Abstract
d-H2Pen-binding behavior to [VO(acac)2] at pH=7.00, 7.50 and 10.0 has been studied in thermodynamic viewpoint using UV/VIS spectroscopy. The optical absorption spectra of [VO(acac)2] were analyzed in order to obtain binding constants and stoichiometries using SQUAD software. The estimation of binding constant at various temperatures enabled us to calculate all of the thermodynamic parameters of binding using the van't Hoff equation. Studies of described reactions at pH=7.00, 7.50 and 10.0 show exothermic, endothermic and exothermic modality, respectively. d-penicillamine is a potentially tridentate ligand, but the pKa for the ‒ COOH, NH3 and S‒ H groups are 1.99–2.00, 8.0 and 10.6, respectively, as a result S‒H can not release proton at pH=7.00 and pH=7.50, so the coordination of S is not possible except at strong alkali mediums. At pH=7.00, d-H2Pen converts [VO(acac)2] to a vanadyl Schiff base complex (coordination mode is N2O2). At pH=7.50, d-H2Pen is converted to HPen− and the exchange of acac− with HPen− produces VO(HPen)2. It is clear that the coordination is formed via amine nitrogen and carboxylate oxygen. At pH=10.0, the main product is a vanadyl complex with the (S2O2) coordination mode. Studies in different ionic strengths of KCl confirm these products according to the number of ions in each medium. The formation constants of the products of VO(acac)2 with d-penicillamine at pH=7.00 and 7.50 are independent of ionic strength, but a Debye–Huckel-type equation was established for the dependence of the formation constant on ionic strength at pH=10.0.
1 Introduction
Numerous studies in experimental animals have shown that tolerable levels of vanadium-containing compounds can normalize plasma glucose, lipids and the thyroid hormone and at least partially counteract the increased oxidative stress that accompanies diabetes mellitus and contributes to secondary complications. Nonetheless, an obviously limited ‘window of optimal utility’[Citation1,Citation2] has spurred continuing efforts to further tailor the ligands used for vanadium complexation such that a greater potency and efficacy could be achieved. The coordination chemistry of vanadium with sulfur-containing ligands is an emerging field of interest with relevance to several disparate biological systems [Citation3–8]. The presence of vanadium–sulfur bonding in the active site of certain nitrogenase enzymes has been well established [Citation9,Citation10] and vanadium–sulfur coordination also appears to be pivotal to the well-known tyrosine phosphatase inhibition through binding to cysteine at the putative active site [Citation11–14]. In this research, the VO(IV)-binding capabilities to a type of amino acid (d-penicillamine) have been briefly surveyed and different coordination modes of the ligand have been suggested via the differences in thermodynamic entities at different pHs. In contrast with previous studies that VO(IV) ion has undergone different coordination modes facilities with different ligands or suggested structures of the coordination of a ligand with multiple kinds of potential donors to a vanadyl ion have been obtained with other spectral techniques [Citation15].
For VO(acac)2 four relatively low-intensity absorption bands in the 12,000–18,000 cm−1 region at low temperature (77°K) has been reported. It was proposed that these might possibly be the four d–d transition expected from the C2v symmetry of the molecule. These bands would be expected to be contained in the unresolved pair of bands in the region 12,000–14,000 cm−1 and the band at 17,000–18,000 cm−1 at room temperature. The second one seems be the last of the four d–d transitions, the exact positions being somewhat dependent upon the particular solvent used. Besides the band in the 25,000–26,000 cm−1 region would be the first of the expected charge transfer bands [Citation16].
On the ionic strength dependence of the formation constants for some stable complexes of amino acids by some metal ions, some interesting features of the function log K=f(I) have been reported [Citation17–23], where K and I refer to the formation constant and ionic media, respectively. In particular, all the formation constants seem to follow the same trend as a function of ionic strength, if allowance is made for different types of reaction stoichiometries and different charges of reactants and products. In determining a formation constant at a fixed ionic strength, in all cases, some uncertainties are always present. This fact is mainly due to the uncertainties in numerical values of stability constants.
d-penicillamine is a potentially tridentate ligand, but the pka for the ‒ COOH, NH3 and S‒H groups are 1.99–2.00, 8.0 and 10.6, respectively [Citation24]. The present work deals with the study of formation constant of the product of VO(acac)2 with d-penicillamine at pH=7.00, 7.50 and 10.0 at first in a constant ionic strength and then comparison of three mediums in an ionic strength range of 0–1.0 mol dm−3 potassium chloride. The parameters that define this dependency were analyzed with the aim of obtaining further information with regard to the differences of reactants and products’ charges at different pHs, in another research it has been shown that the product at neutral pH [Citation25] is a complex with a rather large ligand by joining d-penicillamine with acetylacetonate ligand and complexation at weak alkali and strong alkali has different thermodynamic entities, so it is likely that the formation constants of the products of VO(acac)2 with d-penicillamine at pH=7.00 and 7.50 are independent of ionic strength but a Debye–Huckel-type equation was established for the dependence of the formation constant on ionic strength at pH=10.0.
2 Experimental
2.1. Materials
Vanadyl(IV)acetylacetonate [VO(acac)2], vanadyl sulfate, d-penicillamine and C5H11NO2S were purchased from commercial sources (Aldrich or Fluka), KH2PO4 and were obtained from Merck and were used for making buffer with pH=7.00 and 7.50. Sodium hydroxide, NaOH and orthoboric acid, and H3BO3 (from Merck for making the buffer of Na2B4O7–NaOH with pH=10.0) were used as supplied.
2.2. Absorption spectra
The absorption spectra were recorded on a Shimadzu 1650 spectrophotometer using 1 cm quartz cuvettes, with a thermostat cell compartment that controls the temperature around the cell spectrophotometer. Titrations were carried out by adding 50 μL of aliquots portions of a d-penicillamine solution (about 0.2 M) directly into a quartz cell containing VO(acac)2 (about 0.02 M). The titration experiment was continued until the absorbance of the VO(acac)2 solution in the UV–VIS range remained constant. The spectra were recorded within the range of 500–900 nm. At first, the measurements were performed at buffers with pH=7.00, 7.50 and 10.0 at a constant ionic strength of KCl at six different temperatures (20°C, 25°C, 30°C, 35°C, 40°C and 45°C) and then these measurements were repeated in different ionic strengths (0.05, 0.1, 0.15, 0.2, 0.4, 0.6 and 0.8) mol dm−3 of KCl at 25°C. At the same condition, the absorption spectrum of vanadyl sulfate solution was recorded and titration with d-penicillamine has no effect on its absorbance spectrum.
3 Results and discussion
3.1. Thermodynamic studies
3.1.1. Formation constants
The valve of absorbance of and the concentration range of 0.01–0.025 M indicate that the absorbance obeys Beer's law in the range of 500–900 nm, and there are two peaks in this region. Evidence has shown that the two absorption bands in this region are due to ligand field (or d–d) transitions for V4+.
The general features of VO(acac)2 spectra at various H2Pen concentrations at pH=7.00 and 7.50 have no isobestic point but it has two isobestic points at pH=10.0 () which disappeared after a while confirming the existence of more than two species in the equilibrium which approves multiple access for coordination of Pen2− to vanadyl ion, which tends to produce one type of coordination mode (S2O2) at the end.
Figure 1. Absorption spectra of VO(acac)2 upon titration with d-H2Pen at pH=10.0 and I=0.1 mol dm−3 (KCl) at 25°C.
In order to analyze the spectral data of VO(acac)2 at various concentrations of H2Pen in titration experiments, 50 wavelengths showing suitable absorbance variations upon the addition of H2Pen were selected from the spectrum of VO(acac)2. The values of absorbance of these selected wavelengths at various H2Pen concentrations were analyzed in order to calculate equilibrium formation constants using SQUAD software. This program is designed to calculate the best values for the stability constants of the proposed equilibrium model by employing a non-linear least-square approach. This program is completely general in scope, having the capability to refine stability constants for the general complex , where m, l, n, q≥0 and j are positive for protons, negative (for hydroxyl ions) or zero. The algorithm employed is SQUAD program. The input data for the analysis of the VO(acac)2 H2Pen system were absorbance at 50 different wavelength of 15 VO(acac)2 spectra. These 15 spectra correspond to 15 various concentrations of H2Pen. Reactants and products have been shown in equations (1), (2) and (3) at pH=7.00, 7.50 and 10.0, respectively. The program also calculates the values of uncertainty in log kij. The results show that the best fitting corresponds to 2 : 1 complex model at all studied temperatures with sum of squares of reduced error between 10−2 and 10−3.
As the equations show, the number of ions on either side of equations (1) and (2) are constant but it is different in the case of equation (3). It is clear that the ionic strength of the medium has no effect on the formation constants of the reactions described by Equations (1) and (2) but there are different formation constants at various ionic strengths at pH=10.0 due to the difference in the number of ions on either side of the Equation (3). The outputs are the logarithm of equilibrium formation constant, log Kij, for the formation of [VOL2], [VO(Hpen)2] and [VO(pen)2]2− at pH=7.00 and 7.50 and 10.0, respectively. Equation (4) shows Kij for complex formation at pH=10.0.
Furthermore, according to the relationship between KC and Kγ, equation (5), concentration formation constant (Kc) as a function of (μ)1/2 showed that Kc increases up to 0.1 ionic strength and then decreases (). If μ is the ionic strength, equation (6), the relationship between activity coefficient and ionic strength is according to the extended Debye–Huckel equation at 25°C, equation (7) (ZA and ZB are cation and anion charges). On the other hand, there is an upright relationship between KC and ionic strength up to 0.1 ionic strengths and an inverse relationship in superior ionic strengths because the number of ions in products is more than the reactants at . It seems that a dianionic S–O type complex of VO(II) has been created
Figure 2. Kc vs. μ1/2 for [VO(Pen) 2]2− at 25°C.
![Figure 2. Kc vs. μ1/2 for [VO(Pen) 2]2− at 25°C.](/cms/asset/9c777286-6226-4688-9242-2c527a85def4/tcme_a_883489_f0002_c.jpg)
3.1.2. Thermodynamic constants
The equilibrium of VO(acac)2 with d-H2Pen at different mediums can be conveniently characterized by three familiar thermodynamic parameters: standard Gibbs free energy, Δ G0, enthalpy, Δ H0, and entropy, Δ S0, changes. The Δ G0 can be calculated from equilibrium constant, K, of the reaction using the familiar relationship, in which R and T referring to the gas constant and the absolute temperature, respectively. If heat capacity change of reaction is negligible, the van't Hoff equation (8) gives a linear plot of ln K versus 1/T. The Δ H0 can be calculated from the slope, Δ H0/R, and the Δ S0 from the intercept, Δ S0/R from equation (9)
Figure 3. The Van't Hoff plot for binding of VO(acac)2 to d-H2Pen in a buffer with pH=10.0 and I=0.1 mol dm−3(KCl)at various temperatures.
Table 1. Thermodynamic parameters for binding of [VO(acac)2] to d-H2Pen in a buffer with pH=10.0 and I=0.1 mol dm−3(KCl) at various temperatures.
At , the reaction of [VOL2] formation is an exothermic process, due to a decrease in entropy through converting two molecules to one molecule. Whereas titration of vanadyl sulfate with d-H2Pen showed no changes in UV/VIS spectra at
, it means there is no reaction between vanadyl sulfate(VOSO4) and d-H2Pen at neutral medium. It confirms the joining of acetylacetonate with d-H2Pen to create a Schiff base complex from the reaction of VO(acac)2 and D-H2Pen at
. Some spectroscopic techniques such as infrared and mass spectroscopy have been applied to verify the provided complex [Citation25], besides thermodynamic evidences and comparison with alkaline medium confirm the product.
At , d-H2Pen is converted to HPen− and the exchange of acac− with HPen− produces VO(HPen)2, it is clear that the coordination has been formed via amine nitrogen and carboxylate oxygen. This reaction is an endothermic and entropy-driven process. The hydration of HPen− is more than acac− because HPen− is a hydrophilic anion.
At pH=10, the product of the reaction of VO(acac)2 with d-H2Pen is [, this reaction is an exothermic process and the changes of entropy is negative. In this condition, vanadyl ion prefers thiolate ion to amine nitrogen for coordination.
According to the difference of thermodynamic natures of the reaction between VO(acac)2 and d-penicillamine at and
, the best suggested structure of the constituted complex at
is shown in Scheme 2. The computational calculations have been reported condensation magnitude of all atoms to vanadium ion [Citation26]. In some complexes with ammine, carboxylate and thiolate donors, have been seen that thiolate anion has stronger interaction than carboxylate anion and ammine nitrogen [Citation27].
Scheme 1. Ligand L from joining of D-penicillamine with acetylacetonate ligand.
Scheme 2. The suggested structure of the complex of [VO(Pen) 2]2−.
![Scheme 2. The suggested structure of the complex of [VO(Pen) 2]2−.](/cms/asset/3e4c8754-13ef-4aba-9653-1c304d729284/tcme_a_883489_f0005_b.gif)
4 Conclusion
d-penicillamine is a potentially tridentate ligand, but the pka for the –COOH, NH3 and S–H groups are 1.99–2.00, 8.0 and 10.6, respectively. d-penicillamine can not release H at and the coordination of S is not possible, but in strong alkali mediums for example at
, SH group releases H and creates a competition between carboxylate, ammine and thiolate groups to coordinate to vanadyl ion. According to the hard–soft theory, its coordination to soft sulfur donors is expected to have less possibility, nonetheless sulfur coordination occurs with macromolecules in biological systems and in a competition between thiolate, ammine and carboxylate groups to coordinate to a vanadyl ion, thiolate anion has the strongest interaction because thiolate behaves as an efficient anchoring donor, so that when d-penicillamine acts as a donor with carboxylate, ammine and thiolate groups, a vanadyl complex with the S2O2 coordination mode is expected. Because of the formation of more ions in products than source materials in the equilibrium at
, Kγ decreases and KC increases up to 0.1 ionic strength.
Acknowledgements
The support of the science department of Jahrom azad university is gratefully acknowledged.
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