluminum hydrolysis. The ML, MLH, ML2, M3L3H-4, M2L2H-2, ML2H-1, and ML2H-2 species have been characterized over the whole physiological pH range using as large reactant concentration ratios as possible. Corresponding formation constants have then been used to investigate the role of citrate towards aluminum bioavailability. Blood plasma simulations reveal that citrate can promote aluminum urinary excretion, which substantiates recent clinical observations made on mice. However, the higher plasma aluminum concentrations are, the less effective citrate is to be expected. Gastrointestinal simulations confirm that the electrically neutral ML complex does represent an important risk of aluminum absorption in the upper region of the gastrointestinal tract at usual therapeutic doses. At moderate- and low-aluminum concentrations, citrate is also capable of dissolving the aluminum trihydroxide precipitate, which may combine with the capacity of other ligands to complex Al3+ into absorbable complexes at less acidic pH.     

Secretion of an aluminum chelator, 2-isopropylmalic acid, by the budding yeast, Saccharomyces cerevisiae

An aluminum(III)-binding substance (ABS), that solubilizes Al(III) at neutral pH, was found to be secreted by Saccharomyces cerevisiae. A combination of anion exchange chromatography and preparative high performance liquid chromatography using an octadecylsilane (ODS) column separated ABS from the medium. The structure determination of ABS was performed using 1H and 13C NMR spectroscopy and heteronuclear multiple-bond correlation (HMBC) spectroscopy, and ABS was identified to be 2-isopropylmalic acid (2-iPMA). The structure was further confirmed using high resolution electrospray ionization mass spectrometry. Solubilization of otherwise sparingly soluble Al(III) by 2-iPMA at neutral pH indicated the binding of the compound with Al(III). This is supported by 27Al NMR spectrometry for a solution containing 10 mM Al(III) and 20 mM 2-iPMA at pH 6.6, where four Al(III) species were evident. Although the function of this compound is unclear, it might play a key role in Al detoxification.     

Synthetic, structural and solution speciation studies on binary Al(III)–(carboxy)phosphonate systems. Relevance to the neurotoxic potential of Al(III)

Efforts to delineate the interactions of neurotoxic Al(III) with low molecular mass substrates relevant to neurodegenerative processes, led to the investigation of the pH-specific synthetic chemistry of the binary Al(III)-[N-(phosphonomethyl) iminodiacetic acid] (Al-NTAP), Al(III)-[nitrilo-tris(methylene-phosphonic acid)] (Al-NTA3P), and Al(III)-[1-hydroxy ethylidene-1,1-diphosphonic acid] (Al-HEDP) systems, in correlation with solution speciation studies. Reaction of Al(NO₃)₃·9H₂O with NTAP at pH 7.0 and 4.0 afforded the new species (CH₆N₃)₄[Al₂(C₅H₆NPO₇)₂(OH)₂]·8H₂O (1) and (NH₄)₂[Al₂(C₅H₆NPO₇)₂(H₂O)₂]·4H₂O (2), while reaction of Al(NO₃)₃·9H₂O with NTA3P led to K₈[Al₂(C₃H₆NP₃O₉)₂(OH)₂]·20H₂O (3). Complexes 1–3 were characterized by elemental analysis, FT-IR, ¹³C, ³¹P, ¹H NMR (for 1–2 solid state and solution NMR where feasible), and X-ray crystallography. The structures of 1–3 reveal the presence of uniquely defined dinuclear complexes of octahedral Al(III) bound to fully deprotonated phosphonate ligands, water and hydroxo moieties. The aqueous solution speciation studies on the aforementioned binary systems project a clear picture of the binary Al(III)–(carboxy)phosphonate interactions and species under variable pH-conditions and specific Al(III):ligand stoichiometry. The concurrent solid state and solution work (a) exemplifies essential structural and chemical attributes of soluble aqueous species, reflecting well-defined interactions of Al(III) with phosphosubstrates and (b) strengthens the potential linkage of neurotoxic Al(III) chemical reactivity toward O,N-containing (carboxy)phosphate-rich cellular targets.     

Voltammetry as a virtual potentiometric sensor in modelling of a metal-ligand system and refinement of stability constants. Part 4. An electrochemical study of NiII complexes with methylene diphosphonic acid

The Ni(II)-MDP-OH system (MDP=methylene diphosphonic acid) and stability constants of complexes formed at ionic strength 0.15M at 298K were established by direct current polarography (DCP) and glass electrode potentiometry (GEP). The final M-L-OH model could only be arrived to by employing recent concept of virtual potentiometry (VP). VP-data were generated from non-equilibrium and dynamic DC polarographic technique. The VP and GEP data were refined simultaneously by software dedicated to potentiometric studies of metal complexes. Species distribution diagrams that were generated for different experimental conditions employed in this work assisted in making the final choice regarding the metal-ligand model. The model established contains ML, ML(2), ML(OH) and ML(OH)(2) with stability constants, as logbeta, 7.94+/-0.02, 13.75+/-0.02, 12.04 (fixed value), and 16.75+/-0.05, respectively. It has been demonstrated that virtual potential must be used in modelling operations (predictions of species formed) when a polarographic signal decreases significantly due to the formation of polarographically inactive species (or formation of inert complexes). The linear free energy relationships that included stability constant logK(1) for Ni(II)-MDP established in this work together with other available data were used to predict logK(1) values for Sm(III) and Ho(III) with MDP. The logK(1) values for Sm(III)-MDP and Ho(III)-MDP were estimated to be 9.65+/-0.10 and 9.85+/-0.10, respectively.     

Complexes of vanadium(III) with L-alanine and L-aspartic acid

The equilibria of the complexation processes of V(3+) with L-alanine and L-aspartic acid in aqueous solution over a wide pH range (2-10) were studied by potentiometric and spectroscopic (UV-Vis, CD) methods. The results show that alanine forms complexes with V(3+) in the metal ion concentration range and at the ligand-to-metal ratios investigated, giving mononuclear species only. In ML(2) species, which dominate in the range pH 4-8, alanine acts as a bidendate ligand through O and N atoms. The complexation processes of V(3+) with aspartic acid are more complicated. In acidic solution (up to pH approximately 4) they are similar to those for alanine. In the higher pH region, however, there are complicated equilibria among mono- and various dinuclear species. These dinuclear species consist of carboxylic or mu-oxo bridges and differ from each other by the number of coordinated ligands and OH(-) groups. The solid phase of the V(III) complex with aspartic acid could be isolated from nonaqueous solution only. Spectroscopic (UV-Vis-IR) measurements and magnetic susceptibility data confirm the coordination of vanadium(III) by two carboxylic groups. Both V(III)-L-aspartic acid and V(III)-L-alanine complexes have a significant apoptotic effect on Hepatoma Morris 5123 cells.     

Al(III)-binding ability of an octapeptide and its phosphorylated derivative

A synthetic octapeptide, H-GlyGluGlyGluGlySerGlyGly-OH, and its phosphorylated Ser derivative were synthetized and their solution speciation and binding modes in their complexes with Al(III) were measured. One goal of the work was find a lead compound for the design of a selective peptide-based Al(III) chelator. pH-potentiometry was used to characterize the stoichiometry and the stability of the species formed in the interactions of the metal ion and the peptides, while multinuclear NMR was applied to characterize the binding sites of the metal ion in the complexes. CD spectroscopy revealed a difference in the conformational behaviour of the phosphorylated peptide as compared with its non-phosphorylated parent derivative. The Al(III) is presumed to enhance aggregation through the -PO3H(-)-Al(3+)-PO3(2-)-Al(3+)- intermolecular bindings between the peptide chains. The results of molecular dynamics calculations supported the experimentally obtained secondary structures and the binding position of Al(III).     

Spectroscopic characterization of 2-isopropylmalic acid-aluminum(III) complex

Spectroscopic elucidation of a 2-isopropylmalic acid (2-iPMA)-aluminum(III) complex has been carried out using (1)H, (13)C and (27)Al NMR spectroscopy, diffusion-ordered NMR spectroscopy (DOSY) and electrospray ionization mass spectrometry (ESI-MS). 2-iPMA is secreted by Saccharomyces cerevisiae and can dissolve Al(III) in the culture medium. The (1)H chemical shift perturbation and (1)H DOSY clearly indicated the formation of the 2-iPMA-Al(III) complex. The measurements of (13)C and (27)Al NMR spectroscopy and ESI-MS demonstrated that the major form of a complex is comprised four 2-iPMA and two Al(III) species. This compound is expected to possess strong Al(III)-detoxification capability.