Citrate Speciation Research Articles

Chemical controls on uranyl citrate speciation and the self-assembly of nanoscale macrocycles and sandwich complexes in aqueous solutions.

Uranyl citrate forms trimeric species at pH > 5.5, but exact structural characteristics of these important oligomers have not previously been reported. Crystallization and structural characterization of the trimers suggests the self-assembly of the 3 : 3 and 3 : 2 U : Cit complexes into larger sandwich and macrocyclic molecules. Raman spectroscopy and ESI-MS have been utilized to investigate the relative abundance of these species in solution under varying pH and citrate concentrations. Additional dynamic light scattering experiments indicate that self-assembly of the larger molecules does occur in aqueous solution.

Revision of iron(III)–citrate speciation in aqueous solution. Voltammetric and spectrophotometric studies

A detailed study of iron (III)–citrate speciation in aqueous solution (θ=25°C, Ic=0.7molL−1) was carried out by voltammetric and UV–vis spectrophotometric measurements and the obtained data were used for reconciled characterization of iron (III)–citrate complexes. Four different redox processes were registered in the voltammograms: at 0.1V (pH=5.5) which corresponded to the reduction of iron(III)–monocitrate species (Fe:cit=1:1), at about −0.1V (pH=5.5) that was related to the reduction of FeL25−, FeL2H4− and FeL2H23− complexes, at −0.28V (pH=5.5) which corresponded to the reduction of polynuclear iron(III)–citrate complex(es), and at −0.4V (pH=7.5) which was probably a consequence of Fe(cit)2(OH)x species reduction. Reversible redox process at −0.1V allowed for the determination of iron(III)–citrate species and their stability constants by analyzing Ep vs. pH and Ep vs. [L4−] dependence. The UV–vis spectra recorded at varied pH revealed four different spectrally active species: FeLH (logβ=25.69), FeL2H23− (logβ=48.06), FeL2H4− (logβ=44.60), and FeL25− (logβ=38.85). The stability constants obtained by spectrophotometry were in agreement with those determined electrochemically. The UV–vis spectra recorded at various citrate concentrations (pH=2.0) supported the results of spectrophotometric–potentiometric titration.

Curium(iii) citrate speciation in biological systems: a europium(iii) assisted spectroscopic and quantum chemical study

Citrate complexes are the dominant binding form of trivalent actinides and lanthanides in human urine at pH < 6. Hence, an accurate prediction of the speciation of these elements in the presence of citrate is crucial for the understanding of their impact on the metabolism of the human organism and the corresponding health risks. We studied the complexation of Cm(III) and Eu(III), as representatives of trivalent actinides and lanthanides, respectively, in aqueous citrate solution over a wide pH range using time-resolved laser-induced fluorescence spectroscopy. Four distinct citrate complexes were identified and their stability constants were determined, which are MHCit(0), M(HCitH)HCit(2-), M(HCit)(2)(3-), and M(Cit)(2)(5-) (M = Cm, Eu). Additionally, there were also indications for the formation of MCit(-) complexes. Structural details on the EuHCit(0) and EuCit(-) complexes were obtained with FT-IR spectroscopy in combination with density functional theory calculations. IR spectroscopic evidence for the deprotonation of the hydroxyl group of the citrate ion in the EuCit(-) complex is presented, which also revealed that the complexation of the Eu(3+) ion takes place not only through the carboxylate groups, like in EuHCit(0), but additionally via the hydroxylate group. In both EuHCit(0) and EuCit(-) the carboxylate binding mode is mono-dentate. Under a very low metal : citrate ratio that is typical for human body fluids, the Cm(III) and Eu(III) speciation was found to be strongly pH-dependent. The Cm(III) and Eu(III) citrate complexes dominant in human urine at pH < 6 were identified to be Cm(HCitH)HCit(2-) and a mixture of Eu(HCitH)HCit(2-) and EuHCit(0). The results specify our previous in vitro study using natural human urine samples (Heller et al., Chem. Res. Toxicol., 2011, 24, 193-203).

Mechanism and Kinetics of Ligand Exchange between Ferric Citrate and Desferrioxamine B

The kinetics of ligand exchange between ferric citrate and desferrioxamine B (DFB) was investigated at pH 8.0 and high citrate/Fe molar ratios (500-5000) with particular attention given to understanding the precise mechanism of ligand exchange. Ferric citrate complexes present in a test solution and therefore involved in the reaction with the incoming ligand (DFB) were initially examined by evaluating ferric citrate speciation on the basis of published thermodynamic constants. The speciation analysis indicated that mononuclear (mono- and dicitrate) ferric complexes are the major species responsible for the ligand exchange with DFB under the conditions examined in the present work. Given the tendency of DFB to adjunctively associate with the ferric citrate complexes, we propose a kinetic model containing the following three mechanisms: (i) direct association of DFB to the ferric dicitrate complex prior to any dissociation of citrate molecules from the Fe center, (ii) adjunctive association of DFB toward ferric monocitrate complex following dissociation of one molecule of citrate from the parent complex, and (iii) complexation of hydrated Fe by DFB after sequential dissociation of two molecules of citrate from the Fe center. Overall rates for the ligand exchange were determined by spectrophotometrically monitoring the formation of ferrioxamine B. Further analysis in quantifying the rate of each mechanism by use of published and determined rate constants of relevant elemental reactions suggested that the first and second mechanisms were significant under our experimental conditions where [Cit] ≫ [DFB] with the relative importance of these two pathways depending on citrate concentration.

Iron(iii) citrate speciation in aqueous solution

Citrate is an iron chelator and it has been shown to be the major iron ligand in the xylem sap of plants. Furthermore, citrate has been demonstrated to be an important ligand for the non-transferrin bound iron (NTBI) pool occurring in the plasma of individuals suffering from iron-overload. However, ferric citrate chemistry is complicated and a definitive description of its aqueous speciation at neutral pH remains elusive. X-Ray crystallography data indicates that the alcohol function of citrate (Cit4-) is involved in Fe(III) coordination and that deprotonation of this functional group occurs upon complex formation. The inability to include this deprotonation in the affinity constant calculations has been a major source of divergence between various reports of iron(III)-citrate affinity constants. However the recent determination of the alcoholic pKa of citric acid (H4Cit) renders the reassessment of the ferric citrate system possible. The aqueous speciation of ferric citrate has been investigated by mass spectrometry and EPR spectroscopy. It was observed that the most relevant species are a monoiron dicitrate species and dinuclear and trinuclear oligomeric complexes, the relative concentration of which depends on the solution pH value and the iron : citric acid molar ratio. Spectrophotometric titration was utilized for affinity constant determination and the formation constant for the biologically relevant [Fe(Cit)2]5- is reported for the first time.

Oxygenation of Fe(II) in the Presence of Citrate in Aqueous Solutions at pH 6.0−8.0 and 25 °C: Interpretation from an Fe(II)/Citrate Speciation Perspective

The kinetics of Fe(II) oxidation in the presence of various citrate concentrations have been investigated in aqueous solutions over the pH range 6.0-8.0 using colorimetry and speciation modeling. Oxidation of Fe(II) was interpreted and quantitatively modeled in terms of oxidation of various Fe(II)-citrate species. Using the model, it is possible to predict whether the presence of citrate would dominate the Fe(II) oxidation and thus enhance/retard the oxidation rate of Fe(II) and vice versa. The study also supports the presence of other Fe(II)-citrate species rather than just the monomeric species at circumneutral pH. At low pH and in a system where complexation of Fe(II) by citrate is dominant, oxidation of Fe(II) is controlled by the oxidation of both Fecit- and Fecit24-. As the pH increases, the oxidation of Fe(OH)cit25- becomes increasingly important and dominates the oxidation of Fe(II) at pH 8.0. Rate constants for the oxidation of all five suggested Fe(II)-citrate species have been estimated and may be used to predict the rate of Fe(II) oxidation at any combination of pH and citrate concentration.

Delving into the complex picture of Ti(IV)–citrate speciation in aqueous media: Synthetic, structural, and electrochemical considerations in mononuclear Ti(IV) complexes containing variably deprotonated citrate ligands

The aqueous reaction of TiCl 4 with citric acid at pH ∼ 4 (KOH), led to the surprising isolation of a species assembly K 3[Ti(C 6H 6O 7) 2(C 6H 5O 7)] · K 4[Ti(C 6H 5O 7) 2(C 6H 6O 7)] · 10H 2O ( 1). The same system at pH ∼ 3 (neocuproine), led to the crystalline material (C 14H 13N 2) 2[Ti(C 6H 6O 7) 3] · 5H 2O ( 2), while at pH 5.0 (NaOH), afforded Na 3[Ti(C 6H 6O 7) 2(C 6H 5O 7)] · 9H 2O ( 3). Analytical, spectroscopic and structural characterization of 1, 2 and 3 revealed their distinct nature exemplified by mononuclear complexes bearing variably deprotonated citrates bound to Ti(IV). Solid-state 13C MAS NMR spectroscopy in concert with solution 13C and 1H NMR on 3 provided ample evidence for the existence of bound citrates of distinct coordination mode to the metal ion. Cyclic voltammetry defined the electrochemical signature of complex 2, thereby projecting the physicochemical profile of the species formulated by the aforementioned properties. Comparison of cyclic voltammetric data on available discrete Ti(IV)–citrate species depicts the electrochemical profile and an E 1/2 value trend of the species in that binary system’s aqueous speciation, further substantiating the redox behavior of mononuclear Ti(IV)–citrate species in a pH-sensitive fashion. Collectively, the well-defined discrete species in 1– 3 reflect and corroborate a synthetically challenging yet complex pH-specific picture of the aqueous Ti(IV) chemistry with the physiological citric acid, and shed light on the pH-dependent speciation in the binary Ti(IV)–citrate system.

Gibbsite and Goethite Solubility

2‐Ketogluconate (kG) is a microbial byproduct that has been isolated from the rhizosphere of several plants and has been found to accumulate around bacteria adhering to rock surfaces. It has long been suspected that kG may play a role in enhancing the bioavailability of soil nutrients. However, quantitative information relevant to its behavior in soils, its reactions with mineral surfaces, and its aqueous complexation of metal cations is unavailable. The objectives of this research were to examine the influence of kG on gibbsite and goethite solubility and to describe these effects by considering the aqueous complexation of Al3+ and Fe3+ by kG. Secondary to this was to compare the impact of kG on mineral solubility to that of citrate. The equilibrium solubility of gibbsite and goethite was examined at ambient (20–22°C) or controlled (25°C) temperatures as a function of pH, ionic strength, and kG or citrate concentration. Both ligands were observed to significantly enhance mineral solubility (as determined by the total soluble concentrations of Al and Fe), with kG and citrate having similar impacts on gibbsite solubility, but with kG having a lesser impact than citrate on goethite solubility. The solubility data were employed to characterize the aqueous complexation chemistry of Al– and Fe(III)–kG and citrate. In the 4 &lt; pH &lt; 8.5 range, the mononuclear species AlkG2+(aq), Al(OH)2kG0(aq), and Al(OH)3kG−(aq) describe Al–kG speciation, with the latter two species mechanistically described by the bidentate complexation of AlOH2+(aq) and Al(OH)+2aq [also written as AlOH(H−1kG)0(aq) and Al(OH)2(H−1kG)−(aq)]. Iron(III)–kG complexation is described by the FekG2+(aq) and Fe(OH)3 kG22−aq or FeOHH−1kG22−aq species. For the citrate systems, the chemical models required to describe metal speciation differed from those reported in the literature: Alcit0(aq), Al(OH)2cit2−(aq), and Al(OH)3 cit3−(aq) for Al–citrate speciation and Fecit0(aq), Fe(OH)2 cit2−(aq), and Fe(OH)3cit3−(aq) for Fe(III)–citrate.

PH-Specific Aqueous Synthetic Chemistry in the Binary Cadmium(II)−Citrate System. Gaining Insight into Cadmium(II)−Citrate Speciation with Relevance to Cadmium Toxicity

The involvement of Cd(II) in toxic manifestations and pathological aberrations in lower and higher organisms entails interactions with low and high molecular mass biological targets. To understand the relevant chemistry in aqueous media, we have launched pH-dependent synthetic efforts targeting Cd(II) with the physiological ligand citric acid. Reactions of Cd(II) with citric acid upon the addition of NaOH at pH 2.5 and pyridine at pH 3 and the addition of ammonia at pH approximately 7 led to the new complexes [Cd3(C6H5O7)2(H2O)5] x H2O (1) and (NH4)[Cd(C6H5O7)(H2O)] x H2O (2), respectively. Complexes 1 and 2 were characterized by elemental analysis, spectroscopy (FT-IR and NMR), and X-ray crystallography. Complex 1 crystallizes in the monoclinic space group P2(1)/n, with a = 18.035(6) A, b = 10.279(4) A, c = 12.565(4) A, beta = 109.02(1) degrees, V = 2202(2) A3, and Z = 4. Complex 2 crystallizes in the monoclinic space group P2(1), with a = 9.686(4) A, b = 8.484(4) A, c = 7.035(3) A, beta = 110.28(1) degrees, V = 542.3(4) A3, and Z = 2. Complex 1 is a trinuclear assembly with the citrate ligand securing a stable metallacyclic ring around one Cd(II), with the terminal carboxylates spanning into the coordination sphere of two nearby Cd(II) ions. Complex 2 contains mononuclear units of Cd(II) bound by citrate in an overall coordination number of 8. In both 1 and 2, the participating citrates exhibit three different modes of coordination, thus projecting a distinct yet variable aqueous structural chemistry of Cd(II) with physiological substrates. The pH-dependent chemistry and its apparent structural diversity validate past solution speciation studies, projecting the existence of mononuclear species such as the one in the anion of 2. The spectroscopic and structural properties of 2 emphasize the significance of the information emerging from synthetic studies that otherwise would not have been revealed through conventional solution studies, while concurrently shedding light onto the linkage of the requisite chemistry with the potential biological toxicity of Cd(II).

Titanium(IV) Citrate Speciation and Structure under Environmentally and Biologically Relevant Conditions

The water-soluble complexes of Ti(IV) with citrate are of interest in environmental, biological, and materials chemistry. The aqueous solution speciation is revealed by spectropotentiometric titration. From pH 3-8, given at least three equivalents of ligand, 3:1 citrate/titanium complexes predominate in solution with successive deprotonation of dangling carboxylates as the pH increases. In this range and under these conditions, hydroxo- or oxo-metal species are not supported by the data. At ligand/metal ratios between 1:1 and 3:1, the data are difficult to fit, and are consistent with the formation of such hydroxo- or oxo- species. Stability constants for observed species are tabulated, featuring log beta-values of 9.18 for the 1:1 complex [Ti(Hcit)](+), and 16.99, 20.41, 16.11, and 4.07 for the 3:1 complexes [Ti(H(2)cit)(3)](2-), [Ti(H(2)cit)(Hcit)(2)](4-), [Ti(Hcit)(2)(cit)](6-), and [Ti(cit)(3)](8-), respectively (citric acid = H(4)cit). Optical spectra for the species are reported. The complexes exhibit similar yet distinct spectra, featuring putative citrate-to-Ti(IV) charge-transfer absorptions (lambda(max) approximately 250-310 nm with epsilon approximately 5000-7000 M(-)(1) cm(-1)). The prevailing 3:1 citrate/titanium ratio in solution is supported by electrospray mass spectrometry data. The X-ray crystal structure of a fully deprotonated tris-citrate complex Na(8)[Ti(C(6)H(4)O(7))(3)].17H(2)O (1) (or Na(8)[Ti(cit)(3)].17H(2)O) that crystallizes from aqueous solution at pH 7-8 is reported. Compound 1 crystallizes in the triclinic space group P, with a = 11.634(2) Angstroms, b = 13.223(3) Angstroms, c = 13.291(3) Angstroms, V = 1982.9(7) Angstroms(3), and Z = 2.

PH- and mol-ratio dependent tungsten(VI)–citrate speciation from aqueous solutions: syntheses, spectroscopic properties and crystal structures

Investigation of the aqueous coordination chemistry for citrate and tungsten(VI) resulted in the isolation of three new monomeric and dimeric tungsten(VI) citrate NaK 3[W 2O 5(Hcit) 2]·4H 2O ( 1), (Hphen) 3[WO 2(H 2cit)(Hcit)]·6H 2O ( 2) and K 4[WO 3(cit)]·2H 2O ( 3), (H 4citcitric acid). The three complexes have been characterized by elemental analyses, IR and NMR spectroscopies. The IR and NMR spectra are consistent with monomeric species or a monooxo-bridged dinuclear structure as revealed by single crystal X-ray diffraction study. The dimeric anion of complex 1 contains a bent (O 2W)O(WO 2) core with an angle of WObW 162.3(3)°. Each citrate ligand is tridentately coordinated to one tungsten atom through the α-alkoxyl, α-carboxyl, and one β-carboxyl group, making each metal atom six-coordinate and forming a mesomer with configuration of Δ SΛ R. The mononuclear tungstate 2 consists of a cis-dioxo partially protonated citrato tungstate(VI) anion and three phenanthrolinium cations. The W(VI) atom is coordinated by two acid oxo groups and two bidentate citrate ligands through α-alkoxyl and α-carboxyl groups, while the other two β-carboxyl or carboxylic groups remain free. The tungsten atom in the complex 3 forms an octahedral coordination with three fac-oxo groups and one tridentate citrate, in which the later is coordinated through the α-alkoxyl and α-carboxyl groups, and much more weakly by one of the two terminal β-carboxyl groups [2.348(4) Å]. Interconversion of citrato tungstate(VI) with pH value and the molar ratio of the reactant are discussed.

Synthesis and spectroscopic and structural studies of a new cadmium(II)-citrate aqueous complex. Potential relevance to cadmium(II)-citrate speciation and links to cadmium toxicity.

The presence of cadmium in the environment undoubtedly contributes to an increased risk of exposure and ultimate toxic influence on humans. In an effort to comprehend the chemical and biological interactions of Cd(II) with physiological ligands, like citric acid, we explored the requisite aqueous chemistry, which afforded the first aqueous Cd(II)-citrate complex [Cd(C(6)H(6)O(7))(H(2)O)](n)() (1). Compound 1 was characterized by elemental analysis, and spectroscopically by FT-IR and (113)Cd MAS NMR. Compound 1 crystallizes in the orthorhombic space group P2(1)2(1)2(1), with a = 6.166(2) A, b = 10.508(3) A, c = 13.599(5) A, V = 881.2(5) A(3), and Z = 4. The X-ray structure of 1 reveals the presence of octahedral Cd(II) ions bound to citrate ligands in a molecular crystal lattice. Citrate acts as a tridentate binder promoting coordination to one Cd(II) through the central alcoholic moiety, one terminal carboxylate group, and the central carboxylate group. In addition, the central carboxylate binds to three Cd(II) ions. Specifically, one of the oxygens of the central carboxylate serves as a bridge to two neighboring Cd(II) ions, while the other oxygen binds to a third Cd(II). A bound water molecule completes the coordination requirements of Cd(II). (113)Cd MAS NMR studies project the spectroscopic signature of the nature of the coordination environment around Cd(II) in 1, thus corroborating the X-ray findings. Collectively, the data at hand are in line with past solution studies. The latter predict that other similar low molecular mass Cd(II)-citrate complexes may exist in the acidic pH region, thus influencing the uptake of cadmium by living (micro)organisms, their ability to metabolize organic substrates, and possibly Cd(II) toxicity.

Systematic studies on pH-dependent transformations of dinuclear vanadium(V)–citrate complexes in aqueous solutions: A perspective relevance to aqueous vanadium(V)–citrate speciation

Vanadium(V) involvement in interactions with physiological ligands in biological media prompted us to delve into the systematic pH-dependent synthesis, spectroscopic characterization, and perusal of chemical properties of arising aqueous vanadium(V)–citrate species in the requisite system. To this end, facile reactions led to dinuclear complexes (NH 4) 4[V 2O 4(C 6H 5O 7) 2]·4H 2O ( 1) and (NH 4) 6[V 2O 4(C 6H 4O 7) 2]·6H 2O ( 2). Complex 1 and 2 were characterized by elemental analysis, FT-IR and X-ray crystallography. Complex 1 crystallizes in the monoclinic space group C2/ c with a=16.998(5) Å, b=16.768(5) Å, c=9.546(3) Å, β=105.22(1)°, V=2625(1) Å 3, and Z=4. Complex 2 crystallizes in the triclinic space group P 1 ̄ , with a=9.795(4) Å, b=9.942(4) Å, c=9.126(3) Å, α=90.32(1)°, β=111.69(1)°, γ=108.67(1)°, V=774.5(5) Å 3, and Z=1. The structures of 1 and 2 were consistent with the presence of a V V 2O 2 core, to which citrate ligands of differing protonation state were bound in a coordination mode consistent with past observations. Ultimately, the aqueous pH dependent transformations of a series of three dinuclear complexes, 1, 2 and (NH 4) 2[V 2O 4(C 6H 6O 7) 2]·2H 2O ( 3), all isolated at pH values from 3 to 7.5, were explored and revealed an important interconnection among all species. Collectively, pH emerged as a determining factor of structural attributes in all three complexes, with the adjoining acid–base chemistry unfolding around the stable V V 2O 2 core. The results point to the participation of all three species in aqueous vanadium(V)–citrate speciation, and may relate the site-specific protonations–deprotonations on the dinuclear complexes to potential biological processes involving vanadium(V) and physiological ligand targets.

A novel dinuclear species in the aqueous distribution of aluminum in the presence of citrate.

The chemistry of aluminum was explored in the presence of the physiological ligand citric acid and in low-pH aqueous media. As a result, the first dinuclear aluminum-citrate complex (NH4)4[Al2-(C6H4O7)(C6H5O7)2].4H2O was isolated at low pH (approximately 3.5), and was characterized by FT-IR spectroscopy and X-ray crystallography. The structural analysis reveals the presence of a dinuclear assembly of two aluminum ions octahedrally coordinated to three citrate ligands of differing protonation state. The NMR solution behavior of this complex emphasizes its time-dependent transformation into a number of variable nature species, ultimately leading to the thermodynamically stable trinuclear species. It also establishes the participation of the dinuclear complex as a viable component of the aqueous Al(III)-citrate speciation. The chemical and structural features of this novel low molecular mass species provide considerable insight into citrate's ability, as a natural ligand, to influence the chemistry of aluminum in a pH-dependent fashion, and potentially affect aluminum's (bio)distribution, absorption, accumulation, and biotoxicity at sensitive biological sites.

Correlations of synthetic, spectroscopic, structural, and speciation studies in the biologically relevant cobalt(II)-citrate system: the tale of the first aqueous dinuclear cobalt(II)-citrate complex.

Synthetic efforts targeting soluble species of Co(II) with the low molecular mass physiological ligand citric acid led to the isolation of the first dinuclear complex [Co(2)(C(6)H(5)O(7))(2)(H(2)O)(4)](2-), at pH approximately 5, in the form of its K+ (1) and Na+ (2) salts. Both 1 and 2 were characterized analytically, spectroscopically (FT-IR, UV/visible, EPR), and magnetically. Complex 1 crystallizes in the monoclinic space group P2(1)/n, with a = 10.348(5) A, b = 11.578(6) A, c = 12.138(6) A, beta = 112.62(2) degrees, V = 1342(1) A(3), and Z = 2. Complex 2 crystallizes in the monoclinic space group P2(1)/c, with a = 9.234(4) A, b = 11.913(4) A, c = 11.728(6) A, beta = 99.93(2) degrees, V = 1271(1) A(3), and Z = 2. X-ray crystallography on 1 and 2 reveals the presence of two Co(II) ions, in a dinuclear assembly, octahedrally coordinated by two citrate ligands in a tridentate fashion. The octahedral environment around each Co(II) is complemented by another singly bonded citrate belonging to the adjacent Co(II) unit and two water molecules. Magnetic susceptibility and EPR studies on 1, in the solid state, corroborate the X-ray results, indicating a weak interaction between the two Co(II) ions. Moreover, EPR and UV/visible studies in solution suggest that 1 does not retain its dimeric structure, yielding a mononuclear octahedral Co(II)-citrate species. Detailed speciation studies suggest the presence of a number of species including the mononuclear complex [Co(C(6)H(5)O(7))](-), optimally present around pH approximately 5. In consonance with EPR and UV/visible spectroscopy, [Co(C(6)H(5)O(7))](-) is likely the scaffolding unit on the basis of which the dimer [Co(2)(C(6)H(5)O(7))(2)(H(2)O)(4)](2-) is isolated from aqueous solutions. Collectively, this comprehensive study offers significant structural insight into the Co(II)-citrate speciation and the elucidation of the role of Co(II) in biological fluids.