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Humic substances are poorly known, though they represent a major pool of non-biotic organic carbon on earth. In particular, there is little knowledge on the formation of humic substances by irradiation of organic matter dissolved in waters. Specifically, it is known that humic substances can be formed from proteins by photochemical processes in surface waters, but the role of single amino acids and their transformation pathways are not yet known. Therefore, here we studied the phototransformation of aqueous l-tryptophan under simulated sunlight. Irradiated l-tryptophan solutions were analyzed by absorption, fluorescence, nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopies, chromatography, potentiometry and mass spectrometry (MS). The solutions appeared turbid after irradiation; therefore, nephelometry and dynamic laser light scattering were used to characterize the suspended particles. Results show that about 95% of l-tryptophan was degraded in 8-h irradiation, undergoing deamination and decarboxylation of the amino acidic moieties to release ammonium and formate. The MS signal at m/z 146 suggests the formation of 3-ethylindole, while pH-metric and NMR data revealed the presence of hydroxylated compounds. The phototransformation intermediates of l-tryptophan had fluorescence and absorption spectra similar to those of humic substances, they were able to produce ·OH upon irradiation and tended to aggregate by both ionic and hydrophobic interactions. Overall, our findings reveal for the first time the nature of products formed upon phototransformation of l-tryptophan. Interestingly, the transformation of l-tryptophan is quite different from that of the previously studied l-tyrosine, although both compounds produce humic-like materials under irradiation.

Tannic acid (TA) belongs to the class of hydrolysable tannins that are natural polymers derived from the vegetable kingdom. Although TA is described as a molecule with a central core of glucose esterified with five digallic units, its molecular structure has not yet been completely clarified. Actually, TA is described as a mixture of different compounds. In this work, by using potentiometry, UV-vis and fluorescence spectroscopy, as well as ab initio calculations we identified the protonation properties and spectroscopic features of TA. A preliminary investigation on gallic acid (GA), present as an impurity in the commercial TA mixture, and on methyl 3,4,5-trihydroxybenzoate served as a benchmark for the computational work and gave insight into the contribution of GA to the TA properties. GA principally affects the pH of TA solutions and the fluorescence signals. The data showed the presence of three main types of protogenic groups that can be assigned to the typical TA structure, with pKa values included in the range of 6–8.5, which can be ascribed to the phenolic functions. The least acidic site shows the highest concentrations, and the dissociation of half of the TA phenolic groups takes place at pH ∼ 7.8. The UV-vis spectra of the protonated and deprotonated species were obtained through data elaboration by stoichiometric and chemometric approaches. The results show main absorption maxima (277 and 323 nm, respectively) similar to those obtained using ab initio calculations. Overall, we achieved a remarkable coherence among the outcomes obtained by using different methodologies.

Hydroxypyrone derivatives have a good bioavailability in rats and mice and have been used in drug development. Moreover, they show chelating properties towards vanadyl cation that could be used in insulin-mimetic compound development. In this work, the formation of coordination compounds of oxovanadium(IV) with four kojic acid (5-hydroxy-2-(hydroxymethyl)-4-pyrone) derivatives was studied. The synthetized studied ligands (S2, S3, S4, and SC) have two or three kojic acid units linked through diamines or tris(2-aminoethyl)amine chains, respectively. The chemical systems were studied by potentiometry (25 &deg, C, ionic strength 0.1 mol L&minus, 1 with KCl), and UV-visible and EPR spectroscopy. The experimental data were analyzed by a thermodynamic and a chemometric (Multivariate Curve Resolution&ndash, Alternating Least Squares) approach. Chemical coordination models were proposed, together with the species formation constants and the pure estimated UV-vis and EPR spectra. In all systems, the coordination of the oxovanadium(IV) starts already under acidic conditions (the cation is totally bound at pH higher than 3&ndash, 4) and the metal species remain stable even at pH 8. Ligands S3, S4, and SC form three coordination species. Two of them are probably due to the successive insertion of the kojate units in the coordination shell, whereas the third is most likely a hydrolytic species.

Background: Metabolic and physicochemical evaluation is recommended to manage the condition of patients with nephrolithiasis. The estimation of the saturation state (β values) is often included in the diagnostic work-up, and it is preferably performed through calculations. The free concentrations of constituent ions are estimated by considering the main ionic soluble complexes. It is contended that this approach is liable to an overestimation of β values because some complexes may be overlooked. A recent report found that β values could be significantly lowered upon the addition of new and so far neglected complexes, [Ca(PO4)Cit]4− and [Ca2H2(PO4)2]. The aim of this work was to assess whether these complexes can be relevant to explaining the chemistry of urine. Methods: The Ca–phosphate–citrate aqueous system was investigated by potentiometric titrations. The stability constants of the parent binary complexes [Cacit]− and [CaPO4]−, and the coordination tendency of PO43− toward [Ca(cit)]− to form the ternary complex, were estimated. βCaOx and βCaHPO4 were then calculated on 5 natural urines by chemical models, including or not including the [CaPO4]− and [Ca(PO4)cit]4− species. Results: Species distribution diagrams show that the [Ca(PO4)cit]4− species was only noticeable at pH &gt, 8.5 and below 10% of the total calcium. β values estimated on natural urine were slightly lowered by the formation of [CaPO4]− species, whereas [Ca(PO4)cit]4− results were irrelevant. Conclusions: While [CaPO4]− species have an impact on saturation levels at higher pHs, the existence of ternary complex and of the dimer is rejected.

The stability of sodium complexes with poly-carboxylic and polyamino-carboxylic acids is investigated with ion-selective electrode-Na(+) potentiometry, working at strictly constant ionic strength. It is observed that the formation constants of the Na(+) complexes with monoligand stoichiometry (ML) increase with the number of charges on the ligand. For example, in poly-carboxylic acids this dependency is linear and is well captured by an experimental equation. A different behaviour is observed for the poly-amino carboxylic acids, which show higher complexation capabilities reaching a plateau of the binding energy past a specific ligand size. The experimental results are discussed qualitatively using ab initio calculations based on DFT B3LYP, and the principal electronic characteristics of the ligands under investigation are identified. As a result of the flexibility imparted by the long chains of polyamino-carboxylic ligands, both experimental and theoretical models demonstrate that nitrogen atoms in proximity of Na(+) ions can participate in the metal coordination, thus providing further stabilization for the complexes. Moreover, by increasing the ligand size the stabilization gained in terms of ΔG reached a plateau for EDTA, in agreement with experimental observations.

1,2-Benzenedisulfonimide (1) has previously been found to be an excellent and, importantly, safe Bronsted acid catalyst. In this work we present the results of a search for derivatives of 1 that are more acidic and effective. Instead of blindly synthesizing a series of analogues, we have carried out a screening process in which the pKa of 1 and a set of its derivatives were calculated. The calculated pKa values were confirmed experimentally by carrying out determination through potentiometric titrations of some of the compounds: 1, 4-methyl and 4-nitro derivatives of 1. The calculations indicated that the dinitro and 4-nitro derivatives are among the best candidates for the synthesis. The latter was obtained in good yields and tested as a catalyst. Results were excellent as the reactions took place more quickly and at lower temperatures in all cases, and in a number of cases it was even possible to reduce the amount of catalyst.

International audience; The UVB photolysis of l-tyrosine yields species with fluorescence and absorption spectra that are very similar to those of humic substances. By potentiometric measurements, chemical modeling and the application of NMR, mass spectrometry and laser flash photolysis, it was possible to get insights into the structural and chemical properties of the compounds derived by the l-tyrosine phototransformation. The photolytic process follows aromatic-ring hydroxylation and dimerization. The latter is presumably linked with the photoinduced generation of tyrosyl (phenoxy-type) radicals, which have a marked tendency to dimerize and possibly oligomerize. Interestingly, photoinduced transformation gives compounds with protogenic and complexation capabilities similar to those of the humic substances that occur naturally in surface waters. This finding substantiates a new and potentially important abiotic (photolytic) pathway for the formation of humic compounds in surface-water environments.

The interactions of dioxouranium(VI) cation with different organic and inorganic ligands of environmental and biological interest were carefully examined with the aim to draw a chemical speciation picture of this ion in natural aquatic ecosystems and in biological fluids. Since UO22+ ion shows a significant tendency to hydrolyze, particular attention was paid in considering the hydrolysis species formation both in the presence and in absence of ligands. The results reported in the literature show that formation of the hydrolytic species assumes a great importance in the complexation models for all the UO22+-ligand systems considered. In particular, the following ligands have been taken into account: (i) hydroxyl, chloride, fluoride, sulfate, carbonate and phosphate, as inorganic ligands, and (ii) carboxylates (with particular reference to oxalate and citrate), amines, amino acids, poly(amino carboxylates) (complexones), nucleotides, phosphonates, mercapto compounds and sulfonates, as organic ligands. In order to elucidate the speciation of uranyl in the presence of dissolved natural organic matter, the interactions with humic and fulvic acids were also considered. The strength of interaction in all the systems considered was expressed in terms of stability constants of complex species and, if available, of the relative thermodynamic stability parameters. When possible, if data reported in the literature were sufficiently homogeneous, trends of stability were found for the different ligands of the same class and for ligands of different classes. Moreover, relationships were derived for poly-functional ligands, such as poly-carboxylate, poly-amine and poly(amino carboxylate) ones, useful to predict the stability constants as a function of the number of binding sites per molecule, considering also, as in the case of amino acids, the contribution of the single functional groups to the whole stability of uranyl species formed. In addition, using the stability data collected for the uranyl-ligand systems considered, the sequestering capacity of some classes of ligands towards uranyl was calculated in terms of pL0.5, i.e., the ligand concentration useful to bind at least 50% of the cation. A comparison of pL0.5 of the most important classes of ligands considered was made to point out the different effectiveness in the UO22+ sequestration by the different ligands which can be present in multi-component solutions as natural waters and biological fluids. Finally, some considerations are reported about the different experimental techniques employed to study the complex formation in solution.

In this paper we report an investigation on the interactions between dioxouranium(VI) and citrate using potentiometry (H+-glass electrode) and UV-spectrophotometry. Potentiometric measurements were carried out in NaCl and KNO3 aqueous solutions at t = 25 ºC in a wide range of experimental conditions (concentrations, ligand/metal molar ratio, pH, titrants). Measurements in NaCl were carried out at different ionic strength values (0.1 ≤ I/mol L−1 ≤ 1.0); different procedures were employed for the acquisition of experimental data and careful analysis of these data performed. In all cases the speciation model that best fits experimental data takes into account the formation of the following species: UO2(Cit)−, (UO2)2(Cit)2 2−, (UO2)2(Cit)2(OH) 2 4− , (UO2)2(Cit)2(OH)3−, (UO2)2(Cit)(OH)2 −, (UO2)2(Cit)(OH)0, (UO2)3(Cit)2(OH) 5 5−. The dependence on ionic strength of formation constants was taken into account by using both a simple Debye–Hückel type equation and the SIT (specific ion interaction theory) approach. Moreover, a visible absorption spectrum for each complex reaching a significant percentage of formation in solution (KNO3 medium) has been calculated to characterise the compounds found by pH-metric refinement. Recommended values for the uranyl-citrate species were proposed for each ionic strength values in NaCl aqueous solution. Comparison with literature stability constants is reported too.

The synthesis and the physicochemical and biological characterization of a series of carnosine amides bearing on the amido group alkyl substituents endowed with different lipophilicity are described. All synthesized products display carnosine-like properties differentiating from the lead for their high serum stability. They are able to complex Cu(2+) ions at physiological pH with the same stoichiometry as carnosine. The newly synthesized compounds display highly significant copper ion sequestering ability and are capable of protecting LDL from oxidation catalyzed by Cu(2+) ions, the most active compounds being the most hydrophilic ones. All the synthesized amides show quite potent carnosine-like HNE quenching activity; in particular, 7d, the member of the series selected for this kind of study, is able to cross the blood-brain barrier (BBB) and to protect primary mouse hippocampal neurons against HNE-induced death. These products can be considered metabolically stable analogues of carnosine and are worthy of additional investigation as potential neuroprotective agents.

In this paper we report an investigation on coordination compounds of vanadyl ion with citric, D(+)- threo -isocitric, cis - and trans -aconitic acids in aqueous solution. The different binary systems have been studied by potentiometric and spectroscopic techniques ( t = 25 °C, I = 0.1 mol dm −3 ). Electronic paramagnetic resonance spectroscopy (EPR) at room temperature and molecular absorption spectrophotometry were employed in order to obtain a structural characterization of complexes. A speciation model was proposed for all the metal–ligand systems from potentiometric data. For all the systems studied the chemical model assumes the formation of complexes with stoichiometry MLH r and binuclear species M 2 L 1 H − r . Citric and D(+)- threo -isocitric acids also form stable dimeric species M 2 L 2 H − r . The EPR investigation reveals that the dimeric complexes of hydroxyacids present magnetic interaction supporting the hypothesis of an alcoholate-bridged dinuclear structure. For each system investigated we recorded visible absorption spectra and estimated the individual spectrum of species in solution. A particular spectrophotometric behaviour of citrate system was recognized and the joint examination of our spectroscopic results and the literature data leads us to suppose that the trans position of vanadyl is occupied by the bridged alcoholate group.

The formation constants of dioxouranium(VI)-2,2′-oxydiacetic acid (diglycolic acid, ODA) and 3,6,9-trioxaundecanedioic acid (diethylenetrioxydiacetic acid, TODA) complexes were determined in NaCl (0.1≤I≤1.0 mol⋅L−1) and KNO3 (I=0.1 mol⋅L−1) aqueous solutions at T=298.15 K by ISE-[H+] glass electrode potentiometry and visible spectrophotometry. Quite different speciation models were obtained for the systems investigated, namely: ML0, MLOH−, ML22−, M2L2(OH)−, and M2L2(OH)22−, for the dioxouranium(VI)–ODA system, and ML0, MLH+, and MLOH− for the dioxouranium(VI)–TODA system (M=UO22+ and L = ODA or TODA), respectively. The dependence on ionic strength of the protonation constants of ODA and TODA and of both metal-ligand complexes was investigated using the SIT (Specific Ion Interaction Theory) approach. Formation constants at infinite dilution are [for the generic equilibrium pUO22++q(L2−)+rH+⇌(UO22+)p(L)qHr(2p−2q+r);βpqr]: log 10β110=6.146, log 10β11−1=0.196, log 10β120=8.360, log 10β22−1=8.966, log 10β22−2=3.529, for the dioxouranium(VI)–ODA system and log β110=3.636, log 10β111=6.650, log 10β11−1=−1.242 for dioxouranium(VI)–TODA system. The influence of etheric oxygen(s) on the interaction towards the metal ion was discussed, and this effect was quantified by means of a sigmoid Boltzman type equation that allows definition of a quantitative parameter (pL50) that expresses the sequestering capacity of ODA and TODA towards UO22+; a comparison with other dicarboxylates was made. A visible absorption spectrum for each complex reaching a significant percentage of formation in solution (KNO3 medium) has been calculated to better characterize the compounds found by pH-metric refinement.

This work is aimed at reviewing the chemical literature dealing with thermodynamic aspects of the weak complex formation (species with log K values less than about 3) between alkali and alkaline earth metal ions with low molecular weight inorganic and organic ligands in aqueous solution. The following ligands (up to hexavalent anions) were examined in detail: (i) hydroxide, chloride, sulfate, carbonate and phosphate as inorganic, and (ii) carboxylates, amines, amino acids, complexones and nucleotides as organic ligands. The paper also identifies the main reasons responsible for the dispersion of the stability data on ion pairs in the literature. When possible, the trend of stability for the different metal ions interacting with the same ligand will be considered to find predictive interaction relationships. Since the stability of weak alkali and alkaline earth metal complexes are mainly due to electrostatic interaction, simple empirical relationships were obtained between log K and the charge of the anionic ligand. The interest for alkali and alkaline earth cations rises since they are used in study of basic science as components of the supporting electrolyte and are widely diffused in natural fluids. Some examples of application of this science were presented too, to show the role of weak complex formation in the modelling process of natural systems.

This paper reports evaluation of the behaviour of different combined glass electrodes applied to measurement of the pH of a primary, 0.01 mol kg(-1), tetraborate buffer. Measurements were first performed by use of a primary Harned cell (at 15, 25, and 37 degrees C); these results were then compared with those obtained for the same solution by use of three combined glass electrodes (25 degrees C) with different membranes and liquid-junction designs, calibrated by use of commercial pH-metric buffers. The pH of the same solution was also measured in terms of the molal concentration of hydrogen ions, using acid-base titration to evaluate the formal potential difference K of each cell at fixed ionic strength, I, adjusted by addition of KCl or Et4NI (tetraethylammonium iodide). The reference value from primary measurement, paH = 9.171, was slightly closer to the mean value obtained by determination of concentration, rather than that obtained by direct measurement of activity; the differences were smaller than the extended uncertainty characteristics of the secondary measurements. The importance of evaluation of the ionic strength of the solution under study is emphasised. We verified that for tetraborate buffer slight modification of the value of I used to calculate gamma (i) (the activity coefficient of a single ion) in the calculation of paH from the acidity function at zero molality of chloride can significantly affect the reference value of the calibrator tool. This is true, in general, for low values of the ionic strength, such as those considered in this work; an approximate value of I can then cause distortions along the pH traceability chain. Application of the concepts of thermodynamics to this traceability chain is discussed.

This work reports a study of the proton-binding capacity of biopolymers obtained from different materials supplied by a municipal biowaste treatment plant located in Northern Italy. One material was the anaerobic fermentation digestate of the urban wastes organic humid fraction. The others were the compost of home and public gardening residues and the compost of the mix of the above residues, digestate and sewage sludge. These materials were hydrolyzed under alkaline conditions to yield the biopolymers by saponification. The biopolymers were characterized by 13C NMR spectroscopy, elemental analysis and potentiometric titration. The titration data were elaborated to attain chemical models for interpretation of the proton-binding capacity of the biopolymers obtaining the acidic sites concentrations and their protonation constants. The results obtained with the models and by NMR spectroscopy were elaborated together in order to better characterize the nature of the macromolecules. The chemical nature of the biopolymers was found dependent upon the nature of the sourcing materials.

The charge-transfer complexes have scientific relevance because this type of molecular interaction is at the basis of the activity of pharmacological compounds and because the absorption bands of the complexes can be used for the quantification of electron donor molecules. This work aims to assess the stability of the charge-transfer complexes between the electron acceptor 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and two drugs, procaine and atenolol, in acetonitrile and ethanol. The stability of DDQ in solution and the time required to obtain the maximum complex formation were evaluated. The stoichiometry and the stability of the complexes were determined, respectively, by Job’s plot method and by the elaboration of UV–vis titrations data. The latter task was carried out by using the non-linear global analysis approach to determine the equilibrium constants. This approach to data elaboration allowed us to overcome the disadvantages of the classical linear-regression method, to obtain reliable values of the association constants and to calculate the entire spectra of the complexes. NMR spectra were recorded to identify the portion of the donor molecule that was involved in the interaction. The data support the participation of the aliphatic amino groups in complex formation and exclude the involvement of the aromatic amine present in the procaine molecule.

We attempted to draw some general considerations regarding the ability of the visible absorption spectra to provide information on axial co-ordination around copper(II) ion in aqueous solution. A series of visible spectra was recorded on the copper(II)–2,2′-bipyridine and copper(II)– l -aspartic acid systems. 2,2′-Bipyridine is known for its ability to co-ordinate copper(II) in axial position(s) as well, while l -aspartic acid is a potential tridentate ligand whose ability to chelate in axial position has been here investigated. Combined pH-metric and visible absorption spectrophotometric measurements were performed: visible absorption spectra were recorded during alkalimetric titrations as a function of pH. Further computer-assisted treatment allowed obtaining absorption spectra of single complexes. A series of axially co-ordinated copper(II) complexes found in the literature was also considered. Structural assignment of planar complexes is based on an empirical equation previously published by our research group, which allows an estimation of the ligand field strength of the equatorial donor groups involved. This equation can now help to strengthen the discussion and to assess a criterion to establish the axial co-ordination mode of copper(II) ion. The location of a donor group along the z -axis of the copper(II) distorted octahedron causes a red shift in the absorption spectrum. A quantitative estimation of the red shift per donor group was performed and ranges of red shifts for single donor groups were identified: we calculated a mean value of 36 nm, while extreme values are 19 and 58 nm.

In this paper an attempt is made to model the alkalimetric titration trend (pH Analytical measurements of carboxylic acids, amino acids, inorganic anions and metals were executed to know the concentration of a series of acid–base active reactants (pH

Starting with known analytical concentrations related to the considered Italian red grape table wines, we now expound the methodology which allows us to establish a relationship among (a) the molar concentration of a series of substances of routine measurement, (b) the total acidity and (c) the pH value of each fluid at a thermodynamic level. The experimental pH value of each wine is compared with the calculated one, obtained as a result of a thermodynamic simulation based on the contemporary treatment of a series of chemical equilibria (17 reactants, 77 equilibria). Accurate equilibrium constant values are previously refined on test substances components of red wines (namely: acetic, tartaric, succinic and citric acid) starting by pH-metric readings (at 25°C) recorded in appropriate experimental conditions. In fact, the chemical conditions take care of both the solvent composition, as regards the amount of ethanol, and the ionic strength value, to take into account the effect due to the backgroun...

In this study we considered pi-methyl-L-histidine (pi-methis) and tau-methyl-L-histidine (tau-methis) as ligands for copper(II) ion, in order to clarify, by means of ultraviolet (UV) spectroscopy in aqueous solution (T = 25 degrees C, I = 0.1 M), some aspects of the co-ordination mode with respect to other ligands of a previous study in which copper(II) complexes of L-histidine, N-acetyl-L-histidine, histamine, L-histidine methyl ester or carnosine were investigated. Particularly, UV spectra (300-400 nm) were recorded on solutions at various pH values, containing each binary system Cu-L; afterwards, an UV absorption spectrum for single complexes was calculated, taking into account the chemical model previously assessed, in order to fulfil a correct spectrum-structure correlation. The problem related to the eventual superimposition of the CT shoulder (approximately 330 nm) to copper(II) of OH- and imidazole pyridine nitrogen groups were now solved by means of a comparison of the UV spectra of dimer species formed by both pi-methis or tau-methis. Finally, copper(II) complex formation with 2,2'-bipyridine was taken into account to compare the behaviour of pyridine (from 2,2'-bipyridine) and pyridine imidazole nitrogens (from pi-methis or tau-methis) with respect to the UV charge transfer process to copper(II) ion.

In this study an analysis of visible spectrophotometric data was carried out using more than 100 copper(II) complexes formed by ligands with different coordination environments, and whose aim is the λmax value estimation for equatorially-coordinated copper(II) complexes in solution. Calculations were performed with a least-squares procedure and a bootstrap-based method for accuracy assessment. The results obtained are in good agreement to previous reports describing the coordination of amino, peptide (negatively-charged), pyridine-imidazole nitrogen and carboxylate oxygen, while it was possible to calculate an individual contribution for hydroxide, alcoholate and water oxygen donors. The spectrum–structure correlation for copper(II) complexes in solution is widely discussed in this paper, and an estimation of the uncertainty in relation to the prediction of λmax of copper(II) complexes has also been made.

The protonation of polymethacrylic acid (W∼230 000) has been studied potentiometrically [(H+)-glass electrode] in different aqueous salt solutions (NaCl, KCl, Et4NI; 0≤I≤0.5 mol dm−3) at 25°C. Measurements have been performed also for polymethacrylic acid (PMA)–CaCl2 mixtures (PMA/Ca2+=0.5–4) at I=0.1 mol dm−3 (NaCl). The potentiometric data have been analyzed first by using the three-parameters model of Hogfeldt for apparent protonation constants, and then differences in different media have been interpreted in terms of complex formation. Formation constants are reported for Na+, K+ and Ca2+ complexes.

The complex formation between copper(II) or nickel(II) and the alifatic dipeptides L-γ-glutamyl-L-ϵ-lysine and L-glutamyl-L-ϵ-lysine has been studied in aqueous solution at 25°C and ionic strength 0.1 mol·dm−3 by means of pH-metric and spectroscopic measurements. In L-γ-glutamyl-L-ϵ-lysine, two α-aminoacidic residues are located at a long distance from each other, and are so far from peptide nitrogen atom that the involvement of dissociated peptide group in coordination is not favored. As a consequence, a small number of complexes is formed, either with copper(II) or nickel(II), and the ML° species predominates over the others. In L-glutamyl-L-ϵ-lysine, the amino group of glutamyl residues is adjacent to the peptide group whose dissociation occurs in the presence of both copper(II) and nickel(II). Either potentiometric or spectroscopic data suggest the formation of a high number of complexes with the absence of a dominant species both for copper(II) and nickel(II): MLH22+, MLH+, M2L2H+, M2L2°, M2L2H−1−, M2L2H−22−, ML2H2°, ML2H−, ML22+, M2L2+, and M2LH−1+ (M=copper(II) or nickel(II)). Since both of the ligands can form homobinuclear complexes with each of the metal ions investigated, mixtures containing both metal ions and one of the peptide ligands have been studied too, but the relevance of heterobinuclear species is not remarkable. The spectroscopic data obtained for single species in solution allowed us to propose the probable structure in solution for several copper(II) complexes.

An attempt has been made to identify the characteristic positions, and their eventual displacements with changing co-ordination, of the charge-transfer bands involving donor groups located on molecules of biological interest [L-malic acid, N-acetyl-L-aspartic acid, (1R,2R)-cyclohexane-1,2-diamine, L-alanyl-L-alanine, L-γ-glutamyl-L-Iµ-lysine, N-acetyl-L-histidine, β-alanyl-L-histidine and N-benzoylglycyl-L-histidyl-L-leucine] such as carboxylate, alcoholate, amine, deprotonated peptide and imidazole. Information about the species formed in solution was obtained by means of pH-metric readings while ultraviolet–circular dichroism spectra were recorded, at fixed pH values, 298 K and I= 0.1 mol dm–3, for the proton–ligand and proton–copper(II)–ligand systems, in order to evaluate a spectrum for each complex formed in solution. Intraligand and charge-transfer bands were assigned for each spectrum with the aim of relating spectral features to the structure of the species formed in solution.

The synthesis, physico-chemical, and biological characterisation of a short series of carnosine amides bearing NO-donor nitrooxy functionalities are described. The NO-donor carnosine analogues and their des-NO derivatives display carnosine-like properties, differing from the lead for their high serum stability. The newly-synthesised compounds are able to complex Cu(2+) ions at physiological pH, displaying significant copper ion sequestering ability, and protect LDL from oxidation catalysed by Cu(2+) ions. All products show moderately-potent HNE quenching activity. The NO-donor compounds 7c-f relaxed rat aorta strips via an NO-dependent mechanism. In vivo evaluation of organ protection in a model of cerebral ischaemia/reperfusion injury, using the selected NO-donor 7e and its des-NO analogue 7a, showed that both derivatives protect from hypoxia-induced brain damage, at lower concentrations than carnosine; 7e also decreased serum TNF-α levels. This class of NO-donor carnosine amides is worthy of further study as potential tools for treating a wide range of chronic vascular and neurodegenerative diseases in which NO-bioavailability is reduced.

The copper(II)— l -carnosine (L − ) system has been re-investigated in aqueous solution, at I = 0.1 mol dm −1 , different temperatures (5⩽ t ⩽45°C) and with metal to ligand ratios ranging from 3:1 to 1:3. Both potentiometry and visible spectrophotometry were employed. From an overall consideration of all experiments, [CuLH] 2+ , [CuL] + , [CuLH −1 ]°, [Cu 2 L 2 H −2 ]° and [Cu 2 LH −1 ] 2+ were recognized as the species which provide the best interpretation of experimental data. The complex formation constants, determined at different temperatures, allowed us to obtain reliable values of Δ H ° and good estimates of Δ C ° p . From visible spectrophotometric measurements, carried out at different pH and metal to ligand ratios, it was possible to calculate the electronic spectrum of each complex formed in solution. A structure is also proposed for each species, on the basis of thermodynamic and spectral results.

The reproducibility of Tessier's extractions and the total content of cadmium, chromium, copper, iron, manganese, lead, zinc and calcium in a river sediment have been evaluated. The metals were determined with AAS in flame and in graphite furnaces. The accuracy of the dissolution procedures was evaluated using a reference material (RM) BCR 145; none of the methods applied proved optimal for all the metals determined. The concentrations of metals extracted by the various reagents were characterized by good reproducibility on species bonded to the carbonates, to iron and manganese oxides and in the residual; precision was lower in the other cases. The sequential extractions also showed a satisfactory mass balance.

Protonation constants of o-phosphate were studied potentiometrically, using the (H+)-glass electrode in aqueous NaCl, KCl and tetraethylammonium iodide solutions, at 0≤I≤IM and 10≤T≤50°C. The differences found in the protonation constants for different salt solutions are explained by a complex formation model. The formation of the species MPO 4 2− , MHPO 4 − , MH2PO 4 0 , M2PO 4 − and M2HPO 4 0 (M=Na+, K+) is hypothesized. In mixed NaCl-KCl solutions, it is possible to find the mixed metal species NaKPO 4 − and NaKHPO 4 0 . Ionic strength and temperature dependence parameters are reported for all species. The relevance of Na+ and K+ complexes is discussed in connection with speciation problems of natural fluids, such as marine water and urine.

The complexes formed in aqueous solution by copper(II) with glycyl-L-histidylglycine (L–) have been extensively studied, by using potentiometry, calorimetry, visible spectrophotometry and circular dichroism techniques, with the aim of elucidating their type and probable structure. Only monomeric species are formed in the acidic range of pH ([CuL]+, [CuLH–1], [CuL2] and [CuL2H–1]–), while at higher pH (>8) polynuclear [Cu4L4H–6]4– is formed in addition to [CuLH–2]–. The interpretation of all the experimental data is strongly consistent with a structure involving three nitrogen atoms co-ordinated to copper(II) for [CuLH–1] and [CuL]+ and with the participation of a fourth nitrogen atom in [CuLH–2]– and [Cu4L4H–8]4–. In the tetrameric species very probably there is a dissociation of the N(1)-pyrrole hydrogen ion, with consequent formation of imidazolate bridges.

The hydrolysis of VO 2+ and the complex with sulfate were studied potentiometrically, spectrophotometrically and calorimetrically, in NaCl aqueous solution (0 I ≤ 1 mol L − 1 ) and at t = 25 °C. The formation of two hydrolytic species VO(OH) + and VO 2 (OH) 2 2+ and one complex with sulfate was found, with log β = − 5.65 for the reaction VO 2+ + H 2 O = VO(OH) + + H + , log β = − 7.02 for the reaction 2VO 2+ + 2H 2 O = (VO) 2 (OH) 2 2+ + 2H + and log K = 1.73 for VOSO 4 0 species (at I = 0.1 mol L − 1 and t = 25 °C). For these species, using calorimetric data, Δ H and T Δ S values were also obtained. By using the above values, interactions of VO 2+ with acetate ( ac ), malonate ( mal ), succinate ( suc ), 1,2,3-propanetricarboxylate ( tca ) and 1,2,3,4-butanetetracarboxylate ( btc ) ligands were studied potentiometrically and spectrophotometrically. The formation of ML + , ML 2 0 and MLOH 0 for ac ; ML 0 , MLH + , ML 2 2− and ML 2 H − for mal ; ML 0 , MLH + and MLOH − for suc ; ML − and MLH 0 for tca and ML 2− , MLH − and MLH 2 0 for btc were found. Formation constants are reported at I = 0.1 mol L − 1 , together with SIT parameters for the dependence on ionic strength. By visible spectrophotometric measurements, λ max and e max values for the relevant species in solution were determined.

A method that uses a Harned cell to perform potentiometric pH measurements has been optimized and applied to an aqueous solution of simulated seawater that contains sodium perchlorate, sodium sulfate, sodium hydrogen carbonate and boric acid and has an ionic strength I of 0.57 mol kg−1. The standard metrological approach developed for the measurement of pH in low ionic strength aqueous solutions was maintained, but a few modifications were necessary, and measurement procedures and calculations were modified ad hoc from those adopted in conventional protocols. When determining the standard potential of the cell, E°, NaClO4 salt was added to a 0.01 mol/kg HCl solution to attain the same ionic strength as the test solution and to investigate possible specific effects related to the high levels and the nature of the background electrolyte. An appropriate value of γ ±HCl (0.737) was then selected from the literature, based on a realistic value for I. Finally, in order to convert the acidity function at zero chloride molality into pH, a suitable value of γ Cl (0.929) was calculated. As a result, we obtained pH = 8.18 (T = 25 °C) with an associated expanded uncertainty U = 0.01 (coverage factor k = 2). The aim was to establish a sound basis for the pH measurement of seawater by identifying the critical points of the experimental and theoretical procedure, and to discuss further possible developments that would be useful for achieving a reference solution.

In this paper we report the results of an investigation performed by potentiometric (H+-glass electrode) and visible spectrophotometric measurements on the interaction of UO2(2+) ion towards some carboxylic ligands (acetate, malonate, succinate, azelate). The measurements were carried out at T= 25 degrees C in different ionic media (KNO3 and NaCl) at different ionic strengths (0.1 < or = I/mol L(-1) < or = 1.0, NaCl; I/mol L(-1) = 0.1, KNO3). The dependence on ionic strength of formation constants was taken into account by using both a simple Debye-Huckel type equation and the SIT (Specific ion Interaction Theory) approach. Different speciation models (depending on concentration of reagents, ionic strength, pH-range) both for different carboxylates and different ionic media have been obtained. Linear combinations between formation constants, stoichiometric coefficients and length of alkyl chain of dicarboxylates have been observed and predicted formation constants at I= 0 mol L(-1) are reported for the interaction of UO2(2+) with HOOC-(CH2)n-COOH with 1 < or = n < or = 7. Finally, a visible absorption spectrum for each complex reaching a significant percentage of formation in solution (KNO3 medium) has been calculated to characterise the compounds found by pH-metric refinement.

Natural sediments show sequestering properties that can lead to a process of self-purification of aquatic environment from metal pollution. The study of the interaction between metal ions and sediment particles enhances what is known about the distribution and bioavailability of heavy metals in natural systems. Our contribution concerns the characterisation of the sequestering ability of a River Po sediment with regard to calcium(II), magnesium(II), cadmium(II), nickel(II) and copper(II), in fixed experimental conditions, through pH-metric and spectrometric measurements. A batch titration procedure was adopted and, in each solution, after equilibration, both pH and pM (M = Ca(II), Mg(II), Cd(II), Ni(II), Cu(II)) (via Inductively Coupled Plasma - Optical Emission Spectroscopy, ICP-OES) values were measured. The experimental data were first processed with a specific software to evaluate the concentration and protonation constants of the sediment ligand site(s). The speciation model was then assessed, together with the values of complexation constants, for the different sediment/metal cation systems. In order to better characterise the copper(II)-sediment interaction and to obtain more information about the nature of ligand site(s) involved, EPR (Electronic Paramagnetic Resonance) measurements were also made on the dry sediment before and after reaction with copper(II) ions.

In this paper copper(II) complex formation in aqueous solution with a series of nucleosides (adenosine or guanosine) or nucleotides 5'-monophosphate is studied by means of potentiometry, visible spectrophotometry and ultraviolet circular dichroism. A chemical model has been formulated for each binary system (at T= 25 degrees C and I = 0.1 M), with particular attention to the interaction in the basic field. A spectrum (both visible absorption and ultraviolet circular dichroism) for each complex with a significant percentage of formation (in the adopted experimental conditions) has been calculated, allowing structural details to be hypothesised. The interaction with deprotonated alcoholic group(s) of the ribose moiety has been found to be fundamental in determining the co-ordination chemistry of each ligand considered while cases of co-operation between the purine and ribose donor(s) were also considered. Confirmations were also obtained by an investigation on the corresponding 2'-deoxy compounds as ligands.

In this study, the structure–absorptivity relationship problem is dealt with, on the basis of a considerable mass of data (117 complexes) concerning equatorial complexes of copper(II) in aqueous solution. Some spectrophotometric measurements have been carried out and as well as some molar absorptivity calculations on copper(II)–ammonia (e640=77 M−1 cm−1 for the Cu(NH3)42+ species) and copper(II)–biuret (e505=46 M−1 cm−1 for the Cu(C2H3N3O2)22− species) systems have been made, in order to complete the quantitative information necessary for the discussion. It was found that the qualitative relationship between emax and λmax, for the complexes in question is traceable back to a hyperbolic type of path; the dispersion of the data in the cartesian plane (emax, λmax) is analyzed and discussed. It has been found that the dissociated peptide nitrogen is, amongst those under study (namely: amino nitrogen; peptide nitrogen; imidazole nitrogen; carboxylate oxygen; alcoholate oxygen; hydroxide oxygen; water oxygen), that which corresponds to the major increase of emax for copper(II). On the contrary, the one which corresponds to the minor increase is the imidazole donor (that is pyridine nitrogen of imidazole residue), which manifests the ability to lower the emax values of the equatorial complexes of copper(II). Moreover, contrary to what has been previously demonstrated for λmax, for emax it does not appear to be correct to schematize the ligand with its donor groups in order to assess spectrum–structure correlations. Consequently, this study contains a type of methodological proposal, even beyond the metal ion specifically in question, for the analyses of the selection criteria of the molecular characteristics that a ligand must possess in order to form a co-ordination compound of analytical interest with a determined metal ion.

Organic and inorganic ligand sites on sediment particles were alkalimetrically titrated using a glass electrode as indicating device. Data obtained for suspensions containing known masses of sediment were used to calculate the concentration of surface ligand sites and their stability constants for complex formation with proton and copper(II) ion. The relationship between the concentration of ligand sites and the concentrations of metals (Cd, Cr, Cu, Fe, Mn, Pb, and Zn) and of C, N, and S was used to try to discriminate between the contributions of organic and inorganic components to the total ligand capacity of the sediment. The reliability of the chemical model deduced from potentiometric data was checked by comparing calculated values for aqueous copper(II) as a function of pH with values experimentally determined via atomic absorption spectrometry. The procedure proposed might contribute to the modeling of sediment-contaminant interaction, providing information on the nature of the ligands involved.

This work reports a potentiometric, calorimetric and spectropolarimetric ultraviolet circular dichroism (UV/CD) study of the interaction of l-malic acid with alkaline metals or (poly)ammonium (methylamine, ethylenediamine, diethylenetriamine, triethylenetetramine, spermine, tetraethylenepentamine and pentaethylenehexamine) cations. Stability data (logK, DeltaG(0)) were obtained potentiometrically for the l-malic acid with (poly)ammonium cations systems; calorimetric measurements (25 degrees C) made it possible to obtain DeltaH(0) and TDeltaS(0) values for the complexes formed in the systems under examination. logK values calculated (for the reaction: H(i)A(i+)+H(j)L((j-z))=ALH(r)((i+j-z)), with r=i+j) range between 0.8 and 4.6, i.e., the interactions are from weak to fairly strong while maximum stability for each system is given by the species with the highest z(anion)xz(cation) (z=charge) value. Enthalpy changes associated with reactions H(n)A(n+)+L(2-)=ALH(n)((n-2)) and H(n)A(n+)+HL(-)=ALH(n+1)((n-1)) are always positive and increase progressively with n. The same is valid for T DeltaS(0) values, which indicate that these species are entropically stabilized, as expected for electrostatic interactions. It was verified that the UV/CD signal depends on both ionic medium and ionic strength value; for comparison, we used the l-malic acid signal recorded in tetramethylammonium chloride as baseline background salt (as in potentiometry). UV/CD spectra were recorded for solutions containing both cationic and anionic species. When the cation was a protonated polyamine, CD spectra calculations were performed for most stable ion pairs: the results show remarkable differences in Deltaepsilon (dm(3) mol(-1) cm(-1)) values at 205 nm (which is the l-malate UV/CD lambda(max)) between the chiral ligand and its complex with a polyamine.

Potentiometric measurements in different media and at different temperatures were performed in order to determine protonation constants of glyphosate and formation constants for Ca2+ and Mg2+ complexes, together with their dependence on ionic strength and ▵H0 values. Calorimetric measurements were also performed to determine ▵H0 of protonation. Potentiometric measurements in mixed Ca2+ and Mg2+ solution allowed us to determine the formation of a mixed metal species. Thermodynamic parameters reported in this work can be used for studying the complete speciation of glp in electrolyte solutions containing the major ionic components of natural fluids.

The application of potentiometric titrations to the study of the acid-base properties of a river sediment was investigated. A set of alkalimetric titrations were performed on suspensions containing different sediment-solution ratios and EMF measurements were elaborated by a suitable computer program. After the optimisation of the titration procedure, it was possible to refine conditional acid constants. For the considered sediment it was possible to calculate three protonation constants. Comparing calculated pKa with those reported in literature for the characterisation of humic and fulvic material of various origins and for inorganic hydrous oxides, it appears that calculated values are in the range indicated for the assignment of the constants to organic and inorganic titratable groups that can be present on the surface of a natural sediment.

The hydrolysis of iron(III) was studied potentiometrically at different ionic strengths in KNO(3) aqueous solutions, at 25 degrees C, to determine the dependence of hydrolysis constants on ionic strength (nitrate media), to check the existence of nitrate-ferric ion interactions, and to confirm the formation of high polymeric species. Under the experimental conditions 0.03 I (KNO(3)) 1M, 0.3 C 12 mM, the species Fe(OH)(2+), Fe(2)(OH)(4+)(2), Fe(OH)(+)(2) and Fe(12)(OH)(2+)(34) were found, and the hydrolysis constants log beta(11) = 2.20, log beta(12) = -2.91, log beta(22) = -5.7, log beta(12,34) = -48.9 (I = 0M) were calculated. The ionic strength dependence of hydrolysis constants is quite close to that found for several protonation and metal complex formation constants reported elsewhere.

The complex formation between copper(II) and the tetrapeptide glycyl-glycyl- l -histidyl- l -alanine ([H 3 L] 2+ ) has been studied in aqueous solution at t = 25°C and I = 0.1 mol dm −3 by potentiometric, visible spectrophotometric and circular dichroism measurements. All the experimental techniques show that the complex [CuLH −2 ] − is predominant over a wide pH range, while the monodentate [CuLH] 2+ is formed in the acidic region and a further deprotonated [CuLH −3 ] 2− species exists at pH higher than 10. The formation constants of the three above complexes are reported and a structure is proposed on the basis of spectroscopic results. The structure of [CuLH −2 ] − species very probably involves four nitrogen donors in the plane, excluding coordination by the l -alanine residue, while it seems likely that [CuLH −3 ] 2− is formed by simple dissociation of the N-1 pyrrole hydrogen.

1. The serum oxalate concentration rises in chronic renal failure and it is only partially eliminated by regular dialysis treatment. However, the recent literature is not conclusive on whether progressive oxalate retention and secondary oxalosis should be expected in patients on regular dialysis treatment. 2. To further investigate this, we have estimated the state of saturation with respect to calcium oxalate mono-hydrate in plasma ultrafiltrates from 28 patients on maintenance haemodialysis and eight healthy control subjects, matched for sex and age. Five patients had type I primary hyperoxaluria and histologically proven oxalosis, whereas 23 had oxalosis-unrelated renal diseases. Dialysis efficiency was quantified as the KdTd/V of urea. Samples were obtained from each patient before, immediately after and 48 h after a dialysis session. Fasting samples were obtained from the control subjects. Oxalate was determined in both plasma ultrafiltrates and the whole dialysate by ion-exchange chromatography, after a non-delayed and [14C]oxalate-recovery-controlled procedure. The state of saturation with calcium oxalate monohydrate was estimated by means of a computer system which solved the interactions among 45 complex species. 3. The fasting plasma oxalate concentration (means ± sd) in ultrafiltrates from healthy subjects was 3.8 ± 1.5 (range 1.4–5.8) μmol/l, and the state of saturation with calcium oxalate monohydrate was 0.096 ± 0.04. The 23 patients with oxalosis-unrelated chronic renal failure had a pre-dialysis plasma oxalate concentration of 49.8 ± 14.0 μmol/l, which yielded a state of saturation with calcium oxalate monohydrate of 1.0 ± 0.3; dialysis produced a steep decrease in the state of saturation with calcium oxalate monohydrate and the samples remained undersaturated at 48 h from the end of the session. The pre-dialysis oxalate concentration in plasma ultrafiltrates from patients with primary oxalosis was 170 ± 21 μmol/l, so as to exceed saturation by more than threefold. The samples remained above saturation even at the end of dialysis. 4. In 116 ultrafiltrates the state of saturation with calcium oxalate monohydrate was closely dependent on the plasma oxalate concentration but not on the plasma calcium concentration. The pre-dialysis state of saturation with calcium oxalate monohydrate was poorly influenced by the residual renal function and was independent of the sex, age, body weight or time on dialysis. 5. The results of the present study suggest that, unless chronic renal failure is due to primary hyperoxaluria, the extracellular body fluid of patients on current dialysis is often undersaturated with calcium oxalate and this is inconsistent with progressive oxalate accumulation and secondary systemic oxalosis. However, since some tissues may generate or selectively accumulate oxalate, local formation of crystalline deposits may still occur on regular haemodialysis.

The formation and stability of Li + , Na + and K + complexes with oxalate, malonate, succinate, maleate, DL-malate and phthalate were studied potentiometrically at various ionic strengths. From the data thus obtained, as well as from several literature data on the protonation of the above-mentioned ligands in various ionic media and at various temperatures, the dependence of Na + and K + complex formation on temperature was determined. The dependence on ionic strength, both for the protonation and the complex formation, is also discussed.