Ottavia Giuffrè

Thermodynamic parameters for the binding of inorganic and organic anions by biogenic polyammonium cations

Thermodynamic parameters for the interaction of protonated biogenic polyamines with inorganic or organic polyanions were studied potentiometrically (H(+)-glass electrode) and calorimetrically, at 25 degrees C. No background salt was used in the measurements to avoid interferences, and the formation constants and formation enthalpies were extrapolated to zero ionic strength. Species formed are ALH(r) [L=Cl(-), SO(4)(2-), HPO(4)(2-), P(2)O(7)(4-) and P(3)O(10)(5-); tartrate, malate, citrate, glutamate, 1,2,3-propanetricarboxylate, 1,2,3,4-butanetetracarboxylate], with r=1,2...(n+m-2) and r=1,2...(n+m-1) for inorganic and organic ligands, respectively (n, m=maximum degree of protonation of amine and ligand, respectively). The stability of the various species formed is a function of charges involved in the formation reaction. DeltaH(0) values are generally positive, and therefore these complexes are entropically stabilized. Results are discussed in connection with several previously reported data on similar systems. DeltaG(0) and TDeltaS(0) follow a linear trend as a function of polyammonium cation and inorganic or carboxylic anion charges. DeltaG(0) and TDeltaS(0) charge relationships are reported. In particular, mean values of DeltaG(0) and TDeltaS(0) for single interaction were calculated: DeltaG(0)=7.0 kJ mol(-1) n(-1), TDeltaS(0)=9.1 kJ mol(-1) n(-1) and DeltaG(0)=5.7 kJ mol(-1) n(-1) and TDeltaS(0)=8.7 kJ mol(-1) n(-1), for the species of inorganic and organic polyanions, respectively (n=number of possible salt bridges). A linear relationship was also found for TDeltaS(0) versus DeltaG(0), whose equation is TDeltaS(0)=-7-1.39 DeltaG(0) (with r=0.9409; r, correlation coefficient). The body of correlations found for these thermodynamic parameters shows quite good predictive value.

Thermodynamic data for Pb2+ and Zn2+ sequestration by biologically important S-donor ligands, at different temperatures and ionic strengths

Thermodynamic parameters for the interactions of cysteine (cys) and penicillamine (psh) with Pb(II) and glutathione (gsh) with Pb(II) and Zn(II) were determined in NaNO3 or NaCl aqueous solution by potentiometry, at different ionic strengths (0 < I ≤ 1 mol L−1) and temperatures (15 ≤ t ≤ 45 °C). For Pb2+ systems, the formation of PbL, PbLH, PbLH2 and PbL2 species was found, together with PbLOH for cys and psh, and PbL2H and PbL2H2 for gsh. The speciation models for Pb2+–psh and –gsh systems were confirmed by UV spectrophotometric measurements. For the Zn2+–gsh system, a more complex speciation model was obtained with the formation of ZnL, ZnLH, ZnLH2, ZnL2, ZnL2H, ZnL2H2, ZnLOH and ZnL2OH species. From the dependence of formation constants on temperature, rough ΔH values were evaluated: the main contribution to the complexation free energy is entropic in nature, with small enthalpic values. Moreover, from the dependence on ionic strength, formation constants extrapolated at I = 0 mol L−1 were obtained. The sequestering ability of the ligands towards Pb2+ and Zn2+ was evaluated by determining the pL0.5, i.e. the −log of the concentration of the ligand able to complex half of the metal ion fraction. All these ligands show good sequestering ability. For Pb2+–cys, –psh and –gsh systems, pL0.5 reaches the value of 8.2, 9.0 and 5.9, respectively, at pH = 7, I = 0.1 mol L−1 and t = 25 °C. Under the same experimental conditions, the sequestering ability of gsh towards Zn2+ is lower, with pL0.5 = 4.1.

Binding of carboxylic ligands by protonated amines

The formation and stability of some mixed proton complexes ApLqHr(A = triethylenetetramine, tetraethylenepentamine or pentaethylenehexamine; L = acetate, malonate, citrate, propane-1,2,3-tricarboxylate or butane-1,2,3,4-tetracarboxylate), was studied in aqueous solution at T= 25 °C. It was been found, for all the systems, the formation of different species with p= 1, q= 1…3 and r= 1…(n+m– 1)(n and m are the maximum degrees of protonation for the two ligands). In addition, also the following species were found: for A = tetraethylenepentamine and L = malonate, AL2Hr(r= 4,5,6); for A = pentaethylenehexamine and L = acetate, AL2Hr(r= 5,6) and AL3H6; for A = pentaethylenehexamine and L = malonate, AL2H7. The stability of these species is generally quite high and follows the trends triethylenetetramine < tetraethylenepentamine < pentaethylenehexamine, and acetate < malonate < citrate ≈ propane-1,2,3-tricarboxylate < butane-1,2,3,4-tetracarboxylate. Some proton–amine–carboxylic ligand complexes, previously reported, follow a similar trend. The charges of the ligands involved in the complexation reactions play a fundamental role in the stability of the species, and some linear relationships are reported.

Sequestering ability of some chelating agents towards methylmercury(II)

AbstractA study on the interactions between CH3Hg+ and some S, N and O donor ligands (2-mercaptopropanoic acid (thiolactic acid (H2TLA)), 3-mercaptopropanoic acid (H2MPA), 2-mercaptosuccinic acid (thiomalic acid (H3TMA)), d,l-penicillamine (H2PSH), l-cysteine (H2CYS), glutathione (H3GSH), N,N′-bis(3-aminopropyl)-1-4-diaminobutane (spermine (SPER)), 1,2,3,4,5,6-benzenehexacarboxylic acid (mellitic acid (H6MLT)) and ethylenediaminetetraacetic acid (H4EDTA)) is reported. The speciation models in aqueous solution and the possible structures of the complexes formed are discussed on the basis of potentiometric, calorimetric, UV spectrophotometric and electrospray mass spectrometric results. For the CH3Hg+–S donor ligand systems, the formation of ML1–z and MLH2–z complex species is observed, together with a diprotonated MLH23–z species for CYS2−, PSH2− and GSH3− and the mixed hydrolytic one ML(OH)−z for TLA2− and MPA2−. The dependence of the stability on ionic strength and on temperature is also analysed. In the other CH3Hg+-L systems (L = MLT6−, SPER and EDTA4−), ML1–z, MLH2–z and MLH23–z complex species are formed, together with the MLH34–z species for SPER, the mixed hydrolytic ML(OH)–z one for SPER and EDTA, and the M2L2–z for EDTA only. On the basis of the speciation models proposed, the sequestering ability of the ligands towards methylmercury(II) cation is evaluated. All S donor ligands show a good sequestering power (at 10−11 mol L−1 level, in the pH range 4 to 8) following the trend MPA2− < PSH2− < GSH3− < TLA2− < CYS2− < TMA3−, while significantly lower is the sequestering ability of MLT, SPER and EDTA (at 10−3–10−5 mol L−1 level, in the pH range 4 to 8). FigureSum of fractions of CH3Hg+-Lz– species (L = S, O and N donor ligands vs. pL

Speciation of phytate ion in aqueous solution. Non covalent interactions with biogenic polyamines

Abstract The noncovalent interactions of phytate (Phy6-) with biogenic amines were studied potentiometrically in aqueous solution, at t= 25°C. Several species are formed in the different H+-Phy6--amine (A) systems, which have the general formula Ap(Phy)Hq(12-q)-, with p ≤ 3 and 6 ≤ q ≤ 10. The stability of these species is strictly dependent on the charges involved in the formation equilibria. For the equilibrium pHiAi+ + Hj(Phy)(12-j)- = Ap(Phy)Hq(12q)-, (q = pi + j)we found the relationship logK= aζ (ζ is the charge product of reactants), where a= 0.35(0.03, valid for all the amines; this roughly indicates an average free energy contribution per bond -ΔG0 = 4.0 ± 0.2 kJ mol-1. A slightly more sophisticated equation is also proposed for predicting the stability of these species. Owing to the quite high (partially protonated) phytate charge, the stability of Ap(Phy)Hq(12-q)- species is quite high, making phytate a strong amine sequestering agent in a wide pH range.

Binding ability of glutathione towards alkyltin(IV) compounds in aqueous solution.

The interactions between glutathione (H3GSH) and (CH3)2Sn(2+), (CH3)3Sn(+), (C2H5)3Sn(+) and (C3H7)3Sn(+) cations were studied by potentiometry, UV spectrophotometry, (1)H NMR spectroscopy and electrospray mass spectrometry (ESI MS). Potentiometric measurements evidenced speciation models very similar for all the alkyltin(IV) cation-GSH(3-) systems, with the formation of M(GSH)(z-3), M(GSH)H(z-2) and M(GSH)H2(z-1) species [M(z+)=(CH3)2Sn(2+), (CH3)3Sn(+), (C2H5)3Sn(+), or (C3H7)3Sn(+)] and, for dimethyltin(IV) cation, also the mixed hydrolytic M(GSH)(OH)(2-) one. The equilibrium behavior in NaCl aqueous solution, at different ionic strengths and temperatures, is discussed. The stability of the species for the dimethyl and trialkyl cations is quite different. As an example, for M(GSH)(z-3), logβ=13.33, 6.55, 6.44 and 5.84, for (CH3)2Sn(2+), (C3H7)3Sn(+), (C2H5)3Sn(+) and (CH3)3Sn(+), respectively (at I=0.1M and t=25°C). The speciation models and the possible structures of the complexes formed are discussed on the basis of spectroscopic and spectrometric results. The sequestering ability of glutathione towards alkyltin(IV) cations was evaluated by determining the concentration of the ligand able to complex half of the metal ion fraction (pL0.5). Glutathione shows a fairly good sequestering ability towards alkyltin(IV) cations, in 10(-3)-10(-5)M range.

Potentiometric, 1H NMR and ESI-MS investigation on dimethyltin(IV) cation–mercaptocarboxylate interaction in aqueous solution

The interaction between dimethyltin(IV) cation with three different mercaptocarboxylic acids (thiolactic, 3-mercaptopropanoic and thiomalic acids) was studied in aqueous solution by potentiometry, 1H NMR spectroscopy and electrospray (ESI) mass spectrometry. The speciation model and equilibrium data were determined on the basis of potentiometric evidences, as well as the bonding sites by means of ESI mass spectrometry and the geometry of the species by 1H NMR spectroscopy. The speciation profiles show the formation of different complex species, whose stabilities and formation percentages, in particular for the mononuclear mixed species, are high in a wide pH range. ESI mass spectrometry studies together with 1H NMR investigations fully confirm the speciation model. The former also allows to propose a cyclic structure with O→Sn and S→Sn coordination, confirmed by MS/MS analysis; the latter indicates angles mostly matching with a Tbp arrangement around the metal.

Thermodynamic parameters for the binding of ATP by protonated open-chain polyamines

The thermodynamic parameters of formation of ATP–protonated polyamine (ethylenediamine, 1,3-diaminopropane, putrescine, cadaverine, diethylenetriamine, spermidine, triethylenetetramine, spermine, tetraethylenepentamine and pentaethylenehexamine) complexes at 25 °C have been studied by potentiometry (H+-glass electrode and direct calorimetry). The formation constants of species for two, as yet untested, systems are also included. The thermodynamic parameters were obtained for all the systems studied from log K and ΔH0 values. It was found that ATP-protonated amine complexes are stabilized entropically. Empirical charge–stability relationships are shown

Methylmercury(II)-sulfur containing ligand interactions: a potentiometric, calorimetric and 1H-NMR study in aqueous solution

The interactions between CH3Hg+ and four sulfur containing ligands [2-mercaptopropanoic or thiolactic acid (tla), 3-mercaptopropanoic acid (mpa), 2-mercaptosuccinic or thiomalic acid (tma) and L-cysteine (cys)] are studied in aqueous solution by potentiometry, calorimetry and 1H-NMR spectroscopy. Equilibria are studied at T = 25 °C and I = 0.1 mol L−1 using NaNO3 as an ionic medium in the presence, for potentiometric measurements, of iodide (NaI) as a competitive ligand. Speciation models obtained for the different systems are quite similar, with the formation of ML1−z and MLH2−z species for all systems, and M(tla)(OH)2−, M(mpa)(OH)2− and M(cys)H2+ species. Formation constant values show a very high stability (for ML1−z species, log β ranges from 16.57 to 17.90). For the CH3Hg+-tla and -cys systems, 1H-NMR spectroscopy fully supported the chemical model proposed, providing fairly similar formation constant values to those obtained by potentiometry. As expected for typically soft–soft interactions, the enthalpy values are strongly exothermic, and the contribution to the complexation free energy is mainly of enthalpic origin. Speciation diagrams show that, in a wide pH range, most of the metal fraction is present as complex species.

QUANTITATIVE STUDY OF THE INTERACTIONS OF ATP WITH AMINES AND AMINO ACIDS

Interactions of some amines (ethylenediamine, putrescine, cadaverine, histamine, diethylenetriamine, spermidine, spermine, triethylenetetramine and pentaethylenehexamine) and amino acids (lysine and histidine) with ATP have been studied potentiometrically, at 25 °C. Several species A(ATP)H(q–4)q[A = amine or amino acid, q= 1 …n+ 2; (n= number of aminogroups)] were found. For diamine– and diethylenetriamine–ATP systems, the species A2(ATP)H4 are also formed. The stability of these mixed species is strictly dependent on the charges involved in the formation reaction, and empirical stability–charges relationships are reported. Effects of ionic medium on formation constants are discussed, together with the importance of interferences.