Binding Ability of Sodium Catechol Disulfonate (Tiron) toward Hg2+

Article pubs.acs.org/jced

Binding Ability of Sodium Catechol Disulfonate (Tiron) toward Hg2+, CH3Hg+, (CH3)3Sn+, and (CH3)2Sn2+ Cations Gabriella Falcone, Claudia Foti,* and Silvio Sammartano Dipartimento di Scienze Chimiche, Università di Messina, Viale F. Stagno d’Alcontres 31, I-98166, Messina (Vill. S. Agata), Italy ABSTRACT: A potentiometric study on the interactions between catechol disulfonate (disodium 4,5-dihydroxybenzene-1,3-disulfonate, tiron) and Hg2+, CH3Hg+, (CH3)2Sn2+, and (CH3)3Sn+ cations is reported in aqueous NaCl solution with ionic strength in the range (0 ≤ I ≤ 1) mol·L−1 and at a temperature of 298.15 K. In the presence of tiron, CH3Hg+ and (CH3)3Sn+ form only the two species ML and MLH; Hg2+ forms ML, MLH, and ML(OH) species; and (CH3)2Sn2+ forms ML, MLH, and ML2 species. The interactions of tiron with Hg2+ and (CH3)2Sn2+ are stronger than those with CH3Hg+ and (CH3)3Sn+. As an example, at the ionic strength I = 0.1 mol·L−1, equilibrium constants for the formation of ML species are log β = 18.87, 17.076, 10.39, and 7.55, for M = Hg2+, (CH3)2Sn2+, CH3Hg+, and (CH3)3Sn+, respectively. On the basis of the speciation models proposed, the sequestering ability of tiron toward Hg2+, CH3Hg+, (CH3)2Sn2+, and (CH3)3Sn+ was quantitatively evaluated by determining the concentration of the ligand able to complex half of the metal ion fraction. In the range 5 ≤ pH ≤ 8, the sequestering ability is high toward Hg2+ and (CH3)2Sn2+ (at the (10−11 to 10−6) mol·L−1 level), fairly low toward CH3Hg+ (at the 10−3 mol·L−1 level), and very low toward (CH3)3Sn+ (at the 10−1 mol·L−1 level).

■

radicals.10 In addition, it is a powerful nontoxic chelator of several metals and therefore can be potentially used in chelation therapy. The “chelation therapy” is based on the use of ligands as drugs to treat disorders resulting from the presence of unwanted metal ions arising from intoxication or disease. The chelating agents are compounds able to bind the toxic metal and are therefore useful as antidotes for metal intoxications. Sometimes, these compounds can prevent the toxicity of the metals binding to cellular target molecules. The ability as a chelating agent of tiron has been evaluated toward beryllium,11 vanadium,12 uranium,13 and various others metals.14−17 Knowledge of the interactions with metal cations can give useful information on the possibility to use tiron as a chelating agent to remove cations from biological and environmental systems. Despite this, few studies are reported on its complexing ability. A study on the interactions between tiron and some toxic metals is here reported. We took into account the most important species of mercury(II), Hg2+, and CH3Hg+ and two organic species of tin(IV), (CH 3) 2Sn2+, and (CH3)3Sn+. Among triorgano tin(IV) compounds, considered to be the most toxic compounds of tin(IV), we chose the (CH3)3Sn+ because of the higher solubility and considering that, in general, the behavior of this class of compounds does not depend, or depends only slightly, on the nature of the organic groups.18 The metal−ligand interactions were studied by potentiometry, in aqueous NaCl solution, by varying the

INTRODUCTION Molecules containing catechol functional groups represent a class of compounds of remarkable biological importance. This functional group is present in the siderophores, compounds produced by microbes as a result of an iron deficiency and engaged in the cellular transport of iron.1 The siderophores might be also involved in the uptake of molybdenum in nitrogen-fixing bacteria.2 The ability of catechol ligands to chelate metal ions has been used in different therapies. A major class of these compounds is represented by the catecholamines, which act as neurotransmitters3 and are used in many clinical treatments such as those of Parkinson’s disease,4 hypertension,5 and breast cancer.6 A natural siderophore, the desferrioxamine B, is the only drug actually available for the treatment of aluminum intoxication.7 Many catechol ligands containing a wide variety of substituents have been extensively studied for their strong affinity toward metal ions with high oxidation state (Fe(III), uranium(IV), thorium(IV), ....). In particular, sodium catechol disulfonate, also called tiron (Chart 1), is water-soluble and cellpermeable8,9 and is an antioxidant able to scavenge a variety of Chart 1. Structural Formula of Sodium Catechol Disulfonate or Disodium 4,5-Dihydroxybenzene-1,3-Disulfonate (Tiron)

Received: August 1, 2012 Accepted: October 24, 2012 Published: November 5, 2012 © 2012 American Chemical Society

3636

dx.doi.org/10.1021/je300898d | J. Chem. Eng. Data 2012, 57, 3636−3643

Journal of Chemical & Engineering Data

Article

Table 1. Equilibrium Constants for the Formation of the Hydrolytic and the Chloride Species of Hg2+, CH3Hg+, (CH3)2Sn2+, and (CH3)3Sn+, in NaCl at Different Ionic Strengths and at T = 298.15 K log β I (mol·L−1) reaction

0.1

0.25

0.5

1

ref

Hg2+ + H2O = Hg(OH)+ + H+ Hg2+ + 2H2O = Hg(OH)20 + 2H+ Hg2+ + 3H2O = Hg(OH)3− + 3H+ 2Hg2+ + H2O = Hg2(OH)3+ + H+ Hg2+ + Cl− = HgCl+ Hg2+ + 2Cl− = HgCl20 Hg2+ + 3Cl− = HgCl3− Hg2+ + 4Cl− = HgCl42− Hg2+ + Cl− + H2O = HgCl(OH)0 + H+ CH3Hg+ + H2O = CH3Hg(OH)0 + H+ CH3Hg+ + Cl− = CH3HgCl0 (CH3)2Sn2+ + H2O = (CH3)2Sn(OH)+ + H+ (CH3)2Sn2+ + 2H2O = (CH3)2Sn(OH)20 + 2H+ (CH3)2Sn2+ + 3H2O = (CH3)2Sn(OH)3− + 3H+ 2(CH3)2Sn2+ + 2H2O = [(CH3)2Sn]2(OH)22+ + 2H+ 2(CH3)2Sn2+ + 3H2O = [(CH3)2Sn]3(OH)3+ + 3H+ (CH3)2Sn2+ + Cl− = (CH3)2SnCl+ (CH3)2Sn2+ + 2 Cl− = (CH3)2SnCl20 (CH3)2Sn2+ + Cl− + H2O = (CH3)2SnCl(OH)0 + H+ (CH3)2Sn2+ + Cl− + 2H2O = (CH3)2SnCl(OH)2− + 2H+ (CH3)3Sn+ + H2O = (CH3)3Sn(OH)0 + H+ (CH3)3Sn+ + Cl− = (CH3)3SnCl0

−3.60 −6.34 −21.10 −3.58 6.82 13.36 14.44 15.06 3.68 −4.538 5.25 −3.05 −8.36 −19.4 −5.2 −9.4 0.57 0.55 −2.95 −9.0 −6.19 −0.6

−3.63 −6.36 −21.19 −3.10 6.77 13.30 14.36 15.02 3.64 −4.556 5.19 −3.11 −8.43 −19.4 −5.2 −9.5 0.51 0.46 −3.02 −9.1 −6.17 −0.6

−3.62 −6.33 −21.29 −3.09 6.78 13.31 14.35 15.08 3.68 −4.582 5.16 −3.15 −8.49 −19.5 −5.2 −9.6 0.51 0.46 −3.03 −9.1 −6.22 −0.6

−3.56 −6.20 −21.48 −3.145 6.92 13.50 14.50 15.32 3.88 −4.639 5.13 −3.20 −8.58 −19.6 −5.2 −9.7 0.55 0.53 −3.0 −9.1 −6.28 −0.6

36

ionic strength in the range (0 ≤ I ≤ 1) mol·L−1 and at a temperature of 298.15 K. The ability of tiron to sequester cations was quantitatively evaluated by calculating an empirical parameter (pL0.5) that numerically represents the ligand concentration able to sequester 0.5 of the metal ion fraction.18 The pL0.5 values were evaluated for all the systems, at different pH and ionic strength. The values were compared with those reported in previous papers, concerning the sequestering ability of carboxylates, amines, and thiolates.19−30

34 35

33

equipped with a combined Orion glass electrode Ross type 8102 and a Metrohm 713 potentiometer connected to a Metrohm 665 motorized buret and a Ross type 8102SC (from Orion) combinated glass electrode. The estimated reproducibility for both systems was ± 0.15 mV and ± 0.003 mL for the electromotive force (emf) and titrant volume readings, respectively. The titrations were automatically performed connecting the potentiometric systems to a PC and using a suitable computer program to control titrant delivery, to check for emf stability, and to record data. The measurement cells were thermostatted at (298.15 ± 0.1) K. Independent experiments were performed at least three times. Procedure. To determine the ligand protonation constants, 25 mL of the solution containing tiron at different concentrations [(1 ≤ CL ≤ 5) mmol·L−1] and NaCl to obtain the pre-established ionic strength values [(0 < I ≤ 1) mol·L−1] was titrated with the standard NaOH solution. For the investigation of the metal−ligand interactions, the solutions were prepared by dissolving different amounts of tiron [(0.5 ≤ CL ≤ 4) mmol·L−1] and metal or organometal cation [(0.5 ≤ CM ≤ 1) mmol·L−1] to obtain a concentration CM/CL ratio ranging from 0.25 to 1. To the solutions were added hydrochloric acid to have the fully protonated form of the ligand and NaCl to obtain the predetermined ionic strength value [(0 < I ≤ 1) mol·L−1]. A volume of 25 mL of each solution was titrated with standard sodium hydroxide in the pH range 2.5 to 11. Back titrations were performed to verify the presence of real equilibria in solution. To determine the formal electrode potential, for each measurement independent titrations of hydrochloric acid with a standard solution of NaOH were performed, keeping the same conditions of temperature and ionic strength as the systems under study. The free hydrogen ion concentration scale was used (pH =

■

EXPERIMENTAL SECTION Materials and Methods. Mercury(II), monomethylmercury(II), dimethyltin(IV), and trimethyltin(IV) cations were used in the form of chloride salt and were supplied by Aldrich, Riedel-de-Haen, or Alfa-Aesar companies. Disodium salt of 4,5-dihydroxybenzene 1,3-disulfonate (tiron) was supplied by Fluka. The purity of the ligand was controlled by potenziometry and resulted in > 99.5 %. It was therefore used without further purification. The sodium chloride salt (Aldrich, puriss.), previously dried at 110 °C, was used to prepare the corresponding solution by weighing. The concentrated ampules of hydrochloric acid and sodium hydroxide (Fluka) were used to prepare the corresponding diluted solutions. The solutions of hydrochloric acid were standardized against sodium carbonate and those of sodium hydroxide against potassium hydrogen phthalate. All solutions were protected from atmospheric CO2 using soda lime traps. Grade A glassware and twice-distilled water were employed in the preparation of all solutions. Potentiometric Equipment. The potentiometric titrations were performed independently by two operators using different reagents and two systems to minimize systematic errors. The two systems used were: a 809 Metrohm Titrando apparatus 3637



Article

−log[H+]). Purified N2(g) was bubbled into the solution to remove CO2 and O2. The solutions were magnetically stirred. Calculations. Several computer programs were used. The refinement of the parameters of the acid−base titrations (such as the analytical concentration of the reagent and E0) and the calculation of the formation constants of the complexes were carried out by BSTAC and STACO.31 To calculate the formation percentage of each species and to draw the speciation diagrams, ES4ECI31 was used. The fitting of the linear and nonlinear equations for the ionic strength dependence of the formation constants was performed by LIANA. 31 The concentration and the formation constants are reported in the molar concentration scale (mol·L−1); the conversion to the molal scale (mol·kg−1) can be simply done by32

Hg2+−Tiron Interactions. Inorganic mercury(II) hydrolyzes, and in dilute solutions the neutral mononuclear Hg(OH)20 and small amounts of Hg(OH)+, Hg(OH)3−, and Hg2(OH)3+ species can be found. The equilibrium constants for the formation of Hg2+−OH− and Hg2+−Cl− species36 are reported in Table 1, in NaCl at different ionic strengths. Taking into account these species, the potentiometric measurements on Hg2+−tiron solutions evidenced the formations of HgL2−, HgLH−, and HgL(OH)3− species. The formation constant values, in aqueous NaCl solution at four ionic strength values in the range (0 ≤ I ≤ 1) mol·L−1, are reported in Table 3. As can be observed, the increase of the ionic strength determines a significant variation in the stability of the HgLH− species, with the formation constant which varies from log β = 26.54 (at I = 0.1 mol·L−1) to log β = 29.14 (at I = 1 mol·L−1). This may be due to the high concentration of the chloride ion which gives an extra stability to the species, with probably formation of ion pairs. The distribution diagram is shown in Figure 1. At I = 0.1 mol·L−1 (full lines), most of the metal fraction is present as HgL(OH)3− species at pH > 9, achieving the maximum of 0.9 at pH = 10. The HgL2− and HgLH− species reach the maximum of 0.5 and of 0.2 of the metal fraction, respectively. At higher ionic strength values, i.e., at I = 1 mol·L−1 (Figure 1, dotted lines), the metal fraction present as Hg2+−tiron complexes significantly decreases, with a maximum of 0.5 for HgL2− species at pH = 10. This is probably due to the high chloride concentration and, therefore, to the increase of metal fraction present as chloride species. CH 3 Hg + −Tiron Interactions. In aqueous solution, monomethylmercury(II) forms the main species CH3Hg(OH)0 and, in the presence of chloride ion, the CH3HgCl0 species.34 Both formation equilibria (Table 1) were taken into account in the study on the CH 3 Hg + −tiron (L 4− ) system. The methylmercury cation weakly interacts with tiron, with the formation of two complex species CH 3 HgL 3− and CH3HgLH2−. The formation constants determined at different NaCl concentrations are reported in Table 3. The distribution diagram shown in Figure 2 evidences the negligible formation of the protonated CH3HgLH2− species and the significant formation of the CH3HgL3− species at pH > 9, with ∼0.5 of metal fraction present under this form at pH = 10. The metal fraction of the organometallic cation complexed by tiron decreases at I = 1 mol·L−1. In both cases, the binding ability of tiron toward CH3Hg+ is not very strong, and, under the experimental conditions reported, the formation of the CH3HgCl0 and CH3HgOH0 species prevails. (CH3)2Sn2+−Tiron Interactions. The dimethyltin(IV) cation strongly hydrolyzes over all the pH range considered, with the formation of mono (CH3)2Sn(OH)+, (CH3)2Sn(OH)20, and (CH3)2Sn(OH)3− and dinuclear [(CH3)2Sn]2(OH)22+ and [(CH3)2Sn]2(OH)3+ species.35 The equilibrium constants for the formation of (CH3)2Sn2+−OH− and −Cl− species are reported in Table 1. The potentiometric measurements on (CH3)2Sn2+−tiron mixtures evidenced the formation of three complex species, (CH 3 ) 2 SnL 2− , (CH3)2SnL26−, and (CH3)2SnL(OH)3−, whose formation constants in NaCl at different ionic strengths are reported in Table 3. The stability of species is high with, as an example, log β = 17.08 for the (CH3)2SnL2− complex at I = 0.1 mol·L−1. The strong binding ability of tiron toward this cation totally suppresses the hydrolysis, with a high percentage of formation of complex species over all the pH range. As can be observed in Figure 3, at pH < 7 the formation of (CH3)2SnL2− species

c /mol ·L−1 = d0/g·cm−3 + a(c /mol ·L−1) m /mol ·kg −1 + b(c /mol ·L−1)2

(1)

where d0 is the density of pure water and a and b are empirical parameters (in NaCl: a = 0.017765 and b = −0.0006525, at T = 298.15 K).

■

RESULTS AND DISCUSSION Thermodynamic Parameters for the Tiron Protonation and the Hg2+, CH3Hg+, (CH3)2Sn2+, and (CH3)3Sn+ Hydrolysis. Calculations of the metal−ligand interactions need the knowledge of the thermodynamic parameters regarding the ligand protonation, the metal hydrolysis, and the interaction with the ionic medium, being equal to the ionic strength conditions. As concerns the hydrolysis of Hg2+, CH3Hg+, (CH3)2Sn2+, and (CH3)3Sn+, the literature values of equilibrium constants reported in Table 1 were used.33−36 In the same table are also reported the formation constant values of metal−chloride complexes. Both mercury(II) and methylmercury(II) form stable complexes with Cl−, while dimethyltin(IV) and trimethyltin(IV) form weak chloride ion pairs. To render homogeneous the set of formation data for all the systems studied here, we have taken into account these species.33,35,36 Although many data are reported in the literature on tiron protonation,37−39 protonation constants of this ligand were determined by potentiometry ionic strength conditions being equal to those that will be used for the study of the metal− ligand interactions, i.e., in aqueous NaCl solutions at I = (0.1, 0.25, 0.5, and 1) mol·L−1. The experimental protonation constants of tiron are reported in Table 2. Our data are in agreement with literature values, even if determined in other ionic media. As an example, values of 11.70 ≤ log K1 ≤ 12.55 at I = 0.1 mol·L−1 or log K1 = 11.83 at I = 1 mol·L−1 (NaClO4) are reported.37−39 Table 2. Protonation Constants of Tiron (L4−) in NaCl at Different Ionic Strength Values and T = 298.15 K I

a

mol·L−1

log K1

log β2

0.1 0.25 0.5 1.0

12.18(5)a 11.98(3) 11.74(3) 11.62(3)

19.66(4)a 19.35(3) 18.99(3) 18.77(3)

± 95 % C.I. 3638



Article

Table 3. Equilibrium Constants for the Formation of the Hg2+−, CH3Hg+−, (CH3)2Sn2+−, and (CH3)3Sn+−Tiron (L4−) Species, In NaCl at Different Ionic Strengths and at T = 298.15 K log β I (mol·L−1)

a

reaction

0.1

0.25

0.5

1.0

Hg2+ + L4− = HgL2− Hg2+ + L4− + H+ = HgLH− Hg2+ + L4− + H2O = HgL(OH)3− + H+ CH3Hg+ + L4‑ = (CH3Hg)L3− CH3Hg+ + L4− + H+ = (CH3Hg)LH2− (CH3)2Sn2+ + L4− = (CH3)2SnL2− (CH3)2Sn2+ + 2L4− = (CH3)2SnL26− (CH3)2Sn2+ + L4− + H2O = (CH3)2SnL(OH)3− + H+ (CH3)3Sn+ + L4− = (CH3)3SnL3− (CH3)3Sn+ + L4− + H+ = (CH3)3SnLH2−

18.87(1)a 26.54(5) 9.95(2) 10.39(5) 18.06(3) 17.076(1) 23.78(2) 9.820(2) 7.55(5) 15.31(3)

18.98(1)a 26.95(4) 9.64(2) 10.04(1) 18.05(2) 16.724(1) 23.56(2) 9.586(2) 7.40(9) 14.84(5)

19.14(1)a 27.59(3) 9.39(1) 9.74(1) 17.84(6) 16.416(2) 23.35(2) 9.326(3) 7.27(3) 14.47(2)

19.95(2)a 29.14(3) 9.53(3) 9.50(1) 17.98(4) 16.315(2) 23.29(1) 9.243(3) 7.15(2) 14.14(3)

± 95 % C.I.

Figure 1. Speciation diagram for the Hg2+−tiron (L4‑) system in NaCl at I = 0.1 mol·L−1 (full lines) and at I = 1 mol·L−1 (dotted lines). Experimental conditions: CM = 1 mmol·L−1; CL = 2 mmol·L−1, T = 298.15 K.

Figure 3. Speciation diagram for the (CH3)2Sn2+−tiron (L4‑) system in NaCl at I = 0.1 mol·L−1 (full lines) and at I = 1 mol·L−1 (dotted lines). Experimental conditions: CM = 1 mmol·L−1; CL = 2 mmol·L−1, T = 298.15 K.

negligible. The distribution of the species at I = 1 mol·L−1 is similar, with an increase of (CH3)2SnL26− species and a decrease of (CH3)2SnLOH3−, in the range 8 ≤ pH ≤ 11. (CH3)3Sn+−Tiron Interactions. In aqueous NaCl solution, the trimetyltin(IV) cation forms the hydrolytic (CH3)3Sn(OH)0 and the chloride (CH3)3SnCl0 species33 (see Table 1). In the presence of tiron, the formation of (CH3)3SnL3− and (CH3)3SnLH2− was observed, with formation constant values markedly lower than those of (CH3)2Sn2+. As an example, for ML species, we obtain log β (I = 0.1 mol·L−1) = 7.55 and 17.08 for (CH3)3Sn+ and (CH3)2Sn2+, respectively. If we consider the distribution of the species in NaCl (Figure 4), (CH3)3SnL3− and (CH3)3SnLH− reach at maximum ∼0.1 of the metal fraction at pH > 9 and pH = 7, respectively, and the formation of hydrolytic (CH3)3Sn(OH)0 species prevails in a wide pH range (pH > 6). At pH < 6, owing to the high concentration of the chloride ion (CCl = (0.1 or 1) mol·L−1), most of the metal cation is present as (CH3)3SnCl0. Dependence on Ionic Strength. Formation constants determined in NaCl in the ionic strength range (0.1 ≤ I ≤ 1) mol·L−1 were analyzed using the Debye−Hückel-type equation40

Figure 2. Speciation diagram for the CH3Hg+−tiron (L4‑) system in NaCl at I = 0.1 mol·L−1 (full lines) and at I = 1 mol·L−1 (dotted lines). Experimental conditions: CM = 1 mmol·L−1, CL = 2 mmol·L−1, T = 298.15 K.

prevails, with more than 0.9 of the metal fraction under this form at pH 5 and I = 0.1 mol·L−1. At pH > 8, ∼0.9 of the metal fraction is present as mixed hydrolytic (CH3)2SnL(OH)3− species, while the formation of (CH3)2SnL26− species is 3639



Article

CH3HgL3− species. Less significant are the effects of the ionic strength variation on (CH3)2Sn2+− and (CH3)3Sn+−tiron systems, and increasing the ionic strength from I = 0.1 mol·L−1 to I = 1 mol·L−1, the increase of (CH3)2SnL2− and the decrease of (CH3)2SnL(OH)3− formation can be observed. Sequestering Ability. The sequestering ability of tiron toward Hg2+, CH3Hg+, (CH3)2Sn2+, and (CH3)3Sn+ can be quantitatively defined on the basis of the formation constants and speciation profiles of the different metal−tiron systems. This information is of great importance to evaluate the possibility of using tiron as a detoxifying agent in biomedical applications or to remove toxic metal ions from natural systems. However, in real conditions different factors influence the formation yields of the species: the hydrolysis of the metal, the protonation of the ligand, and the interactions with other components. Therefore, the simple analysis of single sets of stability constants of metal/ligand complexes is not always sufficient to assess the global binding ability of a ligand toward a given cation. This problem can be overcome by the determination of the total fraction of metal complexed (x) as a function of pL (pL = −log [L]tot, with [L]tot = total ligand concentration).18 This function is a typically sigmoidal curve, characterized by a rapid rise even after a small change in concentration, and can be described by a Boltzmann-type equation (with asymptotes of 1 for pL→ ∞ and 0 for pL→ 0)

Figure 4. Speciation diagram for the (CH3)3Sn+−tiron (L4−) system in NaCl at I = 0.1 mol·L−1 (full lines) and at I = 1 mol·L−1 (dotted lines). Experimental conditions: CM = 1 mmol·L−1, CL = 2 mmol·L−1, T = 298.15 K.

log β = log β 0 − 0.51z*

I + CI 1 + 1.5 I

(1)

where z* = Σ (charges)2reactants − Σ (charges)2products; β is the formation constant; β0 is the formation constant at infinite dilution; and C is an empirical parameter. Values of the β0, i.e., formation constants extrapolated at I = 0 mol·L−1, and the empirical C parameter are reported in Table 4. The empirical parameter C depends on the charges involved in the formation reaction, and some ligand classes showed constant value of C/ z*. Considering all the species reported in Table 4, the value C/ z* = 0.10 ± 0.04 was obtained, which can be considered a fairly good predictive value for other ligands of this class. To evaluate the ionic strength effect on the distribution of the species, speciation diagrams are shown in the Figures 1 to 4 at I = 0.1 mol·L−1 and at I = 1 mol·L−1. The effect of the ionic strength variation on the distribution of the species is different for each metal−ligand system. For the Hg2+−tiron system, increasing the ionic strength from (0.1 to 1) mol·L−1, the formation of the complex species is shifted to higher pH values. As an example, HgL2− species reach the maximum of formation at pH = 8.5 (I = 0.1 mol·L−1) and at pH = 10 (I = 1 mol·L−1). For the CH3Hg+−tiron system, the shift of the distribution curves is accompanied by a decrease of metal fraction for

x=

1 1 + 10(pL − pL0.5)

(2)

pL0.5 is an empirical parameter that numerically represents the concentration of ligand able to sequester a fraction of metal ion of 0.5. Therefore, the pL0.5 quantitatively describes the sequestering ability of a ligand toward a metal ion: the higher the pL0.5, the higher the sequestering ability is. Calculated pL0.5 values of tiron toward Hg2+, CH3Hg+, (CH3)2Sn2+, and (CH3)3Sn+ are reported in Table 5, under different experimental conditions, i.e., at pH = 5 and 8 and at I = (0.1 and 1) mol·L−1. As expected, the sequestering ability of tiron is high toward dicharged cations Hg2+ and (CH3)2Sn2+, while it is fairly low toward CH3Hg+ and very low toward (CH3)3Sn+. A comparison among different systems is shown in Figure 5, where the sum of fraction of Hg2+−, CH3Hg+−, (CH3)2Sn2+−, and (CH3)3Sn+−tiron species is reported vs pL being equal to the experimental conditions, i.e., at pH = 8 and I = 0.1 mol·L−1 chosen as an example. Very different pL0.5 values

Table 4. Equilibrium Constants for the Formation of the Hg2+−, CH3Hg+−, (CH3)2Sn2+−, and (CH3)3Sn+−Tiron (L4−) Species at Infinite Dilution and C Parameters for the Dependence on Ionic Strength in NaCl, at T = 298.15 K

a

reaction

log β0

C

H+ + L4− = HL3− 2H+ + L4− = H2L2− Hg2+ + L4− = HgL2− Hg2+ + L4− + H+ = HgLH− Hg2+ + L4− + H2O = HgL(OH)3− + H+ CH3Hg+ + L4− = (CH3Hg)L3− CH3Hg+ + L4− + H+ = (CH3Hg)LH2− (CH3)2Sn2+ + L4− = (CH3)2SnL2− (CH3)2Sn2+ + 2L4− = (CH3)2SnL26− (CH3)2Sn2+ + L4− + H2O = (CH3)2SnL(OH)3− + H+ (CH3)3Sn+ + L4− = (CH3)3SnL3− (CH3)3Sn+ + L4− + H+ = (CH3)3SnLH2−

13.05(4)a 21.22(4) 20.23(1) 27.95(5) 10.76(2) 11.23(6) 19.58(7) 18.81(6) 23.71(7) 10.90(5) 8.45(3) 16.83(2)

0.20(4)a 0.42(7) 4.41(3) 7.07(6) 1.36(4) −0.1(1) 1.3(1) 0.79(9) −0.5(2) 0.38(7) 0.35(6) 0.17(3)

± 95 % C.I. 3640



Article

Table 5. Values of pL0.5 for Hg2+−, CH3Hg+−, (CH3)2Sn2+−, and (CH3)3Sn+−Tiron (L4−) Systems in NaCl at Different Ionic Strength and pH Values and T = 298.15 K I M

z+

Hg2+ CH3Hg+ (CH3)2Sn2+ (CH3)3Sn+

mol·L

varying the experimental conditions we have strong variations in the total fraction of metal complexed, resulting in significant differences in the sequestering ability, with pL0.5 values that vary from pL0.5 = 5.12 (at pH = 8 and I = 0.1 mol·L−1) to 7.69 (at pH = 8 and I = 1 mol·L−1), to 8.22 (at pH = 5 and I = 0.1 mol·L−1), and to 11.49 (at pH = 5 and I = 1 mol·L−1). Apparently, the high sequestering capacity of tiron toward Hg2+ at pH = 5 is in contradiction with the speciation diagram shown in Figure 1. In reality, one must consider the different experimental conditions: speciation diagram of Figure 1 refers to millimolar metal concentration (CHg = 1 mmol·L−1), while the sequestering ability is calculated considering the metal trace. Under these conditions, at pH = 5, more than 0.9 of the metal fraction is present as HgLH− species, resulting in the high pL0.5 value. The influence of pH and ionic strength on the sequestering ability of tiron toward the other metals is much lower but not negligible with pL0.5 values in the range 5.36 ≤ pL0.5 ≤ 6.90 for (CH3)2Sn2+, 2.44 ≤ pL0.5 ≤ 3.07 for CH3Hg+, and 0.35 ≤ pL0.5 ≤ 1.82 for (CH3)3Sn+. The sequestering ability of tiron can be compared with that of other ligand classes. As an example, pL0.5 values of some Odonor, N-donor, and S-donor ligands are reported in Table 6.

pL0.5 −1

0.1 1 0.1 1 0.1 1 0.1 1

pH = 5

pH = 8

8.22 11.49 2.69 2.44 5.36 6.16 0.62 0.35

5.12 7.69 2.76 3.07 6.03 6.90 1.57 1.82

Table 6. Sequestering Ability of Different Ligands towards Hg2+, CH3Hg+, and (CH3)2Sn2+, at I = 0.1 mol·L−1, pH = 5, and T = 298.15 K pL0.5 L

Figure 5. Sum of fractions of Hg −, CH3Hg −, (CH3)2Sn −, and (CH3)3Sn+−tiron species vs pL in NaCl at pH = 8, I = 0.1 mol·L−1, and T = 298.15 K. Curves: 1, (CH3)2Sn2+; 2, Hg2+; 3, CH3Hg+; 4, (CH3)3Sn+. 2+

+

2+

tiron succinic acid malonic acid mellitic acid ethylendiamine spermine glycine thiomalic acid cysteine glutathione EDTA

were obtained for each system, following the trend: (CH3)2Sn2+ > Hg2+ ≫ CH3Hg+ > (CH3)3Sn+. Under the other conditions of pH and ionic strength, the sequestering ability of tiron toward Hg2+ is always higher than that toward the other metals, and the pL0.5 values follow the trend: Hg2+ > (CH3)2Sn2+ ≫ CH3Hg+ > (CH3)3Sn+. Figure 6 shows the effect on the sequestering ability of the change in the pH and ionic strength values, for the Hg2+−tiron system. As can be observed, by

a

CH3Hg+

(CH3)2Sn2+

ref

8.22 5.58

2.69

5.36

11.00 5.94

3.83a

this work 29 18 29,30 18,29 18,30 18,29 18,26 28,30 28,30 30

Hg

2+

1.16

2.97 5.76 26.01 24.32c 22.89c

a

11.32a 11.13a 10.79a 5.72

2.10b 2.76b 1.31b 5.47 4.1d

pH = 4. bpH = 6.5. cI = 0.25 mol·L−1; dI = 0 mol·L−1.

The sequestering ability of tiron toward Hg2+ is higher compared to that of succinic acid, ethylenediamine, and glycine, i.e., O- and N-donor ligands, with differences of about 2 orders of magnitude in the pL0.5 values. The highest sequestering capacity toward Hg2+ is that of S-donor ligands, with pL0.5 = 26.01, 24.32, and 22.89 for thiomalic acid, cysteine, and glutathione, respectively. Slightly different is the trend for CH3Hg+, with comparable values of pL0.5 for tiron, mellitic acid, and spermine and very high values of pL0.5 for S-donor ligands. Toward (CH3)2Sn2+, tiron shows a sequestering ability comparable only to that of thiomalic acid and higher than that of other ligands.

■

CONCLUSION This paper reports a thermodynamic study on the interactions between tiron and four metals of environmental importance: Hg2+, CH3Hg+, (CH3)2Sn2+, and (CH3)3Sn+. The findings may be summarized in the following:

Figure 6. Sum of fractions of the Hg2+−tiron (L4‑) species vs pL in NaCl, at T = 298.15 K. 1, I = 0.1 mol·L−1, pH = 8; 2, I = 1 mol·L−1, pH = 8; 3, I = 0.1 mol·L−1, pH = 5; 4, I = 1 mol·L−1, pH = 5. 3641



■

Article

− The speciation models of metal−tiron systems are relatively different with the formation of ML and MLH species for Hg2+, CH3Hg+, and (CH3)3Sn+, ML2 for (CH3)2Sn2+, and ML(OH) for Hg2+ and (CH3)2Sn2+. − Formation constant values were determined in aqueous NaCl solution at different ionic strength values [(0.1 ≤ I ≤ 1) mol·L−1] and at a temperature of 298.15 K. Tiron strongly interacts with Hg2+ and (CH3)2Sn2+, and most of the metal fraction is present as a tiron complex in a wide pH range. Significantly lower is the complexing ability of tiron toward CH3Hg+ and (CH3)3Sn+, and the formation of the hydrolytic MOH species prevails in a wide pH range. − The sequestering ability, evaluated under different conditions of pH and ionic strength, is high toward Hg2+ and (CH3)2Sn2+, fairly low toward CH3Hg+, and very low toward (CH3)3Sn+. The influence of pH and ionic strength on the sequestering ability is strong for Hg2+, while it is much lower, but not negligible, for CH3Hg+, (CH3)2Sn2+, and (CH3)3Sn+. − Data here reported can give useful information on the possibility to use tiron as a chelating agent to remove Hg2+, CH3Hg+, (CH3)2Sn2+, and (CH3)3Sn+ from biological and environmental systems. − The knowledge of these interactions is essential when making studies of speciation, considering that it is important to know not only the binding capacity of the metal to be removed but also against other metals present that can potentially interfere.

(12) Domingo, J. L.; Bosque, M. A.; Luna, M.; Corbella, J. Prevention by Tiron (sodium 4,5-dihydroxybenzene-1,3-disulfonate) of vanadateinduced developmental toxicity in mice. Teratology 1993, 48, 133− 138. (13) Bosque, M. A.; Domingo, J. L.; Llobet, J. M.; Corbella, J. Effectiveness of sodium 4,5-dihydroxybenzene- 1,3-disulfonate (Tiron) in protecting against uranium-induced developmental toxicity in mice. Toxicology 1993, 79, 149−156. (14) Avdeef, A.; Sofen, S. R.; Bregante, T. L.; Raymond, K. N. Coordination Chemistry of Microbial Iron Transport Compounds. 9. Stability Constants for Catechol Models of Enterobactin. J. Am. Chem. Soc. 1978, 100, 5362−5370. (15) Desroches, S.; Biron, F.; Berthon, G. Aluminum speciation studies in biological fluids Part 5. A quantitative investigation of Al (III) complex equilibria with desferrioxamine, 2,3-dihydroxybenzoic acid, Tiron, CP20 (Ll), and CP94 under physiological conditions, and computer-aided assessment of the aluminum-mobilizing capacities of these ligands in vivo. J. Inorg. Biochem 1999, 75, 27−35. (16) Farkas, E.; Enyedy, E. A.; Micera, G.; Garribba, E. Coordination modes of hydroxamic acids in copper(II), nickel(II) and zinc(II) mixed-ligand complexes in aqueous solution. Polyhedron 2000, 19, 1727−1736. (17) Sharma, P.; Mishra, K. P. Aluminum-induced maternal and developmental toxicity and oxidative stress in rat brain: Response to combined administration of Tiron and glutathione. Reprod. Toxicol. 2006, 21, 313−321. (18) Gianguzza, A.; Giuffrè, O.; Piazzese, D.; Sammartano, S. Aqueous solution chemistry of alkyltin(IV) compounds for speciation studies in biological fluids and natural waters. Coord. Chem. Rev. 2012, 256, 222−239. (19) De Stefano, C.; Foti, C.; Gianguzza, A. Interactions of Alkyltin(IV) Compounds with Ligands of Interest in the Speciation of Natural Fluids: Complexes of (CH3)3Sn+ with Carboxylates. Ann. Chim. (Rome) 1999, 89, 147−155. (20) Fiore, T.; Foti, C.; Gianguzza, A.; Orecchio, S.; Pellerito, L. Dglucuronate complexes of mono-, di- and triorgano tin(IV) compounds. Potentiometric and Mössbauer spectroscopic investigations. Appl. Organomet. Chem. 2002, 16 (6), 294−301. (21) Cardiano, P.; Giuffrè, O.; Pellerito, L.; Pettignano, A.; Sammartano, S.; Scopelliti, M. Thermodynamic and spectroscopic study of the binding of dimethyltin(IV) by citrate at 25°C. Appl. Organomet. Chem. 2006, 20, 425−435. (22) Cardiano, P.; De Stefano, C.; Giuffrè, O.; Sammartano, S. Thermodynamic and spectroscopic study for the interaction of dimethyltin(IV) with L-cysteine in aqueous solution. Biophys. Chem. 2008, 133, 19−27. (23) Bretti, C.; Giacalone, A.; Gianguzza, A.; Sammartano, S. Speciation of dimethyltin(IV)- and trimethyltin(IV)- carbocysteinate and -glutamate systems in aqueous media. Chem. Speciation Bioavailability 2008, 20 (3), 137−148. (24) Casale, A.; De Stefano, C.; Manfredi, G.; Milea, D.; Sammartano, S., Sequestration of alkyltin(IV) compounds in aqueous solution: formation, stability and empirical relationships for the binding of dimethyltin(IV) cation by N- and O-donor ligands. Bioinorg. Chem. Appl. 2009, 2009, ID 219818. (25) Cardiano, P.; Giuffrè, O.; Napoli, A.; Sammartano, S. Potentiometric, 1H-NMR, ESI-MS investigation on dimethyltin(IV) cation-mercaptocarboxylate interaction in aqueous solution. New J. Chem. 2009, 33, 2286−2295. (26) Cardiano, P.; Cucinotta, D.; Foti, C.; Giuffrè, O.; Sammartano, S. Potentiometric, calorimetric and 1H-NMR investigation on Hg2+mercaptocarboxylate interaction in aqueous solution. J. Chem. Eng. Data 2011, 56, 1995−2004. (27) Cardiano, P.; Falcone, G.; Foti, C.; Giuffrè, O.; Sammartano, S. Methylmercury(II)-sulphur containing ligand interactions: a potentiometric, calorimetric and 1H-NMR study in aqueous solution. New J. Chem. 2011, 35 (4), 800−806.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Tait, G. H. The identification and biosynthesis of siderochromes formed by Micrococcus denitrificans. Biochem. J. 1975, 146, 191−204. (2) Duhme, A. K. Synthesis, characterisation, and solution behaviour of two dioxomolybdenum(VI) complexes of a bis(catecholamide) siderophore analoque. J. Chem. Soc., Dalton Trans. 1997, 773−778. (3) Shyer, L. Biochemistry; Freeman, W.H.: San Francisco, 1975. (4) Cotzias, G. C.; Van Woert, M. H.; Schiffer, L. M. Aromatic amino acids and modification of parkinsonism. N. Engl. J. Med. 1967, 276, 374−379. (5) Fermaglich, J.; Chase, T. N. Methyldopa or methyldopahydrazine as levodopa synergists. Lancet 1973, 1, 1261−1262. (6) Stoll, B. A. Hypothesis: Breast Cancer Regression under Oestrogen Therapy. Lancet 1973, 1, 431−432. (7) Hider, R. C.; Hall, A. D. In Perspectives in Bioinorganic Chemistry; Hay, R. W., Dilworth, J. R., Nolan, K. B., Eds.; JAI Press: London, 1991; Vol. 1, pp 209−253. (8) Jones, M. M. New developments in therapeutic chelating agents as antidotes for metal poisoning. Crit. Rev. Toxicol. 1991, 21, 209−233. (9) Domingo, J. L. The use of chelating agents in the treatment of aluminum overload. Clin. Toxicol. 1989, 27, 355−367. (10) Bors, W.; Saran, M.; Michel, C. Pulse-radiolytic investigations of catechols and catecholamines. II. Reactions of Tiron with oxygen radical species. Biochim. Biophys. Acta 1979, 582, 217−223. (11) Sharma, P.; Shah, Z. A.; Shukla, S. Protective effect of Tiron (4,5-dihydroxybenzene-1,3-disulphonic acid disodium salt) against beryllium-induced maternal and fetal toxicity in rats. Arch. Toxicol. 2002, 76, 442−448. 3642



Article

(28) Cardiano, P.; Falcone, G.; Foti, C.; Sammartano, S. Sequestration of Hg2+ by some biologically important thiols. J. Chem. Eng. Data 2011, 56, 4741−4750. (29) Foti, C.; Giuffrè, O.; Lando, G.; Sammartano, S. Interaction of Inorganic Mercury(II) with Polyamines, Polycarboxylates, and Amino Acids. J. Chem. Eng. Data 2009, 54, 893−903. (30) Falcone, G.; Foti, C.; Gianguzza, A.; Giuffrè, O.; Napoli, A.; Pettignano, A.; Piazzese, D. Sequestering ability of some chelating agents towards methylmercury(II). Anal. Bioanal. Chem. 2013, DOI: 10.1007/s00216-012-6336-5. (31) De Stefano, C.; Sammartano, S.; Mineo, P.; Rigano, C. Computer Tools for the Speciation of Natural Fluids. In Marine Chemistry - An Environmental Analytical Chemistry Approach; Gianguzza, A., Pelizzetti, E., Sammartano, S., Eds.; Kluwer Academic Publishers: Amsterdam, 1997; pp 71−83. (32) De Stefano, C.; Foti, C.; Sammartano, S.; Gianguzza, A.; Rigano, C. Equilibrium Studies in Natural Fluids. Use of Synthetic Seawater and Other Media as Background Salts. Ann. Chim. (Rome) 1994, 84, 159−175. (33) Cannizzaro, V.; Foti, C.; Gianguzza, A.; Marrone, F. Hydrolysis of Trimethyltin(IV) Cation in NaNO3 and NaCl Aqueous Media at Different Temperatures and Ionic Strengths. Ann. Chim. (Rome) 1998, 88, 45−54. (34) De Robertis, A.; Foti, C.; Patanè, G.; Sammartano, S. Hydrolysis of (CH3)Hg+ in Different Ionic Media: Salt Effects and Complex Formation. J. Chem. Eng. Data 1998, 43, 957−960. (35) De Stefano, C.; Foti, C.; Gianguzza, A.; Martino, M.; Pellerito, L.; Sammartano, S. Hydrolysis of (CH3)2Sn2+ in Different Ionic Media: Salt Effects and Complex Formation. J. Chem. Eng. Data 1996, 41, 511−515. (36) Powell, K. J.; Brown, P. L.; Byrne, R. H.; Gajda, T.; Hefter, G.; Sjoberg, S.; Wanner, H. Chemical speciation of environmentally significant heavy metal with inorganic ligands. Part 1: the Hg2+-Cl−, OH−, CO32‑, SO42‑, and PO43‑ aqueous systems. Pure Appl. Chem. 2005, 77 (4), 739−800. (37) Martell, A. E.; Smith, R. M.; Motekaitis, R. J. Critically Selected Stability Constants of Metal Complexes; National Institute of Standard and Technology, NIST, PC-based Database: Gaithersburg, 2004. (38) Pettit, L.; Powell, K. J. The IUPAC Stability Constants Database; Academic Software: U.K., 2001. (39) May, P. M.; Murray, K. Database of Chemical Reactions Designed To Achieve Thermodynamic Consistency Automatically. J. Chem. Eng. Data 2001, 46, 1035−1040. (40) Bretti, C.; De Stefano, C.; Foti, C.; Sammartano, S. Critical evaluation of protonation constants. Literature analysis and experimental potentiometric and calorimetric data for the thermodynamics of phthalate protonation in different ionic media. J. Solution Chem. 2006, 35 (9), 1227−1244.

3643


Binding Ability of Sodium Catechol Disulfonate (Tiron) toward Hg2+

Recommend Documents