Conditional Formation Constants Research Articles

Didactic experiment for voltametric determination the complexing capacity of natural waters

The titration method can be applied to determine the complexation capacity between metal ions and ligands in natural water samples. The process involves the incremental addition of the metal to the sample containing a specific ligand, resulting in a titration curve that reflects the formation of complexes. The change in the curves slope indicates the saturation of the complexation capacity. In this study, the copper (II) ion was used due to its stability with organic ligands. The titration curve was analyzed to determine the complexation capacity (CC) and conditional formation constant (K) of the complex. Different mathematical treatments are proposed for this analysis. In the described experiment, differential pulse anodic stripping voltammetry was used to determine the complexation capacity of humic substances in natural waters. Experimental parameters such as pH, supporting electrolyte concentration, and volume of additions were previously optimized for the voltammetry technique. The obtained data were processed and analyzed, resulting in titration curves and voltammograms. The analysis of the curves allowed for the calculation of the complexation capacity and conditional stability constant of the formed complex. The results demonstrate the feasibility of the method as a didactic experiment for the voltammetric determination of natural water complexation capacity.

Experimental and computational evidence of U(VI)–OH–Si(OH)4 complexes under alkaline conditions: Implications for cement systems

The complexation of uranyl hydroxides with orthosilicic acid was investigated by experimental and theoretical methods. Spectroluminescence titration was performed in a glovebox under argon atmosphere at pH 9.2, 10.5 and 11.5, with [U(VI)] = 10−6 and 5 × 10−6 mol kgw−1. The polymerization effects of silicic acid were minimized by ruling out samples with less than 90 % monomeric silicic acid present, identified via UV–Vis spectrometry using the molybdate blue method. Linear regression analysis based on time-resolved laser-induced fluorescence spectroscopy (TRLFS) results yielded the conditional stepwise formation constants of U(VI)–OH–Si(OH)4 complexes at 0.05 mol kgw−1 NaNO3. The main spectroscopic features – characteristic peak positions and decay-time – are reported for the first time for the UO2(OH)2SiO(OH)3− species observed at pH 9.2 and 10.5 and UO2(OH)2SiO2(OH)22− predominant at pH 11.5. Quantum chemical calculations successfully computed the theoretical luminescence spectrum of the complex UO2(OH)2SiO(OH)3− species, thus underpinning the proposed chemical model for weakly alkaline systems. The conditional stability constants were extrapolated to infinite dilution using the Davies equation, resulting in log10β°(UO2(OH)2SiO(OH)3−) and log10β°(UO2(OH)2SiO2(OH)22−). Implications for U(VI) speciation in the presence and absence of competing carbonate are discussed for silicate-rich environments expected in certain repository concepts for nuclear waste disposal.

Binding Models of Copper(II) Thiosemicarbazone Complexes with Human Serum Albumin: A Speciation Study.

Copper(II) complexes of thiosemicarbazones (TSCs) often exhibit anticancer properties, and their pharmacokinetic behavior can be affected by their interaction with blood transport proteins. Interaction of copper(II) complexes of an {N,N,S} donor α-N-pyridyl TSC (Triapine) and an {O,N,S} donor 2-hydroxybenzaldehyde TSC (STSC) with human serum albumin (HSA) was investigated by UV–visible and electron paramagnetic resonance spectroscopy at physiological pH. Asp-Ala-His-Lys and the monodentate N-methylimidazole were also applied as binding models. Conditional formation constants were determined for the ternary copper(II)-TSC complexes formed with HSA, DAHK, and N-methylimidazole based on the spectral changes of both charge transfer and d-d bands. The neutral N-methylimidazole displays a similar binding affinity to both TSC complexes. The partially negatively charged tetrapeptide binds stronger to the positively charged Triapine complex in comparison to the neutral STSC complex, while the opposite trend was observed for HSA, which demonstrates the limitations of the use of simple ligands to model the protein binding. The studied TSC complexes are able to bind to HSA in a fast process, and the conditional constants suggest that their binding strength is only weak-to-moderate.

Solubility of PuO2(am,hyd) and the Formation of Pu(IV) Carbonate Complexes in Carbonate Solutions Containing 0.1–5.0 mol·dm−3 NaNO3

The solubility of plutonium hydrous oxide (PuO2(am, hyd)) was investigated in the presence of carbonate in neutral to alkaline, dilute to concentrated sodium nitrate (NaNO3) solutions. The observed dependence of the Pu(IV) solubility in low to medium concentration NaNO3 agreed with previously reported data and was well reproduced by the thermodynamic model curves, containing the species \({\text{Pu(CO}}_{3} )_{4}^{4 - }\), \({\text{Pu(CO}}_{3} )_{5}^{6 - }\), and \({\text{Pu(OH)}}_{{2}} {\text{(CO}}_{3} )_{2}^{2 - }\). Significant deviations between experimental and modeled Pu(IV) solubility were, however, observed for 2 mol·dm−3 NaNO3 solutions. A more comprehensive chemical model including the complex \({\text{Pu(OH)}}_{{2}} {\text{(CO}}_{3} )_{3}^{4 - }\) was established, that reproduces the experimental results reasonably well. The conditional complex formation constants for \({\text{Pu(OH)}}_{{2}} {\text{(CO}}_{3} )_{3}^{4 - }\) were extrapolated to I = 0 using the specific ion interaction theory (SIT) and Pitzer approaches, and the corresponding ion interaction parameters were evaluated.

Aggregation of poly(N-isopropylacrylamide) homopolymer by Cu2+ and Zn2+: Significance for ratiometric metal ion indicators

Aqueous transition metal dications Cu2+ and Zn2+ bind to the amide groups of the responsive polymer poly(N-isopropylacrylamide) (PNIPAm). Ratiometric detection by the response of PNIPAm is based on the effect of metal ion binding to ligands that are copolymerized into PNIPAm. Metal ions bound to copolymerized ligands cause a polarity change that shifts the ratio of coil and globule polymer phases, whereby the coil:globule state is monitored by the FRET process of a pair of fluorophores distributed throughout the polymer. This work shows that aqueous Cu2+ or Zn2+ ([M2+] = 2.5 × 10−2 M to 2.5 × 10−4 M) cause aggregation of the PNIPAm homopolymer (Mn = 8400 and 10,300) in the globule phase above 32 °C, as determined by second-order and dynamic light-scattering. Conditional formation constants (pH 6.3) for Cu2+ binding PNIPAm were measured as log Kc = 2.64 ± 0.07 and 4.02 ± 0.05 for PNIPAm concentrations of 1 mg/mL and 0.05 mg/mL, respectively. The interaction of Cu2+ and Zn2+ with PNIPAm is therefore in competition with the copolymerized ligands and when detecting millimolar to high micromolar concentrations, this interaction poses a potential interference to metal ion detection. Several aggregation processes are implicated based on Cu2+ or Zn2+ from 8 × 10−5 to 2.5 × 10−2 M. These processes are fully reversible by reducing the sample temperature.

Synthesis and structural studies of copper(ii) complex with N2S2 based N-substituted pendant phosphonic acid arms.

The synthesis of a heteromacrocyclic bifunctional chelator with phosphonic acid pendent arms is presented along copper(ii) complexation. Ligand N2S2-POH featuring N,N'-bis-substituted phosphonate pendent arms was isolated in respectable yields, characterized, and chelated to copper(ii). Implementation of both Moedritzer-Irani and Kabachnik-Fields conditions using aza-thia macrocycle 1,8-dithia-4,11-diazacyclotetradecane afforded 1,8-dithia-4,11-diazacyclotetradecane-4,11-diyl-bis-(methylene)-bis-(phosphonic acid) (N2S2-POH). Kinetic NMR studies provided four acid dissociation constants with respect to hydronium ion concentration. Benesi-Hildebrand binding experiment provided a conditional formation constant of 2.8 × 104 M-1. Heteromacrocycle N2S2-POH readily formed an encapsulated copper(ii) chelate at room temperature, which was examined through EPR analysis.

A NEW METHOD FOR CONSTANT ESTIMATE STABILITY OF COMPLEX WITH EDTA

The present work describes a new methodology in analytical chemistry, which makes it possible to calculate the stability constant (equilibrium or formation) of the coordination compounds generated between all metallic cations in the periodic table, except for alkali metals (group 1 ) together with the ethylene diamine tetraacetic acid binder. The system is simple to use, inexpensive, and allows the knowledge of the size of the conditional formation constants for all metals coordinated with the complexing agent to be investigated at any pH value investigated. In addition, the method is also extensive for other binders having the same chemical reaction property as ethylenediamine tetraacetic acid performs with metal ions in the same stoichiometric bonding ratio. For the execution of this study, solutions of copper (II) cation was employed, at a concentration of 1.0x10-2 mol L-1 and the complexing agent, also presenting the same concentration of metal. The assay was conducted in a buffering system consisting of ammonium hydroxide and ammonium chloride salt at a constant pH of 10.0. The method allows its use in experimental procedures developed both in a chemical laboratory and in a classroom, used during the theoretical activities. The chelating agent has a wide spectrum of application in the food product industries, also in the generation of cleaning materials in general, as well as in the photography industry, textile production, paper manufacture and the like, rubbers and the like, polymers and derivatives, among other production lines intended for its manufacture for human consumption.

Bichromophoric naphthalene derivatives of diethylenetriaminepentaacetate (DTPA): fluorescent response to H+, Ca2+, Zn2+, and Cd2+

ABSTRACTAs an effort to design selective fluorescent sensors toward Ca2+, Zn2+ and Cd2+, synthetic and fluorometric studies were performed on four bichromophores, each of which consists of two naphthyl or methynaphthyl units (1- and 2-isomers) linked with a diethylenetriaminepentaacetate (DTPA) chain. Every bichromophore exhibits naphthalene-monomer emission at 370 nm and excimer emission at 405 nm. Emission intensities show sensitive pH dependence, from which protonation constants were determined. Fluorometric titrations with the metal ions were performed at the physiological pH and the conditional formation constants were determined. Naphthyl rings define the stoichiometry and stability of the complexes. The insertion of CH2 spacer intensifies the emission and enhances the selective response to metal ions: the excimer emission is strengthened by 70−100 % with Cd2+ coordination, weakened by 60 % with Zn2+, and insensitive to Ca2+. The high response of methylnaphthyl bichromophores to Cd2+ is advantageous in fluorometric analyses.

β-Cyclodextrin-copper (II) complex as chiral selector in capillary electrophoresis for the enantioseparation of β-blockers

The Copper (II) complex of β-cyclodextrin (β-CD), β-CDCu2, was prepared and used as a chiral selector for the enantioseparation of 11 β-blockers by capillary electrophoresis. Resolution for 10 pairs of enantiomers was greater than 1.9 and for the four stereoisomers of labetalol (with two chiral centers), the closest two peaks had a resolution of 1.2. The resolution was superior to that obtained by β-CD itself as well as using anionic cyclodextrin, sulfated-β-cyclodextrin (S-β-CD). In order to elucidate the separation mechanism, the conditional binding constants between the enantiomers and the complex chiral selector and the elctrophoretic mobilities of the diastereomeric complexes were calculated by nonlinear regression. It was found that binding difference and mobility difference are two effects that are responsible for enantiomeric separation. When the concentration of selector is low, the differences in binding were dominant in the separation, while at high concentrations, differences in mobility were dominant in the separation. It was possible to achieve changes in migration orders for two of the drugs tested, with the remainder showing robust resolution due to compound effects of mobility and conditional formation constant. The complex chiral selector provides good separation for the enantiomers or stereoisomers of all the drugs tested which cannot be resolved by β-CD itself.

Binding of vanadium to human serum transferrin - voltammetric and spectrometric studies

Previous studies generally agree that in the blood serum vanadium is transported mainly by human serum transferrin (hTF). In this work through the combined use of electrochemical techniques, matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry and small-angle X-ray scattering (SAXS) data it is confirmed that both VIV and VV bind to apo-hTF and holo-hTF. The electrochemical behavior of solutions containing vanadate(V) solutions at pH=7.0, analyzed by using two different voltammetric techniques, with different time windows, at a mercury electrode, Differential Pulse Polarography (DPP) and Cyclic Voltammetry (CV), is consistent with a stepwise reduction of VV→VIV and VIV→VII. Globally the voltammetric data are consistent with the formation of 2:1 complexes in the case of the system VV-apo-hTF and both 1:1 and 2:1 complexes in the case of VV-holo-hTF; the corresponding conditional formation constants were estimated. MALDI-TOF mass spectrometric data carried out with samples of VIVOSO4 and apo-hTF and of NH4VVO3 with both apo-hTF and holo-hTF with V:hTF ratios of 3:1 are consistent with the binding of vanadium to the proteins. Additionally the SAXS data suggest that both VIVOSO4 and NaVVO3 can effectively interact with human apo-transferrin, but for holo-hTF no clear evidence was obtained supporting the existence or the absence of protein-ligand interactions. This latter data suggest that the conformation of holo-hTF does not change in the presence of either VIVOSO4 or NH4VVO3. Therefore, it is anticipated that VIV or VV bound to holo-hTF may be efficiently up-taken by the cells through receptor-mediated endocytosis of hTF.

Study the Molar Ratio of the Reaction between Thalassemia s Drug (Deferoxamine) with Iron Over loud

Iron-Deferoxamine complex have been synthesized in vitro by the reaction of deferoxamine (which are use as therapeutic agent for removing iron overload from the bodies of thalassemia patients) with Fe(II) salt in aqueous medium. The geometry of the prepared complex as well as the mode of bonding were determined through FTIR, UV-Vis, mass spectral studies, metal content, and molar conductance. These studies revealed octahedral geometry for the complex with the general formula [FeII (DEF)]Cl2. Complex formation via molar ratio method and variation continuous method in aqueous solution have been also investigated and results were consistent to those found in the solid complexes with a ratio of (M:L) as (1 : 1). Conditional formation constant could also be calculated for [FeII (DEF)]Cl2 as 2.0 × 103. The effect of time on the formation of Iron – Deferoxamine complex has been also studied. This study showed that each 1×10-3M of deferoxamine needs 2.5 – 3 hrs to remove 1×10-3M of Iron (II) by formation coordination complex.

The Determinatıon of The Stability Constant of Finasteride Complexes with Various Metals by Potentiometric and Spectrophotometric Methods

In this study, the stability constants of the cobalt(II), zinc(II), cadmium(II) and copper(II) complexes of finasteride were determined potentiometrically by using Irving-Rossotti and Calvin-Bjerrum methods and supported by spectrophotometric way [1,2,3]. The evaluated protonation constant of finasteride was found as logK=2.21. Potentiometrically the logarithms of the stability constants of the finasteride-metal complexes were found by Calvin-Bjerrum and Irving-Rossotti methods: for cobalt(II) complex logK=3.01; for zinc(II) complex logK=2.82; for cadmium(II) complex logK=3.04 and for copper complex logK=2.85 at 25oC (I=0.10). Conditional formation constants of the complexes were calculated and pH ranges of the complexation were found. Spectrometric measurements were recorded for cobalt(II)-finasteride, zinc(II)-finasteride, cadmium(II)-finasteride and copper(II)-finasteride solutions and the results of the measurement supported the formation of complexes.

Determination of formation constants of uranyl(VI) complexes with a hydrophilic SO3-Ph-BTP ligand, using liquid-liquid extraction

Abstract Complex formation between uranyl ion, UO2 2+, and a hydrophilic anionic form of SO3-Ph-BTP4- ligand, L4-, in water was studied by liquid-liquid extraction experiments performed over a range of the ligand and HNO3 concentrations in the aqueous phase, at a constant concentration of nitrate anions at 25°C . The competition for UO2 2+ ions between the lipophilic TODGA extractant and the hydrophilic L4- ligand leads to the decrease in the uranyl distribution ratios, D, with an increasing L4- concentration. The model of the solvent extraction process used accounts - apart from uranyl complexation by TODGA and SO3-Ph-BTP4- - also for uranyl complexation by nitrates and for the decrease in the concentration of the free L4- ligand in the aqueous phase, due to its protonation, bonding in the uranyl complex and the distribution between the two liquid phases. The unusually strong dependence of the D values on the acidity, found in the experiment, could hardly be explained as due to L4- protonation merely. Three hypotheses were experimentally tested, striving to interpret the data in terms of additional extraction to the organic phase of ion associates of protonated TODGA cation with either partly protonated anionic L4- ligands or anionic UO2 2+ complexes with NO3 - or L4-. None of them has been confirmed. The analysis of the results, based on the formal correction of free ligand concentrations, points to the formation of 1 : 1 and 1 : 2 uranyl - SO3-Ph-BTP complexes in the aqueous phase. The conditional formation constant of the 1:1 complex has been determined, logßL,1 = 2.95 ± 0.15.

Understanding Complexometric Titrations of Metal Cations with Aminopolycarboxylic Acids (EDTA and Analogs) within the frame of the Notion of Reactions between Groups of Chemical Species

In spite of the apparent technical simplicity with which visual complexometric titrations of metal cations with aminopolycarboxylic acids titrants are performed, a complex chemistry takes place in the titrated solution during the titration, due to the chemical environment and to the insuppressible chemical properties of metal cations, aminopolycarboxylic acids and metallochromic indicators. This chemical complexity makes rigorous exposition and evaluations of complexometric titrations arduous. Nonetheless, by the introduction of the notions of groups of chemical species and reactions between groups of chemical species (with the connected concept of conditional formation constant), a frame is created within which complexometric titrations with aminopolycarboxylic acids can be collocated and which allows a reasonably simple presentation and evaluation of the analytically relevant aspects of this type of titrations. In a sense, the well known concept of conditional complex formation constant is updated in a way that facilitates understanding and use.

On pH and Ca2+ Oscillations Monitored by pH Electrode and Ca‐ISE in Bromate–Sulfite–Ferrocyanide System Introduced Ca‐EDTA

The pH and Ca2+ oscillations of KBrO3–Na2SO3–K4Fe(CN)6 (BSF) system in presence of EDTA or Ca‐EDTA were investigated. Both EDTA and Ca‐EDTA can depress the amplitude and period of pH oscillation, and the effects of Ca‐EDTA are more obvious. These effects are attributed to the buffer effect in pH change. The Ca2+ oscillation appears as Ca‐EDTA introduced into the BSF system, which is ascribed to the stability of Ca‐EDTA being pH‐dependent. The pH and pCa (pCa = −log[Ca2+]) oscillations are synchronous in case of higher concentration of Ca‐EDTA; however, the chaotic behavior emerged in pCa oscillation arises at lower pH as the concentration of Ca‐EDTA is lower. Such observations can be understood by taking into account the interference of H+ on the response of Ca‐ISE to Ca2+. Both amplitude and period for pH and pCa oscillations as KCl introduced into the system can be obviously shortened due to the effect of ion strength on the reaction rate of the core oscillator and on the conditional formation constant of Ca‐EDTA. The results are significant for determining whether an experimentally observed chaos in chemical systems is of deterministic origin or is due to experimental noise.

Universal reaction mechanism of boronic acids with diols in aqueous solution: kinetics and the basic concept of a conditional formation constant.

To establish a detailed reaction mechanism for the condensation between a boronic acid, RB(OH)2, and a diol, H2L, in aqueous solution, the acid dissociation constants (Ka(BL)) of boronic acid diol esters (HBLs) were determined based on the well-established concept of conditional formation constants of metal complexes. The pKa values of HBLs were 2.30, 2.77, and 2.00 for the reaction systems, 2,4-difluorophenylboronic acid and chromotropic acid, 3-nitrophenylboronic acid and alizarin red S, and phenylboronic acid and alizarin red S, respectively. A general and precise reaction mechanism of RB(OH)2 with H2L in aqueous solution, which can serve as a universal reaction mechanism for RB(OH)2 and H2L, was proposed on the basis of (a) the relative kinetic reactivities of the RB(OH)2 and its conjugate base, that is, the boronate ion, toward H2L, and (b) the determined pKa values of HBLs. The use of the conditional formation constant, K', based on the main reaction: RB(OH)2 + H2L (K1)⇌ RB(L)(OH)(-) + H3O(+) instead of the binding constant has been proposed for the general reaction of uncomplexed boronic acid species (B') with uncomplexed diol species (L') to form boronic acid diol complex species (esters, BL') in aqueous solution at pH 5-11: B' + L' (K')⇌ BL'. The proposed reaction mechanism explains perfectly the formation of boronic acid diol ester in aqueous solution.

Synthesis, spectral, thermal studies and electrical conductivity of Co (II) and Ni (II) complexes 3-[4-dimethylaminophenyl]-1-(2-pyridyl) prop-2-en-1-one (DMAPP)

Co(II) and Ni(II) complexes of 3-[4-dimethylaminophenyl]-1-(2-pyridyl)prop-2-en-1-one have been prepared and characterized on the basis of elemental analyses, molar conductance, magnetic susceptibility measurements, IR, electronic spectra as well as thermal studies. The magnetic and spectral studies suggested the octahedral geometry for Co(II) and Ni(II) complexes. The kinetic parameters of the thermal decomposition stages have been evaluated using Coats–Redfern method. The electrical conductivity of the titled ligand and its Co(II) complexes was studied. The effects of different alcoholic solvents, pH and temperature on the complexation formation were considered. Also, the effect of Co(II) and Ni(II) ions on the emission spectrum of the free DMAPP was assigned. The stoichiometry of the metal complexes, the conditional formation constant, free energy, Beer׳s law, molar extinction coefficient as well as specific absorptivity were evaluated. The ability of using the titled ligand as metalochromic indicator in complexometric titration was studied.

The Dissolution of Amorphous Silica in the Presence of Tropolone Under Acidic Conditions

AbstractDissolution of amorphous silica or silicate is an important reaction to release silicic acid in natural water and this reaction is affected by several factors. The existence of a natural organic compound, tropolone, because of its abundance, is of particular importance. In the present study, the dissolution of amorphous silica in the presence of tropolone (HL) was investigated in an aqueous solution under acidic conditions. The dissolution is controlled by the reaction between silicic acid and tropolone (Si(OH)4 + 3HL + H+ ⇌ SiL3+ + 4H2O), where the conditional formation constant K = [SiL3+]/[Si(OH)4][HL]3[H+]). The solubility of amorphous silica in the presence of tropolone was significantly greater than that in pure water due to the formation of the Si-tropolonate complex in the solution. The acceleration of silica dissolution by tropolone depends on the reaction pH and tropolone concentration. The solubility of the amorphous silica in the tropolone solution at pH 1 was approximately ten times greater than in the 0.1 mol/dm3 HCl used as a reference. This increase in the solubility correlates to a K value of the Si-tropolone complex at each pH. From the solubility experiments, the conditional formation constants (log K) of the Si-tropolonate complex at each reaction pH 1, 2, and 3 were 6.39, 5.88, and 5.77, respectively. The significant acceleration of the dissolution of amorphous silica by tropolone at pH 1 can be attributed to the large formation constant of the Si-tropolonate complex.

Acid–base behavior and Al3+ complex formation of synthesized 2,3-dihydroxyterephthalic acid (DHTPA) at pH 3 as a model compound of Inogashira fulvic acid (IFA)

The interaction between the aluminum ion (Al3+) and Inogashira fulvic acid (IFA) at pH 3 was investigated using the calibration curve method for 27Al NMR spectra. The average conditional stability constant (logK) can be calculated to be 2.00–2.04 (M−1) (bidentate–monodentate) from the results of 27Al NMR measurements. In addition, because IFA has various coordination sites, including COOH, phenolic and/or alcoholic OH groups, we attempted to synthesize 2,3-dihydroxyterephthalic acid (DHTPA) as a model compound with the functional groups of IFA for the investigation of the microscopic coordination mechanism between Al3+ and IFA by potentiometric titration and 27Al NMR measurements. The pKai values of DHTPA could be determined successfully (pKa1=2.3, pKa2=3.4 and pKa3=7.2; pKa4 was not determined), and these values indicate that DHTPA can act as a powerful chelating ligand, even at pH 3. DHTPA can interact with Al3+, and it predominantly forms 1:1 and 2:1 Al–DHTPA complexes. The calculated average conditional formation constant (logK1 and K2) of each complex can be determined as 1.09 (1:1) and 4.81 (M−1) (2:1). The results obtained showed that both the 1:1 and 2:1 Al–DHTPA complexes are formed and Al3+ interacts with IFA at pH3.

A novel VIVO–pyrimidinone complex: synthesis, solution speciation and human serum protein binding

The pyrimidinones mhcpe, 2-methyl-3H-5-hydroxy-6-carboxy-4-pyrimidinone ethyl ester (mhcpe, 1), 2,3-dimethyl-5-benzyloxy-6-carboxy-4-pyrimidinone ethyl ester (dbcpe, 2) and N-methyl-2,3-dimethyl-5-hydroxy-6-carboxyamido-4-pyrimidinone (N-MeHOPY, 3), are synthesized and their structures determined by single crystal X-ray diffraction. The acid-base properties of 1 are studied by potentiometric and spectrophotometric methods, the pK(a) values being 1.14 and 6.35. DFT calculations were carried out to determine the most stable structure for each of the H2L(+), HL and L(-) forms (HL = mhcpe) and assign the groups involved in the protonation-deprotonation processes. The mhcpe(-) ligand forms stable complexes with V(IV)O(2+) in the pH range 2 to 10, and potentiometry, EPR and UV-Vis techniques are used to identify and characterize the V(IV)O-mhcpe species formed. The results are consistent with the formation of V(IV)O, (V(IV)O)L, (V(IV)O)L2, (V(IV)O)2L2H(-2), (V(IV)O)L2H(-1), (V(IV)O)2L2H(-3), (V(IV)O)LH(-2) species and V(IV)O-hydrolysis products. Calculations indicate that the global binding ability of mhcpe towards V(IV)O(2+) is similar to that of maltol (Hmaltol = 3-hydroxy-2-methyl-4H-pyran-4-one) and lower than that of 1,2-dimethyl-3-hydroxy-4-pyridinone (Hdhp). The interaction of V(IV)O-complexes with human plasma proteins (transferrin and albumin) is studied by circular dichroism (CD), EPR and (51)V NMR spectroscopy. V(IV)O-mhcpe-protein ternary complexes are formed in both cases. The binding of V(IV)O(2+) to transferrin (hTF) in the presence of mhcpe involves mainly (V(IV)O)1(hTF)(mhcpe)1, (V(IV)O)2(hTF)(mhcpe)1 and (V(IV)O)2(hTF)(mhcpe)2 species, bound at the Fe(III) binding sites, and the corresponding conditional formation constants are determined. Under the conditions expected to prevail in human blood serum, CD data indicate that the V(IV)O-mhcpe complexes mainly bind to hTF; the formation of V(IV)O-hTF-mhcpe complexes occurs in the presence of Fe(III) as well, distinct EPR signals being clearly obtained for Fe(III)-hTF and to V(IV)O-hTF-mhcpe species. Thus this study indicates that transferrin plays the major role in the transport of V(IV)O-mhcpe complexes under blood plasma conditions in the form of ternary V(IV)-ligand-protein complexes.