Thermodynamic and spectroscopic study of Al3+ interaction with glycine, l -cysteine and tranexamic acid in aqueous solution

Biophysical Chemistry 230 (2017) 10–19

Contents lists available at ScienceDirect

Biophysical Chemistry journal homepage: www.elsevier.com/locate/biophyschem

Thermodynamic and spectroscopic study of Al3+ interaction with glycine, L-cysteine and tranexamic acid in aqueous solution

MARK

Paola Cardiano, Fausta Giacobello, Ottavia Giuffrè⁎, Silvio Sammartano Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche ed Ambientali, Università di Messina, Viale F. Stagno d'Alcontres 31, 98166 Messina, Italy

H I G H L I G H T S

G RA P H I C A L AB S T R A C T

of Al -amino acids interaction • inStudy NaCl at different ionic strengths. ΔH values of Al -amino acids species • determined by titration calorimetry. and speciation models con• Stability firmed by spectrophotometric and H 3+

3+

1

• •

NMR titrations. Empirical equation for the individual contribution estimate of complex stability of carboxylate, amino and thiolate groups. Sequestering ability of different amino acids towards Al3+ at different pH values.

A R T I C L E I N F O

A B S T R A C T

Keywords: Al3+ complexes Amino acids Speciation in aqueous solution Thermodynamic parameters

In this paper a thermodynamic and spectroscopic study on the interaction between Al3+ and glycine (Gly), Lcysteine (Cys), tranexamic acid (Tranex) is reported. Speciation models have been obtained by processing potentiometric titration data to determine stability constants of the species formed in aqueous solution at T = 298.15 K, 0.15 ≤ I/mol L− 1 ≤ 1 in NaCl. Thermodynamic formation parameters have been obtained from calorimetric titration data, at T = 298.15 K, I = 0.15 mol L− 1 using NaCl as ionic medium. Al3+-Cys system was also investigated by spectrophotometric and 1H NMR measurements. 1H NMR experiments were performed on Al3+-Tranex system as well. Different speciation models have been observed for the three systems. The results showed the formation of MLH, ML and M2L2(OH)2 species for Gly, ML, M2L and MLOH for Cys, MLH and MLOH for Tranex. The formed species are quite stable, i.e. for ML, logβ = 7.18, 11.91 for Gly and Cys, respectively, at I = 0.15 mol L− 1 and T = 298.15 K. For all the systems the dependence of formation constants on ionic strength over the range 0.1–1 mol L− 1 is reported. The sequestering ability of the ligands under study was also evaluated by pL0.5 empiric parameter. For Gly, Cys and Tranex, pL0.5 = 2.51, 3.74, 3.91 respectively, at pH = 5, I = 0.15 mol L− 1 and T = 298.15 K.

1. Introduction Aluminium is the most abundant metal and the third constituent of the earth's crust [1]. It is present ubiquitously in the environment in salts and oxides forms [2]. Its abundance is due to the large numbers of

⁎

Corresponding author. E-mail address: ogiuff[email protected] (O. Giuffrè).

http://dx.doi.org/10.1016/j.bpc.2017.08.001 Received 19 July 2017; Received in revised form 7 August 2017; Accepted 7 August 2017 Available online 08 August 2017 0301-4622/ © 2017 Elsevier B.V. All rights reserved.

aluminium-containing minerals and precious stones [1]. Because of its reactivity, all of the earth's aluminium has combined with other elements, such as oxygen, silicon and fluorine to form a variety of compounds. Aluminium production has grown constantly, as well as its applications related to modern lifestyle. This metal is widely used in

Biophysical Chemistry 230 (2017) 10–19

P. Cardiano et al.

titration data to determine formation constants of the species in NaCl aqueous solution at 0.15 ≤ I/mol L− 1 ≤ 1 and T = 298.15 K. Thermodynamic formation parameters have been obtained from calorimetric titration data, at I = 0.15 mol L− 1 in NaCl and T = 298.15 K. Moreover, Al3+-Cys system has been also investigated by spectrophotometric and 1H NMR measurements. 1H NMR experiments has been performed to investigate the Al3+-Tranex interaction as well. For all the systems the dependence of formation constants on ionic strength over the range 0.1–1 mol L− 1 is reported.

medicine, pharmacy, food technology and cosmetics. It is also widespread in the environment due to effects connected with human activities and natural phenomena as well, such as acid rain [3]. Aluminium is not an essential element in biological processes, so that its cationic form is known to be toxic for plants and animals. Aluminium compounds are poorly absorbed from the gastrointestinal tract and derma (under 1%) and by inhalation (up to 3%), but excessive intake caused by over-medication can be an issue [4]. Kidney eliminates all the absorbed aluminium under ordinary health conditions [4]. Its toxicity is known in patients having chronic renal failure and it is proved that aluminium overload causes neurological, blood and skeletal effects [4,5]. Aluminium can be absorbed from the gastrointestinal tract by paracellular passive diffusion in chemical conditions favourable to its solubilization and charge neutralization [6]. These processes significantly depend on complex species formed by Al3+ with endogenous and exogenous ligands present in the most important biofluids [6]. In order to evaluate the aluminium toxicity in humans, the identification and the quantification of the chemical species formed in the body are of fundamental importance [4,6,7]. The risk evaluation is directly related to the assessment of aluminium bioavailability. Since aluminium was classified as a “hard-sphere” metal ion, it shows a good affinity towards O-containing ligands, such as carboxylates, phosphate, phenolate, catecholate [2,7–11]. In the last decades, significant attention has been paid to the aqueous speciation of Al3+, especially for its tendency to form quite stable species with a variety of ligands [7,12–16]. As underlined by several researchers (Parker, Daydè et al., Hagvall et al.), solution studies and, in particular, the knowledge of the aluminium speciation, is a key factor to predict and to explain its toxicity, bioavailability and to model the geochemical behavior [2,17,18]. The interest of the study towards Al3+ interaction with amino acids is twofold: (i) biomolecules containing these potential binding sites may be directly involved in the uptake, transport, physiological and biological action of Al3+; (ii) these small biomolecules can be considered as model compounds for the study of metal ion binding ability of larger biomolecules, such as proteins, in biological fluids. Essential amino acids are present in the chyme, as product of vegetal and animal protein digestion and also as constituents of many industrial foods and drinks [13,17]. Amino acids can be considered potential ligands for aluminium metabolism, especially in the gastrointestinal tract where they can reach appreciable levels [13,17,19]. The most common essential amino acids are glycine-like amino acids. Among amino acids, cysteine, largely widespread in many food and biological fluids, also plays a crucial role in many life processes. One of the moieties of the antioxidant glutathione is cysteine, which is the only naturally occurring sulfur containing amino acid. A living organism, under normal physiologic conditions, may synthesize cysteine from the essential amino acid methionine. Cysteine and its derivatives are involved in many important biological processes and represent an active site both in several peptides and proteins and in the catalytic function of the enzymes cysteine proteases [20]. Besides the two amino acids glycine and L-cysteine, the ability of tranexamic acid (Trans‑4-aminomethylcyclohexane carboxylic acid, i.e. a derivate of lysine) to interact with Al3+ has been investigated as well. It is largely used for its hemostatic, antiplasminic, anti-infiammatory, antiallergic activities [21]. This compound has been included in the WHO (World Health Organization) list of essential medicines [22]. There are relatively few literature data on the interaction of Al3+ with amino acids [7,11–13,17,19]. In this paper a thermodynamic and spectroscopic study on the interaction between Al3+ and glycine, Lcysteine and tranexamic acid (Scheme 1) is reported. Since the chemistry of Al3+ in aqueous solution is characterized by its tendency to hydrolyze to form mono- and polynuclear hydroxide species [23,24], the pH range investigated experimentally is significantly limited by the formation of sparingly soluble species, although the formation of complex species increases aluminium solubility and transport [25]. Speciation models have been obtained by processing potentiometric

2. Materials and methods 2.1. Materials The aluminium solution has been prepared by weighing the salt AlCl3·6H2O (Sigma-Aldrich product, purity ≥ 99%). Its concentration has been checked by a back-titration with a standard EDTA (ethylenediaminetetraacetic acid) solution, using a standard CuSO4 solution as titrant and Eriochrome Black T, as indicator. Ligands solutions have been obtained from Aldrich commercial products without any further purification. Their purity (always > 99.5%) has been verified by alkalymetrical titration. Hydrochloric acid and sodium hydroxide solutions, prepared from concentrated Fluka ampoules, have been standardised using sodium carbonate and potassium biphtalate previously dried at 383.15 K in an oven. NaOH has been stored in dark bottles and preserved from CO2 through soda lime traps. Sodium chloride solutions have been prepared by weighing the pure salt (Fluka, puriss.), pre-dried in an oven at 383.15 K. Ultrapure water (conductivity < 0.1 μS cm− 1) has been used for the preparation of each solution. 2.2. Potentiometric equipment and procedure Potentiometric measurements have been performed by an automatic titration system consisting of a Metrohm model 809 coupled with Metrohm 800 dispenser and equipped with a Metrohm 750 combined glass electrode. This device has been connected to a PC and the titrations have been carried out using the Metrohm TIAMO 2.2 software to control titrant delivery, data acquisition and e.m.f. stability. The estimated accuracy of the potentiometric system was ± 0.15 mV for e.m.f. and ± 0.002 mL for titrant volume readings. A volume of 25 mL of the solution containing the ligand under study at CL = 6–12 mmol L− 1, the aluminium cation at CM = 2–6 mmol L− 1 and metal/ligand ratios (0.25 ≤ CM/CL ≤ 1), together with NaCl as supporting electrolyte and HCl in the same condition of the ligand concentration has been titrated with standard NaOH until precipitation occurred (pH ∼ 5). All measurements have been performed into glass jacket cells thermostated at T = 298.15 ± 0.1 K under magnetic stirring and bubbling pure N2 through the solutions to avoid O2 and CO2 inside. For each experiment, independent titrations of HCl with standard NaOH have been carried out to get the standard electrode potential E0 and the pKw values in the same experimental conditions of temperature and ionic strength. 2.3. UV–vis equipment and procedure Spectrophotometric measurements on Al3+-Cys containing solutions have been carried out using a Varian Cary 50 UV–Vis spectrophotometer, equipped with an optical fiber probe, having a path length equal to 1 cm, able to investigate the ultraviolet and visible electromagnetic spectrum area, and a Metrohm 750 combined glass electrode in order to register the pH values. This system is connected to a PC used to collect the spectra. The measurements have been performed on 25 mL solution containing the metal cation (1 ≤ CM/mmol L− 1 ≤ 2) and the ligand (2 ≤ CL/mmol L− 1 ≤ 4) at different metal ligand ratios (0.25 ≤ CM/CL ≤ 1) at different pH, in the spectral range from 220 to 280 nm, at T = 298.15 K and I = 0.15 mol L− 1, using NaCl as 11

Biophysical Chemistry 230 (2017) 10–19

P. Cardiano et al.

Scheme 1. Ligands under study.

Glycine (Gly)

L-Cysteine (Cys)

Tranexamic acid (Tranex) reagents in the solution and pH [28,29]. ES5CM99 program has been used to determine the enthalpy changes of ligand protonation (only for Tranex) and species formation. LIANA program has been selected to study the formation constants dependence on ionic strength. The speciation diagrams and the formation species percentages have been obtained with ES4ECI program. Details on computer programs used in the multicomponent system speciation are reported on Ref. [30].

supporting electrolyte. Each experiment was carried out into the same glass jacket cells and apparatus described in the previous potentiometric equipment and procedure paragraph. 2.4. Calorimetric equipment and procedure Calorimetric measurements have been performed at 298.150 ± 0.001 K using a CSC 4300 Isoperibol Titration calorimeter coupled with a computer for the acquisition of calorimetric data. The titrant has been delivered by a 2.5 mL capacity Hamilton syringe, model 1002TLL. Accuracy has been checked by titrating a Tris [tris-(hydroxymethyl) amino-methane] buffer with HCl [26]. The accuracy of calorimetric apparatus for the measured heat was Q ± 0.008 J and for the volume v ± 0.001 mL. For all the measurements with Al3+, a 25 mL solution containing the metal cation (CM = 4 mmol L− 1), HCl (CH = 4 mmol L− 1) and NaCl as supporting electrolyte (I = 0.15 mol L− 1) was titrated with the ligand salt (CL = 0.1 mol L− 1). For determination measurements of protonation enthalpy change of Tranex, the ligand salt at C = 5–10 mmol L− 1 has been titrated with standard HCl. Before each experiment the heat of dilution was measured.

3. Results and discussion 3.1. Complexes of Al3+ with Gly, Cys and Tranex The stoichiometries and the stability constants of the species formed in Al3+-Gly, -Cys, -Tranex systems have been firstly determined by potentiometric titrations on solutions containing different metal/ligand ratios, in NaCl at different ionic strength values. The stability constants of the Al3+-Gly, -Cys, -Tranex have been calculated by considering the ligand protonation constants and aluminium hydrolysis formation constants. Analysis on literature data of Al3+ hydrolysis thermodynamic parameters has been performed [23,31–33]. Al3+ hydrolysis thermodynamic parameters recalculated from analysis of literature data and ligand protonation constants used in the calculations are reported in Tables 1s, 2s of Supplementary information. Protonation constants of Gly and Cys have been already published [34,35], Tranex ones are unpublished data from this laboratory. Complex formation constants of Al3+ (M3+) with the ligands (Lz −) under study are expressed as βpqr, according to the following equilibrium reaction:

2.5. NMR equipment and procedure 1

H NMR solution spectra on Cys, Al3+-Cys, Tranex and Al3+-Tranex systems have been recorded on a Bruker AMX R-300 spectrometer, operating at 300 MHz. The chemical shifts have been measured with respect to 1,4-dioxane as reference and converted relative to TMS (δdioxane = 3.70 ppm). All the measurements have been carried out at T = 298.15 K, in a 9:1 H2O/D2O solution, using presaturation technique for water signal suppression. Depending on the investigated system, NMR titrations have been performed in different pH ranges, by varying metal and ligand concentrations as well as metal/ligand ratios. In detail, Tranex titrations have been carried out in the pH range between approx. 3 and 11.5, whilst Cys titrations in the pH range between ca. 2 and 11. For the metal containing systems, the investigations have been performed at different metal cation concentrations (3 ≤ CM/ mmol L− 1 ≤ 8), employing 0.4 ≤ CM/CL ≤ 0.8 metal/ligand ratios, in the pH range between ca. 2 and 5.

M3 + + qLz − + rH+ ⇄ ML q Hr (3 − qz + r)

(1)

Some stability constants were also expressed as stepwise equilibria:

M3 +

+ L q Hr (−qz + r) = ML q Hr (3 − qz + r)

(2)

The calculated speciation models for the three systems are quite different since they are featured by the formation of MLH, ML and M2L2(OH)2 species for Gly, ML, M2L and MLOH for Cys, MLH and MLOH for Tranex. More in detail, for the Al3+-Tranex system, two distinct speciation models with the best fit have been obtained by potentiometric measurements, i.e. the first featured by two species only, MLH and MLOH, and the second with an additional ML species, as shown in Table 1. However, in the latter speciation model, the ML species does not reach significant formation percentages (< 10%) and the formation costant values, referring to MLH and MLOH species, in the two models are fairly close. The selection between the two cited models has been carried out on the basis of calorimetric and 1H NMR titrations which confirmed only the first model characterized by two species, i.e. MLH and MLOH. Experimental stability constants of the Al3+-Gly, -Cys, -Tranex species, obtained by potentiometric measurements are collected in Table 2. As it is well established, the most likely Al binding sites are O-donors, such as carboxylates; amines and thiolates have not a good affinity towards Al3+. However, if N- and S-donor groups are not alone, but in a chelating position with respect to an Odonor group, their binding ability may considerably increase [11]. Multidentate ligands with O-donor groups bind Al3+ with affinity that increases with the number and the basicity of donor groups [9].

2.6. Calculations BSTAC4 and STACO4 programs have been used to calculate the formation constants of the ligand protonated species and to refine all the parameters related to an acid-base titration, such as the standard potential E0, the potential junction and the analytical concentration of the reagents. Hyspec2014 program, used for spectrophotometric titration, has been employed to calculate the stability constants and the molar absorbance of the single species, knowing the analytical concentration of the reagents [27]. The individual chemical shifts belonging to each species present at equilibria and the relative formation constants have been calculated by HypNMR software assuming fast exchange on the NMR time-scale, such that the chemical shift for a nucleus is a mol fraction-weighted average over all the chemical species in which this nucleus is present. Data input include the chemical shifts of the NMR peaks in relation to the analytical concentrations of 12

Biophysical Chemistry 230 (2017) 10–19

P. Cardiano et al.

1.0

Table 1 Experimental formation constants for Al3+-Tranex species (in NaCl) obtained by potentiometry at different ionic strengths and at T = 298.15 K.

1

0.8 logβa I = 0.5b

I = 1b

MLH MLOH

Model 1 13.31 (1)c 4.61 (1)

13.33 (2)c 4.66 (2)

13.26 (2)c 4.52 (1)

MLH ML MLOH

Model 2 13.18 (2) 8.39 (4) 4.44 (3)

13.08 (3) 8.55 (3) 4.33 (3)

12.68 (4) 8.82 (1) 3.81 (1)

a b c

3+

I = 0.15b

Al fraction

Species stoichiometry

0.0 2.0

Tranex

a b c

I = 0.5b

I = 1b

11.698 (7)c 7.18 (1) 8.62 (2) 11.91 (3) 15.17 (1) 6.96 (2) 13.31 (1) 4.61 (1)

11.55 (6)c 7.212 (7) 7.84 (5) 11.60 (5) 14.89 (1) 6.39 (4) 13.33 (2) 4.66 (2)

11.694 (4)c 7.245 (5) 8.29 (1) 11.48 (4) 14.24 (7) 6.70 (5) 13.26 (2) 4.52 (1)

0.8

3+

Al fraction

4.5

5.0

0.4

4

2.5

3.0

3.5

4.0

4.5

5.0

of 0.4; in the range of pH = 4.0–4.5, ML and M2L2(OH)2 are also present yet in much lower fractions. The increase of the ionic strength from 0.15 to 1 mol L− 1 leads a significant lowering of MLH fraction (at pH = 3.5, from 0.4 to 0.3). In the Al3+-Cys system, shown in Fig. 2, the metal containing species formation is relevant in a small pH range between 3.5 and 5.0. The main species is M2L with a maximum Al3+ fraction of almost 0.6 at pH = 4.2. ML species reaches a metal fraction of almost 0.3 at pH = 4.5. The effect of the ionic strength is very relevant on the M2L species, since by ionic strength increasing a very large decrease of the Al3+ fraction is observed (at pH = 4.3 from 0.6 to 0.3). The Al3+ fractions of the species formed in Al3+-Tranex system, reported in Fig. 3, are significantly higher than the ones gained in the other two systems. MLH species prevails in the range of pH = 2.5–4, with a maximum fraction of 0.5 at pH = 4.0. At pH = 4.5, MLOH species also reaches a fraction of 0.5. Moreover, in this system the ionic strength increase leads to a substantial lowering of the molar fractions especially for the MLH species (at pH = 4, from 0.5 to 0.4). Similarly to our previous studies on other systems, spectroscopic techniques have been employed with the aim of further confirm the speciation model and to gain more information on the systems understudy [36–40]. In order to investigate the UV spectral properties of the Al3+-Cys species and to confirm the reliability of the formation constants obtained by potentiometry, a wide number of spectra were performed by varying the pH, in the 2.4 ≤ pH ≤ 7.0 range, and the stoichiometric ratio between Al3+ and Cys. Some experimental spectra at

5

2

0.2 3 4

4.0

3 2

pH

1

3.5

1

Fig. 3. Speciation diagram for Al3+-Tranex system in NaCl at I = 0.15 mol L− 1 (solid line) and I = 1 mol L− 1 (dotted line). T = 298.15 K, experimental conditions: CM = 4 mmol L− 1; CL = 8 mmol L− 1. Species: 1. M; 2. MLH; 3. MLOH; 4. M13(OH)32.

0.8

3.0

4.0

0.6

0.0 2.0

1.0

0.0 2.5

3.5

0.2

Accordingly, the results here obtained show, for example, for ML species at T = 298.15 K and I = 0.15 mol L− 1 in NaCl, logβ = 7.18, 11.91 for Gly and Cys, respectively. This evidence clearly indicates that the presence of an additional S-donor group in the cysteine molecule, besides O- and N-donor groups, strongly affects complex stability. As shown by the speciation diagrams, reported in Figs. 1–3, the investigations have been carried out in a small pH range, due to the formation, for all the investigated systems, of slightly soluble species approximately at pH = 5. For the Al3+-Gly system, shown in Fig. 1, in the range of pH = 2–4, the main species is MLH, with an Al3+ fraction

0.4

3.0

1.0

I = 0.15b

According to the reaction (1). In mol L− 1. Least-squares errors on the last significant figure are given in parentheses.

0.6

2.5

3+

Cys

logβa

MLH ML M2L2(OH)2 ML M2L MLOH MLH MLOH

4

Fig. 2. Speciation diagram for Al3+-Cys system in NaCl at I = 0.15 mol L− 1 (solid line) and I = 1 mol L− 1 (dotted line), T = 298.15 K. Experimental conditions: CM = 4 mmol L− 1; CL = 8 mmol L− 1. Species: 1. M; 2. M2L; 3. ML; 4. MLOH; 5. M13(OH)32.

Al fraction

Gly

3

pH

Table 2 Experimental formation constants for Al3+-Gly, -Cys, -Tranex species (in NaCl) obtained by potentiometry at different ionic strengths and at T = 298.15 K. Species stoichiometry

2

0.4 0.2

According to the reaction (1). In mol L− 1. Least-squares errors on the last significant figure are given in parentheses.

Ligand

5

0.6

4.5

pH Fig. 1. Speciation diagram for Al3+-Gly system in NaCl at I = 0.15 mol L− 1 (solid line) and I = 1 mol L− 1 (dotted line), T = 298.15 K. Experimental conditions: CM = 4 mmol L− 1; CL = 8 mmol L− 1. Species: 1. M; 2. MLH; 3. ML; 4. M2L2(OH)2; 5. M13(OH)32.

13

Biophysical Chemistry 230 (2017) 10–19

P. Cardiano et al.

1.4 1.2

1. pH = 2.4 2. pH = 5.6 3. pH = 6.1 4. pH = 6.5 5. pH = 6.7 6. pH = 6.9 7. pH = 7.0

A

7

1.0

6

0.8

5

0.6

4

0.4

3 2

0.2

1

0.0 220

230

featured by an upfield shift, being the CH(1) shielding the most pronounced (from 2.3 to 2.08 ppm). A similar behavior, as already stated, has been observed also for cycloaliphatic CH2 signals, i.e. CH2(2,6) and CH2(3,5), which undergo comparable slight shift towards lower ppm. All of the mentioned resonances, as a common feature, remain almost unchanged up to a pH = 8.5, after which they start to shift again towards higher fields. On the contrary, the peaks due to the proton adjacent to the amino group, namely CH2(7) and CH(4), show an opposite behavior since they undergo a slight but continuous shielding effect up to 2.83 and 1.6 ppm, respectively, at pH = 8.5. From this pH value on, the signals of the mentioned protons start to shift towards lower ppm owing to the deprotonation of the ammonium group. Moreover, the spectra recorded in the pH range between 7 and 8 can be considered due to the LH individual species only, since in this conditions it reaches a formation percentage > 99%. As an example, a 1H NMR spectrum of Tranex is shown in Fig. 6. Once again, as already found in the Al3+-Cys systems, the presence on the metal does not significantly change the spectra. As stated, although a wide number of spectra have been collected, both for Cys and Tranex, by varying metal/ligand ratios and the relative concentrations as well, all the data recorded display a single set of resonances; therefore it is not possible to discriminate free and bound ligand directly from the experimental data, as all the species present at equilibria are involved in a fast exchange on the NMR time-scale. Nevertheless, the HypNMR software is a useful tool to overcome this experimental limitation since it provides the formation constants as well as the chemical shifts for each individual complex, starting from observed data, thus allowing the recalculation of the average chemical shifts at each experimental pH. As an example, in Fig. 7, is reported the excellent agreement between the calculated and observed chemical shifts, for CH and CH2 protons of Al3+-Cys system. Furthermore, the calculated formation constants of both Al3+-Cys and Al3+-Tranex complexes, are reported in Tables 4, 5 (where the formation constant value of ML species, for L = Cys, obtained by potentiometry, has been kept constant in the calculations), together with the ligand protonation constants and the individual chemical shifts of the species. They are in agreement with potentiometric findings, thus confirming the formed species as well as the magnitude of the relative interactions. For example, the formation constant of MLH species of the Al3+-Tranex system is the same of the one achieved by potentiometry. The whole of the data such collected allow us to conclude that NMR experiments are fully consistent with the model employed to rationalize the data coming from potentiometric and spectrophotometric investigations.

240

250

260

270

280

/ nm 3+

Fig. 4. Experimental spectra of Al -Cys solutions at different pH values, I = 0.15 mol L− 1 (NaCl), T = 298.15 K. Experimental conditions: CM = 1 mmol L− 1; CL = 4 mmol L− 1.

Table 3 Experimental formation constants of Al3+-Cys species obtained by spectrophotometry at I = 0.15 mol L− 1 (NaCl) and T = 298.15 K. Species stoichiometry

logβa

ML M2L MLOH

12.20 (4)b 15.17 6.93 (4)

a

According to the reaction (1). Least-squares errors on the last significant figure are given in parentheses. b

different pH values are shown in Fig. 4, where it is clear that the absorbance increases with the increasing of the pH. Results obtained by spectrophotometric titrations on Al3+-Cys solutions, beside to confirm the speciation model achieved by potentiometry, allow detecting formation constant values of ML and MLOH species, collected in Table 3. The formation constant value of M2L species, obtained by potentiometry, has been kept constant in the calculations. The values referring to ML and MLOH species result very close to the ones already obtained by potentiometry, especially in the case of MLOH. Titrations on Cys, Al3+-Cys, Tranex and Al3+-Tranex solutions have been followed by means of 1H NMR spectroscopy in solution. As far the titration of Cys is concerned, as already observed [39,41,42], the methyne group shows an upfield shift at low pH (from 4.1 to 3.9 ppm), thus confirming the deprotonation of eCOOH below pH 3.5; then, upon pH increasing, CH signal keeps constant up to a pH of ca. 7, i.e. in a pH range where LH2 is the most abundant species. After neutral conditions the above resonance at ~3.9 ppm starts to decrease steadily reaching a value of 3.2 ppm at pH = 11. Moreover, the influence of pH increasing on methylene group resonance results in a deshielding effect as well, although less pronounced. The 1H NMR spectrum of Cys, at pH = 4.0, is shown in Fig. 5. Differently from other studied systems [37,39,41], where the stronger affinity of the investigated metal cations towards sulfur-containing ligands was clearly detectable from the NMR spectra, the presence of Al3+ leaves almost unaltered the chemical shifts of both CH and CH2 Cys signals, regardless of the metal/ligand ratio employed. The only difference resulting in the spectra of Al3+-containing systems is a slight deshielding effect on CH signal, with respect the Cys alone titration data. However, it is worth to mention that, in order to avoid the precipitation of hydrolytic compounds, the investigations on the metal-containing solutions have been carried out in smaller pH ranges with respect to the Cys solutions. The titration of Tranex has been followed by 1H NMR spectroscopy in solution as well. At lower pH (pH < 6) the signals due to the protons closer to eCOOH moiety, i.e. methyne and methylenic groups, are

3.2. Stability constants dependence on ionic strength With the purpose of rationalizing the dependence of stability constants on ionic strength, the experimental values reported in Table 2, were analyzed by using the following Debye-Hückel type equation [43]:

logβ = log Tβ − 0.51⋅z∗

I + CI 1 + 1.5 I

(3)

where β is the formation constant at infinite dilution, z* = Σ(charge)2reactants − Σ(charge)2products, C an empirical parameter. More in detail: T

C = (c0 p* + c1 p*)I

(4)

And p* = Σpreagents − Σpproducts, where p = stoichiometric coefficients. C parameter depends on the electric charges (z) of reagents and products involved in the formation reaction and on the stoichiometry of the species [44–46]. Values of logTβ and the empirical parameter C, calculated by Eq. (3), are reported in Table 6 for all the species in the investigated systems. The knowledge of the thermodynamic constants and the C parameters for each species is useful to calculate the constants at any ionic strength over the range investigated. Calculated C parameters are all positive (except for the M2L species of Cys) and in the range between 0.1 and 1. From the comparison of the results here 14

Biophysical Chemistry 230 (2017) 10–19

P. Cardiano et al.

Fig. 5. 1H NMR spectrum of Cys at C = 8 mmol L− 1, pH = 4.0, T = 298.15 K and I = 0.15 mol L− 1 in NaCl.

Fig. 6. 1H NMR spectrum of Tranex at C = 8 mmol L− 1, pH = 7.07, T = 298.15 K and I = 0.15 mol L− 1 in NaCl.

reported with those obtained for the complex species of Al3+ with 2mercaptopropanoic acid and 3-mercaptopropanoic acid, it appears that also the latter are all positive (except for the ML2OH species of 3mercaptopropanoic acid) and in the range between 0.3 and 1.18. On the contrary, those of the species formed by 2-mercaptosuccinic acid are all negative [47]. Calculated formation constant values at different ionic strengths are collected in Table 3s of the Supplementary information.

3.3. Stability constants dependence on temperature In order to complete the framework of the thermodynamic parameters, it is of fundamental importance to determine enthalpy changes referring to complex species, as performed for several other systems [37–39,41,48–50]. In Table 7 the thermodynamic formation values referring to Al3+-Gly, -Cys and Tranex are collected. Tranex enthalpy 15

Biophysical Chemistry 230 (2017) 10–19

P. Cardiano et al.

Table 6 Formation constants for Al3+-Gly, -Cys, -Tranex species at infinite dilution, together with C parameter of Eq. (3) in NaCl, at T = 298.15 K.

3.948

a

CH

3.944

3.940

Ligand

Species stoichiometry

logTβa

C

Gly

MLH ML M2L2(OH)2 ML M2L MLOH MLH MLOH

11.9 (2)b 7.9 (2) 10.1 (2) 13.31 (7) 16.0 (1) 8.2 (1) 13.57 (8) 5.6 (1)

0.2 (3)b 0.6 (3) 1.0 (3) 0.6 (1) − 0.6 (1) 0.9 (2) 0.1 (1) 0.6 (2)

Cys

Tranex

3.936

a b

3.5

4.0

4.5

5.0

5.5

6.0

6.5

pH

According to the reaction (1). Least-squares errors on the last significant figure are given in parentheses.

Table 7 Thermodynamic protonation parameters of Gly, Cys, Tranex and formation parameters of Al3+-Gly -Cys, -Tranex species obtained by titration calorimetry at I = 0.15 mol L− 1 in NaCl and T = 298.15 K.

3.020

b CH2

3.018

Ligand

Species stoichiometry

− ΔGa,b

ΔHa,b,c

TΔSa,b

Gly

LH LH2 MLH LH LH2 LH3 ML M2L MLOH LH LH2 MLH MLOH

54.8 68.2 66.8 58.0 104.5 116.0 68.0 86.6 39.7 59.8 84.3 76.0 26.3

− 44.8d − 48.9d − 41 (5) − 42e − 77e − 81e 35 (3) − 11 (1) 52 (3) − 52.0 (5) − 49.8 (8) − 25 (3) − 51 (1)

10 19.3 26 16 27.5 35 103 76 92 7.8 34.5 51 − 25

Cys

3.016

3.014 Tranex

3.5

4.0

4.5

5.0

5.5

6.0

6.5

pH a

Fig. 7. Experimental (□) and calculated (○) values of the chemical shifts for CH (a) and CH2 (b) of the ligand in Al3+-Cys mixtures at CM = 6 mmol L− 1, CL = 10 mmol L− 1 (a), CM = 8 mmol L− 1, CL = 10 mmol L− 1 (b) and T = 298.15 K.

b c d e

Table 4 Formation constants and calculated chemical shifts of Al3+-Cys species obtained by 1H NMR at I = 0.15 mol L− 1 (NaCl) and T = 298.15 K. Species stoichiometry

logβa

δCH

L LH LH2 LH3 ML M2L MLOH

10.61 (2)b 18.91 (2) 20.80 (4) 11.91 15.0 (4) 7.0 (5)

3.020 3.516 3.939 4.307 3.913 3.969 3.926

a b

As expected for hard-hard interactions, i.e. like those occurring between Al3+ and O-donor ligands, the contribution to the free energy is mainly of entropic origin. For example, for the ML species of Al3+-Cys system, TΔS = 103 kJ mol− 1 and ΔH = 35 kJ mol− 1 have been obtained, the latter being quite similar to the one found for ML species formed in the Al3+-2-mercaptopropanoic acid system (ΔH = 28 kJ mol− 1) [47]. For the MLH species, TΔS = 16, 43 kJ mol− 1 and ΔH = 3.8, 27 kJ mol− 1 have been calculated, for the Al3+-Gly and -Tranex systems, respectively (on the basis of the partial reaction: M + LH = MLH, charges omitted for simplicity). The enthalpy change values, obtained at T = 298.15 K, can be used to calculate the protonation or species formation constant values at other temperatures, by means of Van't Hoff equation. Thermodynamic formation parameters are useful to assess the temperature effect on the speciation.

δCH2 (1)b (1) (1) (3) (3) (3) (3)

In kJ mol− 1. According to the reaction (1). Least-squares errors on the last significant figure are given in parentheses. Ref. [35]. Ref. [34].

2.613 (1)b 2.831 (1) 3.017 (1) 3.113 (1) 3.00 (2) 3.03 (2) 3.01 (2)

According to the reaction (1). Least-squares errors on the last significant figure are given in parentheses.

protonation values have been determined by calorimetric titrations and here reported for the first time, together with literature enthalpy protonation values of Gly and Cys.

Table 5 Formation constants and calculated chemical shifts of Al3+-Tranex species obtained by 1H NMR at I = 0.15 mol L− 1 (NaCl) and T = 298.15 K. Species stoichiometry

logβa

δCH2(7)

L LH LH2 MLH MLOH

10.72 (2)b 15.15 (4) 13.31 (6) 5.39 (5)

2.353 2.814 2.829 2.831 2.824

a b

(2)b (2) (2) (2) (2)

δCH(4) 1.238 1.592 1.614 1.620 1.603

δCH2(2, 6)′

δCH(1) (2)b (2) (2) (2) (2)

2.024 2.059 2.305 2.317 2.183

According to the reaction (1). Least-squares errors on the last significant figure are given in parentheses.

16

(2)b (2) (2) (2) (2)

1.818 1.872 1.974 1.978 1.931

(2)b (2) (2) (2) (2)

δCH2(2, 6)″ 1.737 1.775 1.811 1.812 1.795

(8)b (8) (8) (8) (8)

δCH2(3, 5)′ 1.267 1.303 1.366 1.366 1.339

(5)b (5) (5) (5) (5)

δCH2(3, 5)″ 0.852 0.992 1.029 1.030 1.009

(4)b (4) (4) (4) (4)

Biophysical Chemistry 230 (2017) 10–19

P. Cardiano et al.

already found in a previous investigation as well [47], the contribution to the coordination due to thiolate group results, from empirical calculations, higher than the one related to the amino group and even higher than the one displayed by carboxylate group.

3.4. Empirical relationships Since multicomponent real aqueous solutions, such as natural waters and biological fluids, are characterized by very variable composition, ionic strength, temperature, pH, pressure, the determination of the significant interactions of all components of a system under a particular condition would be a very difficult, if not impossible, task. Although the experimental determination is the best approach, the modeling studies are very useful. The stability of metal-ligand complexes, for the same metal cation and homogeneous ligand classes, can be considered as a function of the number and kind of functional groups of the ligand. As reported in a previous paper, the stability of aluminium complex species can be expressed as the sum of the contributions of the different donor groups present on the ligand molecule [47]:

log K =

∑ nX‐donor

3.5. Literature comparisons As regard the formation of Al3+-Gly complexes, a paper of Daydè et al. [13] reports at T = 310.15 K and I = 0.15 mol L− 1 in NaCl, a speciation model with only a species having stoichiometry M2L(OH)2, different from the one here proposed featured by three species, namely MLH, ML and M2L2(OH)2. In another paper of Kiss et al. the speciation model shows the formation of three species, ML, MLH− 1, M2LH− 1, with logβ = 5.91, 1.08, 4.35, respectively at T = 298.15 K and I = 0.2 mol L− 1 in KCl. However, in the mentioned study, both metal cation and ligand concentrations as well as metal/ligand ratios are substantially different (CM = 20 or 40 mmol L− 1 and M:L from 1:10 to 1:40) with respect to the ones here employed (CM = 2–6 mmol L− 1, CL = 6–12 mmol L− 1and M:L from 1:1 to 1:4) [12]. Consequently, the speciation model as found by Kiss et al. is not comparable to the one here reported. The only species found in both models, ML, displays a logK = 5.91 (at T = 298.15 K and I = 0.2 mol L− 1 in KCl, for Kiss et al.), whereas it shows here a logK = 7.18 (at I = 0.15 mol L− 1 and T = 298.15 K). In the review of Rubini et al. [7] a logK = 5.8–5.9 was estimated by LFER approach for ML species in the Al3+-Gly system. In the literature, to the best of our knowledge, only one paper deals with an investigation on Al3+-Cys system [19]. In this case, the range of concentration employed (metal ligand ratios equal to 1:1, 1:2, 1:3, with CL = 8.1 mmol L− 1) is very similar to the one here investigated, but honestly the pH range (4.0–11.5) appears too large, although the authors admit that “a slight turbidity was observed, from pH 4.6 to 10.0”. Consequently the proposed speciation model includes species, such as ML(OH)2 and ML(OH)3, in addition to ML and MLOH. Furthermore, for ML and MLOH species, logβ = 6.45, 6.15, respectively, at T = 298.15 K and I = 0.1 mol L− 1. From these results it is clear that the ML species formation constant value is very different from the one here proposed, whilst MLOH value is fairly similar (logβ = 6.96, at T = 298.15 K and I = 0.15 mol L− 1 in NaCl).

(5)

where nX-donor could be a S-, N- or O-donor group. By taking into account logK values (as stepwise equilibria, Eq. (2)) at I = 0 mol L− 1 and T = 298.15 K for MLH2, MLH and ML complex species of Al3+ with Gly, Cys, Tranex, thiocarboxylates (namely 2-mercaptopropanoic acid, 3-mercaptopropanoic acid, 2-mercaptosuccinic acid) [47] and carboxylates (namely malonate, 1,2,3-propanetricarboxylate, 1,2,3,4-butanetetracarboxylate) [51], it was possible to calculate the single contribution values. The contributions for carboxylate and thiolate groups have been already published, resulting equal to 2.93 and 6.5, respectively [47]. In addition, the contribution to the stability of amino group has been obtained by adding the results on Al3+-amino acid species here reported, resulting equal to 4.5, with a logK ± 0.8 (95% confidence interval) estimated precision, without considering chelation or other specific effects. Fig. 8 shows the fair agreement obtained by plotting the formation constants calculated by Eq. (5) vs. the corresponding experimental data (correlation coefficient, r2 = 0.92; slope = 0.99 ± 0.03). As an example, by taking into account this contribution values, stability constant of ML species for Cys, is given by the sum of three contributions (carboxylate, amino and thiolate) and results equal to 13.9 at I = 0 mol L− 1, quite comparable to 13.31 (see Table 6). As already discussed in a previous paragraph (Results and discussion), in the case of multidentate ligands containing O-donor groups the affinity towards Al3+ is a function of the number and the basicity of donor groups [9,52] and if S- and/or N-donor groups are present in a chelating position, their binding ability may significantly change [11]. Moreover, it is known that carboxylates with an additional donor group in α or β position may act as bidentate ligands forming cyclic structures [53]. Accordingly, for the multidentate ligands here described, and as

3.6. Sequestering ability From a biological and environmental point of view, some fundamental parameters are of great importance, such as temperature, ionic strength, and pH, since their variations also influence the ligands sequestering ability [54,55]. Sequestration is crucial to evaluate the use of a ligand to remove toxic metal cations from natural systems or as detoxificant in biomedical applications [56]. Often a simple comparison among the stability constants and the formation percentage values of different metal-ligand systems is not enough to establish the sequestering ability of a ligand towards different metal cations. On the basis of these considerations, an empirical parameter, pL0.5, has been proposed by this research group [57,58]. This value is calculated through the following Boltzmann type equation:

20 16

logK calc

12 8

χ=

4

4

8

12

16

(6)

where χ represents the sum of molar fraction of the different species and pL is the cologarithm of the total ligand concentration. More in detail, this parameter expresses the ligand concentration required to sequester 50% of the metal cation when this one is present in traces. For the ligands under study, this parameter at pH = 4 and 5, I = 0.15 mol L− 1 and T = 298.15 K has been determined. The calculated values are collected in Table 8. Fig. 9 shows that the pL0.5 values, at pH = 5, I = 0.15 mol L− 1 and T = 298.15 K, of Cys (3.74) and Tranex (3.91) are fairly similar, but higher than the one of Gly (2.51) of

0 0

1 1 + 10(pL − pL0.5)

20

logKexp Fig. 8. Calculated formation constants, by Eq. (5), of MLHr species (as stepwise equilibria, Eq. (2)) formed by Al3+ with Gly, Cys, Tranex, thiocarboxylates and carboxylates, at I = 0 mol L− 1 and T = 298.15 K, vs. experimental ones.

17

Biophysical Chemistry 230 (2017) 10–19

P. Cardiano et al.

ability towards Al3+ of larger biomolecules, such as proteins, in biological fluids.

Table 8 pL0.5 values of Gly, Cys, Tranex towards Al3+, at different pH values, I = 0.15 mol L− 1 and T = 298.15 K. Ligand

pH

pL0.5

Gly

4 5 4 5 4 5

2.19 2.51 1.62 3.74 2.44 3.91

Cys Tranex

Acknowledgements The author Ottavia Giuffrè thanks MIUR (Ministero dell'Istruzione, dell'Università e della Ricerca) for financial support (co-funded PRIN project with Prot. 2015MP34H3). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.bpc.2017.08.001.

1.0

References

0.8

[1] Aluminium, in: J.K. Aronson (Ed.), Meyler's Side Effects of Drugs, Sixteenth edition, Elsevier, 2016. [2] Aluminum speciation, in: D.R. Parker, D. Hillel (Eds.), Encyclopedia of Soils in the Environment, Elsevier, 2005, pp. 50–56. [3] W. Wang, H. Yang, X. Wang, J. Jiang, Z. Wanpeng, Effects of fulvic acid and humic acid on aluminum speciation in drinking water, J. Environ. Sci. 22 (2010) 211–217. [4] A. Sanz-Medel, A.B. Soldado Cabezuelo, R. Milacic, T.B. Polak, The chemical speciation of aluminium in human serum, Coord. Chem. Rev. 228 (2002) 373–383. [5] G. Crisponi, V.M. Nurchi, Thermodynamic remarks on chelating ligands for aluminium related diseases, J. Inorg. Biochem. 105 (2011) 1518–1522. [6] G. Berthon, Chemical speciation studies in relation to aluminium metabolism and toxicity, Coord. Chem. Rev. 149 (1996) 241–280. [7] P. Rubini, A. Lakatos, D. Champmartin, T. Kiss, Speciation and structural aspects of interactions of Al(III) with small biomolecules, Coord. Chem. Rev. 228 (2002) 137–152. [8] T. Kiss, P. Zatta, B. Corain, Interaction of aluminium(III) with phosphate-binding sites: biological aspects and implications, Coord. Chem. Rev. 149 (1996) 329–346. [9] A.E. Martell, R.D. Hancock, R.M. Smith, R.J. Motekaitis, Coordination of Al(III) in the environment and in biological systems, Coord. Chem. Rev. 149 (1996) 311–328. [10] G. Crisponi, V.M. Nurchi, V. Bertolasi, M. Remelli, G. Faad, Chelating agents for human diseases related to aluminium overload, Coord. Chem. Rev. 256 (2012) 89–104. [11] T. Kiss, From coordination chemistry to biological chemistry of aluminium, J. Inorg. Biochem. 128 (2013) 156–163. [12] T. Kiss, I. Sovago, I. Toth, A. Lakatos, R. Bertani, A. Tapparo, G. Bombi, R.B. Martin, Complexation of aluminium (III) with several bi- and tri-dentate amino acids, J. Chem. Soc. Dalton Trans. (1997) 1967–1972. [13] S. Daydé, D. Champmartin, P. Rubini, G. Berthon, Aluminium speciation studies in biological fluids. Part 8. A quantitative investigation of Al(III)/amino acid complex equilibria and assessment of their potential implications for aluminium metabolism and toxicity, Inorg. Chim. Acta 339 (2002) 513–524. [14] E. Furia, R. Porto, 2–Hydroxybenzamide as ligand. Complex formation with dioxouranium(VI), aluminium(III), neodymium(III) and nickel(II) ions, J. Chem. Eng. Data 53 (2008) 2739–2745. [15] E. Furia, T. Marino, N. Russo, Insights on the coordination mode of quercetin with Al(III) ion from a combined experimental and theoretical study, Dalton Trans. 43 (2014) 7269–7274. [16] A. Beneduci, E. Furia, N. Russo, T. Marino, Complexation behaviour of caffeic, ferulic and p-coumaric acids towards aluminium cation: a combined experimental and theoretical approach, New J. Chem. 41 (2017) 5182–5190. [17] S. Daydé, V. Brumas, D. Champmartin, P. Rubini, G. Berthon, Aluminum speciation studies in biological fluids. Part 9. A quantitative investigation of aluminum (III)–glutamate complex equilibria and their potential implications for aluminum metabolism and toxicity, J. Inorg. Biochem. 97 (2003) 104–117. [18] K. Hagvall, P. Persson, T. Karlsson, Speciation of aluminum in soils and stream waters: The importance of organic matter, Chem. Geol. 417 (2015) 32–43. [19] D. Bohrer, V. Gabbi Polli, P. Cicero do Nascimento, J.K.A. Mendonca, L. Machado de Carvalho, S. Garcia Pomblum, Ion-exchange and potentiometric characterization of Al-cystine and Al-cysteine complexes, J. Biol. Inorg. Chem. 11 (2006) 991–998. [20] S.S. Khaloo, M.K. Amini, S. Tangestaninejad, S. Shahrokhian, R. Kia, Voltammetric and potentiometric study of cysteine at cobalt(II) phtalocyanine modified carbonpaste electrode, J. Ir. Chem. Soc. 1 (2004) 128–135. [21] M.F. Khan, M.F. Khan, M. Ashfaq, G.M. Khan, Synthesis and characterization of some novel tranexamic acid derivatives and their copper(II) complexes, Science 1 (2001) 327–333. [22] Z. Li, L. Fang, J. Wang, L. Dong, Y. Guo, Y. Xie, An improved and practical synthesis of tranexamic acid, Org. Process. Res. Dev. 19 (2015) 444–448. [23] C.F. Baes, R.E. Mesmer, The Hydrolysis of Cations, John Wiley & sons, New York, 1976. [24] R.M. Cigala, C. De Stefano, A. Giacalone, A. Gianguzza, Speciation of Al3+ in fairly concentrated solutions (20–200 mmol L− 1) at I = 1 mol L− 1 (NaNO3), in the acidic pH range, at different temperatures, Chem. Speciat. Bioavailab. 23 (2011) 33–37. [25] R.A. Yokel, Aluminum, in: E. Merian, M. Anke, M. Ihnat, M. Stoeppler (Eds.), Elements and Their Compounds in the Environment - Occurrence, Analysis and Biological Relevance, 2nd, completely and enlarged edition, Vol. 2 - Metals and

0.6 0.4

Gly

Tranex

Cys

0.2 0.0 2

4

6

8

pL 3+

Fig. 9. Sequestration diagram for Al I = 0.15 mol L− 1 in NaCl, T = 298.15 K.

-Gly, -Cys, -Tranex species at pH = 5,

over one logarithmic unit in the same conditions. This trend, at pH = 5, reflects the order of basicity of the three ligands, i.e. Tranex > Cys > Gly (logKH 1 = 10.481, 10.160, 9.571 for Tranex, Cys and Gly, respectively, at I = 0.15 mol L− 1 and T = 298.15 K). The pL0.5 values obtained with Al3+-amino acids can be compared with those achieved for the Al3+-thiocarboxylates systems under the same pH and ionic strength conditions [47] (pL0.5 = 3.18, 2.99, for 2mercaptopropanoic acid and 3-mercaptopropanoic acid, respectively). This indicates that the sequestering ability of two ligands bearing single O- and S-donor groups is significantly lower than the one of Cys, having an additional N-donor group. 4. Conclusions In this investigation the results of a thermodynamic and spectroscopic study on Al3+-Gly, -Cys and -Tranex have been reported. The potentiometric findings have been confirmed by spectrophotometric titrations (for Al3+-Cys system) and 1H NMR experiments (for Al3+-Cys and -Tranex systems). Unlike some literature papers in which some of these systems (Al3+Gly and -Cys) have been studied, in different concentration conditions and at a single ionic strength, in this study the dependence on ionic strength and temperature has been investigated with the purpose of calculating the species formation constant values under the conditions of different biological fluids. The assessment of the formation constants both at different temperatures and at different ionic strengths is very important for the application to real systems, such as natural waters or biological fluids, characterized by very variable composition, ionic strength, temperature, pH. The study of Al3+ interaction with ligands containing O-donor and N-donor groups (and also S-donor for Cys) is useful to clarify the function of biomolecules, such as amino acids, in the uptake, transport, physiological and biological action of this metal cation. Moreover Gly and Cys can be considered model compounds for the study of binding 18

Biophysical Chemistry 230 (2017) 10–19

P. Cardiano et al.

[43] C. Bretti, O. Giuffrè, G. Lando, S. Sammartano, Solubility protonation and activity coefficients of some aminobenzoic acids in NaClaq and (CH3)4NClaq, at different salt concentrations, at T = 298.15 K, J. Mol. Liq. 212 (2015) 825–832. [44] C. De Stefano, O. Giuffrè, S. Sammartano, Protonation constants of ethylenediamine, diethylenetriamine and spermine in NaClaq, NaIaq, (CH3)4NClaq and (C2H5)4NIaq, at different ionic strengths and t = 25 °C, J. Chem. Eng. Data 50 (2005) 1917–1923. [45] C. Foti, O. Giuffrè, S. Sammartano, Thermodynamics of HEDPA protonation in different media and complex formation with Mg2+ and Ca2+, J. Chem. Thermodyn. 66 (2013) 151–160. [46] C. Bretti, C. De Stefano, C. Foti, O. Giuffrè, S. Sammartano, Thermodynamic protonation parameters of some sulphur-containing anions in NaClaq and (CH3)4NClaq at t = 25 °C, J. Solution Chem. 38 (2009) 1225–1245. [47] P. Cardiano, F. Giacobello, O. Giuffrè, S. Sammartano, Thermodynamics of Al3+thiocarboxylate interaction in aqueous solution, J. Mol. Liq. 222 (2016) 614–621. [48] F. Crea, P. Crea, C. De Stefano, O. Giuffrè, A. Pettignano, S. Sammartano, Thermodynamic parameters for the protonation of poly(allylamine) in concentrated LiCl(aq) and NaCl(aq), J. Chem. Eng. Data 49 (2004) 658–663. [49] A. De Robertis, C. Foti, O. Giuffrè, S. Sammartano, The dependence on ionic strength of enthalpies of protonation for polyamines in NaCl(aq), J. Chem. Eng. Data 47 (2002) 1205–1212. [50] C. De Stefano, A. Gianguzza, O. Giuffrè, A. Pettignano, S. Sammartano, Interaction of methyltin(IV) compounds with carboxylate ligands. Part 2: formation thermodynamic parameters, predictive relationships and sequestering ability, Appl. Organomet. Chem. 22 (2008) 30–38. [51] P. Cardiano, F. Giacobello, O. Giuffrè, S. Sammartano, Thermodynamic and spectroscopic study on Al3+-polycarboxylate interaction in aqueous solution, J. Mol. Liq. 232 (2017) 45–54. [52] R.A. Yokel, Aluminum chelation principles and recent advances, Coord. Chem. Rev. 228 (2002) 97–113. [53] P.G. Harrison, K. Lambert, T.J. King, J. Chem. Soc. Dalton Trans. (1983) 363–369. [54] C. De Stefano, A. Gianguzza, O. Giuffrè, D. Piazzese, S. Orecchio, S. Sammartano, Speciation of organotin compounds in NaCl aqueous solution: Interaction of mono-, di- and triorganotin(IV) cations with nucleotides 5′ monophosphates, Appl. Organomet. Chem. 18 (2004) 653–661. [55] A. De Robertis, A. Gianguzza, O. Giuffrè, A. Pettignano, S. Sammartano, Interaction of methyltin(IV) compounds with carboxylate ligands. Part 1: Formation and stability of methyltin(IV)-carboxylate complexes and their relevance in the speciation of natural waters, Appl. Organomet. Chem. 20 (2006) 89–98. [56] P. Cardiano, C. Foti, P. Mineo, M. Galletta, F. Risitano, S. Lo Schiavo, Sequestration ability of task specific ionic liquids towards cations of environmental interest, J. Mol. Liq. 223 (2016) 174–181. [57] A. Gianguzza, O. Giuffrè, D. Piazzese, S. Sammartano, Aqueous solution chemistry of alkyltin(IV) compounds for speciation studies in biological fluids and natural waters, Coord. Chem. Rev. 256 (2012) 222–239. [58] G. Falcone, C. Foti, A. Gianguzza, O. Giuffrè, A. Napoli, A. Pettignano, D. Piazzese, Sequestering ability of some chelating agents towards methylmercury(II), Anal. Bioanal. Chem. 405 (2013) 881–893.

Their Compounds, Wiley-VCH, 2004, pp. 635–658. [26] C. Sgarlata, V. Zito, G. Arena, Conditions for calibration of an isothermal titration calorimeter using chemical reactions, Anal. Bioanal. Chem. 405 (2013) 1085–1094. [27] P. Gans, A. Sabatini, A. Vacca, Determination of equilibrium constants from spectrotophometric data obtained from solutions of known pH: the program pHab, Ann. Chim. 89 (1999) 45–49. [28] C. Frassineti, S. Ghelli, P. Gans, A. Sabatini, M.S. Moruzzi, A. Vacca, Nuclear magnetic resonance as a tool for determining protonation constants of natural polyprotic bases in solution, Anal. Biochem. 231 (1995) 374–382. [29] C. Frassineti, L. Alderighi, P. Gans, A. Sabatini, A. Vacca, S. Ghelli, Determination of protonation constants of some fluorinated polyamines by means of 13C NMR data processed by the new computer program HypNMR2000. Protonation sequence in polyamines, Anal. Bioanal. Chem. 376 (2003) 1041–1052. [30] C. De Stefano, S. Sammartano, P. Mineo, C. Rigano, Computer tools for the speciation of natural fluids, in: A. Gianguzza, E. Pelizzetti, S. Sammartano (Eds.), Marine Chemistry - An Environmental Analytical Chemistry Approach, Kluwer Academic Publishers, Amsterdam, 1997, pp. 71–83. [31] P.M. May, K. Murray, Database of chemical reactions designed to achieve thermodynamic consistency automatically, J. Chem. Eng. Data 46 (2001) 1035–1040. [32] L.D. Pettit, K.J. Powell, IUPAC Stability Constants Database, Academic Software, IUPAC, 2001. [33] A.E. Martell, R.M. Smith, R.J. Motekaitis, Critically Selected Stability Constants of Metal Complexes, National Institute of Standard and Technology, NIST, Gaithersburg, 2004. [34] V.K. Sharma, F. Casteran, F.J. Millero, C. De Stefano, Dissociation constants of protonated cysteine species in NaCl media, J. Solut. Chem. 31 (2002) 783–792. [35] C. Bretti, O. Giuffrè, G. Lando, S. Sammartano, Modeling solubility and acid-base properties of some amino acids in aqueous NaCl and (CH3)4NCl aqueous solutions at different ionic strengths and temperatures, SpringerPlus 5 (2016) 928. [36] P. Cardiano, O. Giuffrè, A. Napoli, S. Sammartano Potentiometric, 1H-NMR, ESI-MS investigation on dimethyltin(IV) cation-mercaptocarboxylate interaction in aqueous solution, New J. Chem. 33 (2009) 2286–2295. [37] P. Cardiano, D. Cucinotta, C. Foti, O. Giuffrè, S. Sammartano, Potentiometric, calorimetric and 1H-NMR investigation on Hg2 +-mercaptocarboxylate interaction in aqueous solution, J. Chem. Eng. Data 56 (2011) 1995–2004. [38] F. Crea, G. Falcone, C. Foti, O. Giuffrè, S. Materazzi, Thermodynamic data for Pb2 + and Zn2 + sequestration by biologically important S-donor ligands, at different temperatures and ionic strengths, New J. Chem. 38 (2014) 3973–3983. [39] P. Cardiano, C. De Stefano, O. Giuffrè, S. Sammartano, Thermodynamic and spectroscopic study for the interaction of dimethyltin(IV) with L-cysteine in aqueous solution, Biophys. Chem. 133 (2008) 19–27. [40] P. Cardiano, G. Falcone, C. Foti, O. Giuffrè, A. Napoli, Binding ability of glutathione towards alkyltin(IV) compounds in aqueous solution, J. Inorg. Biochem. 129 (2013) 84–93. [41] P. Cardiano, G. Falcone, C. Foti, O. Giuffrè, S. Sammartano, Methylmercury(II)sulphur containing ligand interactions: a potentiometric, calorimetric and 1H-NMR study in aqueous solution, New J. Chem. 35 (2011) 800–806. [42] P. Cardiano, G. Falcone, C. Foti, S. Sammartano, Sequestration of Hg2 + by some biologically important thiols, J. Chem. Eng. Data 56 (2011) 4741–4750.

19