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Influence of DMSO organic liquid media on the solution equilibria of 2,3-dihydroxybenzic acid
Journal of Molecular Liquids 300 (2020) 112349
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Influence of DMSO organic liquid media on the solution equilibria of 2,3-dihydroxybenzic acid Ahmed E. Fazary a,⁎, Nasser S. Awwad b, Hala A. Ibrahium b, Ali A. Shati c, Yi-Hsu Ju d,e,f a
Applied Research Department, Research and Development Sector, Egyptian Organization for Biological Products and Vaccines (VACSERA Holding Company), 51 Wezaret El-Zeraa St., Agouza, Giza, Egypt Research Centre for Advanced Materials Science (RCAMS), King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia c Biology Department, Faculty of Science, King Khalid University, Abha 9004, Saudi Arabia d Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, 43 Section 4, Keelung Road, Taipei 10607, Taiwan e Department of Chemical Engineering, National Taiwan University of Science and Technology, 43 Section 4 Keelung Road, Taipei 10607, Taiwan f Taiwan Building Technology Center, National Taiwan University of Science and Technology, 43 Section 4 Keelung Road, Taipei 10607, Taiwan b
a r t i c l e
i n f o
Article history: Received 28 October 2019 Received in revised form 13 December 2019 Accepted 18 December 2019 Available online 20 December 2019 Keywords: 2,3-Dihydroxybenzic acid DMSO Protonation constants Potentiometry Spectrophotometry
a b s t r a c t The protonation equilibrium constants of 2,3-dihydroxybenzic acid (DA) in aqueous solutions was achieved by the HYPERQUAD 2008 software estimation from the pH-potentiometric and spectrophotometeric titration data, which present a multiplicity of statistics arrangements. Aa assessment of the influence of the organic solvent dimethylsulfoxide (DMSO) on the association and protonation processes was also testified and elucidated. The solution equilibria of 2,3-dihydroxybenzic acid was studied at 298.15 K in a water + DMSO liquid mixtures [100 wDMSO = 20%, 40%, 60%, and 80%] with an ionic strength of 0.16 mol·dm−3. The effect of DMSO on the protonation processes was measured and explained. The spectrophotometry evaluation was also done for 2,3-dihydroxybenzic acid bioligand at different pH stages in aqueous solutions and its absorbance ratio was also detected. © 2019 Elsevier B.V. All rights reserved.
1. Introduction 2,3-Dihydroxybenzic acid (DA) is a phenolic bioactive compound which can be found in natural sources such as aquatic fern (Salvinia molesta), lobi-lobi fruit (Flacourtia inermis), and gooseberry (Phyllanthus acidus) [1,2]. DA is also found in human blood plasma and urine as the product of aspirin metabolism [3]. Study by George et al. showed that DA has antimicrobial activity against microorganisms such as Serratia marcescens, Escherichia coli, and Staphylococcus aureus [1]. DA is also known for its ability to form stable chelate complexes with metal ions such as La3+, Al3+, Cr3+, Zn2+, Cu2+, Ni2+, and Co2+ [4–7]. The water and qua-DMSO organic liquid equilibria studies of 2,3dihydroxybenzic acid have attracted our attention to give details about their chemical structure-function association, from which the information of the protonation equilibrium constants and diverse ionization and dissociation states of 2,3-dihydroxybenzic acid, would be precious in drug research and development to understand DA bioligand pharmacokinetics and pharmacodynamics to manage its solubility, absorption, and delivery in the human body, and how much DMSO organic solvent influence the drug biofunction and stability. Throughout the last ten years, there is a many publications concerned in an incessant research ⁎ Corresponding author. E-mail address: [email protected] (A.E. Fazary).
https://doi.org/10.1016/j.molliq.2019.112349 0167-7322/© 2019 Elsevier B.V. All rights reserved.
work leaning towards the study of the aqueous and non-aqueous solution equilibria of many bimolecules with diverse chelating ligand property [8–17]. Consequently, the solution equilibria of 2,3-dihydroxybenzic acid is of particular attention in the present work presented in this paper. 2. Experimental section 2.1. Materials, chemicals, and solutions All chemicals and solvents were of analytical grade and used as provided by the suppliers without further purification (Table 1). The primary material used in this study was 2,3-dihydroxybenzoic acid (DA, C7H6O4, 99% purity) was obtained from Sigma Aldrich (St. Louis, MO, USA). The organic solvent DMSO was of high quality and analytical GC/Spectro grade and supplied from Sigma Aldrich, UK. Sodium hydroxide (NaOH, 96% purity, Yakuri Pure Chemical, Kyoto, Japan) used in titration was first standardized by using potassium hydrogen phthalate (KHP, 99.95% purity, Sigma Aldrich St. Louis, MO). Hydrochloric acid (37.6% purity, Fischer Scientific, Bridgewater, NJ) used to acidify the solution was standardized against NaOH. Ionic strength of the solutions was maintained by using sodium chloride (NaNO3, 99.5% purity, Showa, Tokyo, Japan). All solutions were prepared freshly before use, using ultra-pure water obtained from an ultrapure water system with a resistance of 18.3 MΩ·cm−1.
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A.E. Fazary et al. / Journal of Molecular Liquids 300 (2020) 112349
Table 1 Supplier, purities and pretreatments of the chemicals used in this work. Chemical substance
Supplier
Purity % (mass fraction)
Pretreatment
2,3-Dihydroxybenzoic acid (DA, C7H6O4) Dimethyl sulfoxide (DMSO) Hydrochloric acid Sodium nitrate Sodium hydroxide Standard pH buffering solutions Potassium hydrogen phthalate Nitric acid
Sigma Aldrich, St. Louis, MO, USA Sigma Aldrich, UK Fischer Scientific, Bridgewater, NJ, USA Showa, Tokyo, Japan Yakuri Pure Chemical, Kyoto, Japan Across Organics, USA Sigma Aldrich, USA Panreac, Spain
≥99% ≥99.95% ≥37.6% ≥99.50% ≥96% ~98.00% ≥98.00% ~65.00%
Storage in vacuum desiccator at room temperature Storage in room temperature Storage in room temperature Storage in vacuum desiccator at room temperature Storage in vacuum desiccator at room temperature Storage at room temperature Storage in vacuum desiccator at room temperature Storage at room temperature
2.2. Potentiometry and spectrophotometry measurements As done in our previously published solution and molecular liquid studies [8–17], pH-Potentiometric measurements were done using an autotitrator Metrohm 888 Titrando, supported by a 805 Dosimat, ion selective electrodes “Ecotrode Plus” and a 802 rod stirrer with 804 Ti stand [18]. As well, spectrophotometry titrations and measurements were achieved by using a Jasco V-550 spectrophotometer. The tested solutions were located in a standard 10 mm path length quartz glass cuvettes and the spectrum were measured in the wavelength range of 200–400 nm. For the measurements of protonation equilibrium constants, the solutions of bioligand DA were prepared. pH-potentiometric titrations were carried out in a 150 cm3 double walled glass vessel connected with a refrigerated circulating bath to uphold constant temperature. The tested solutions contained an appropriate proportion (w/w) of the DMSO organic solvent studied. Each solution was set in a volume of 50 mL and was major acidify to pH ≈ 2.5 by adding drops of 3 × 10−2 mol·L−1 HCl acid. The ionic strength (I) was preserved at 0.16 mol·L−1 by using NaNO3 solution. The solution was left to stand at 25 °C for 5 min before titration. Free carbonate sodium hydroxide was prepared at 1 × 10−1 mol·L−1 as the titrant to alter the pH until 11.0. In pHpotentiometric technique, the volume increase injected from microsyringe was sustained at 5 μL to obtain the titration data points up to 100 point with an addition of 5 s. Most of the titration measurements were monitored and recorded by TiNet 2.4 titration software [18]. Calibration of the glass electrode was done before and after each runs of the measurements by using three buffer solutions; pH 4.01 (0.05 mol·L−1 potassium hydrogen phthalate), 6.87 (0.05 mol dm−3 disodium hydrogen phosphate + 0.025 mol dm−3 potassium dihydrogen phosphate) and 9.18 (0.01 mol·dm−3 sodium tetraborate decahydrate) as known to IUPAC reference and by the aid of Glee Software [19]. All organizing buffer solutions were prepared in similar ionic strength as in the reputable solutions (0.16 mol·L−1 NaNO3). Triplicate titrations were done for each process with a reproducibility ± 0.02 in pH unit. The equilibrium constants from pH-potentiometric methods were developed by using the Hyperquad 2008 program [20]. As well, HypSpec software overtakes both Hyperquad 2006 and pHab software's for the calculations of equilibrium constants (pKa values) from spectrophotometric titration data as it combines and improves the functionality of both programs [21].
2.3. Quantum chemical calculations The alteration of protonation equilibrium constants of DA by Hyperquad 2006 are a non-linear process. The constraints in these programs that should be applied to obtain a good refinement result are: (1) The goodness of fitting is stated as standard deviation value obtained from the difference between theoretical calculated and experimental data, in which the value should be b10% to obtain a valid log10β; (2) For Hypequad 2008, the electrode calibration always obeys the Nernst Law: E = E0 + f (RT/F) log10[H+], where E is the standard potential, f is slope factor, F is Faraday constant. To interpretation for the alterations in acidity, basicity, dielectric constant, and ion activities for
moderately aqua-organic media comparative to the pure aqueous ones, pH values of each former solutions were modified by gathering the usage of the procedure designated by Douheret [22,23]. Computational density functional theory and quantum calculations were done using Gaussian 09 program to study the dissociation processes behavior of DA molecule in aqueous and non-aqueous solutions [22–32].
3. Results and discussions DA is a triprotic ligand with three binding oxygen donor functional groups starts to dissociate at the carboxyl (\\COO) group with pKa1 of 2.72, followed by two hydroxyl (OH\\) groups at meta and ortho positions with pKa2 value of 10.03 and pKa3 value of 13.00, respectively (Table 2). This dissociation process occurred stepwise with increasing pH, and can be expressed by the following equations: H3DA ↔ H2DA− + H+; [H+][H2DA−] = Ka1[H3DA]
H2DA− ↔ HDA2− + H+; [H+][HDA2−] = Ka2[H2DA−]
HDA2− ↔ DA3− + H+; [H+][DA3−] = Ka3[HDA2−] The dissociation process of DA could be schematically depicted in Fig. 1. From these results, we stated that protonation equilibrium constants are precise by the electronic effects of the substituent groups. The first and second proton dissociation constants (pKa1, and pKa2) values compared with those of previous research in aqueous solutions displayed that, the present results agreed within a very rational range. The maximum deviation in all values (Table 2) was found to be 0.19 units. Such deviations between the results could be attributed to the variety of experimental conditions [6,33,34].As shown in Table 2, the first dissociation occurs at lower value (pKa1) than the second dissociation value (pKa2). While for ortho\\O, the pKa3 occurs at highly basicenvironment and cannot be obtained accurately by potentiometric method due to accuracy limitation. The ortho OH group tends to be more nucleophilic due to benzene substituent effect and not easily dissociated [6,33–35].
Table 2 Acid dissociation constants (pKa's values) of 2,3-dihydroxybenzoic acid. Functional group
COOH meta OH ortho OH a b c d e
pKa Our work
Ref. [6]c
Ref. [34]d
Ref. [35]e
2.72 ± 0.03a 10.03 ± 0.02a 13.00 ± 0.02b
2.66 9.80 N14
2.70 9.76 13.0
2.70 10.06 13.1
Potentiometric, I = 0.16 mol∙dm−3 NaNO3, T = 298 K. Spectrophotometric, I = 0.16 mol∙dm−3 NaNO3, T = 298 K. Potentiometric, I = 0.2 mol∙dm−3 KCl, T = 25 °C. Potentiometric, I = 1.0 mol∙dm−3 NaClO4, T = 25 °C. Potentiometric, I = 0.02–0.13 mol∙dm−3 NaCl, T = 25 °C.
A.E. Fazary et al. / Journal of Molecular Liquids 300 (2020) 112349
HO
3
OH
HO
O
Fig. 1. Molecular Structure of 2,3-Dihydroxybenzic Acid (DA), its Computer-Generated Model, and its Protonation Equilibria.
Thus, spectrophotometric method was chosen instead of potentiometric to determine the pKa3 value which was found to be 13.00. The effect of DMSO organic solvent on the pKa values of DA can be explained by the solvation chromic values of Kamlet–Taft hydrogen bond and dipolarity polarizability π⁎ of DMSO organic solvent [36–38]. The investigational reliability in dissociation constants values (pKa 's) for DA in a variety of non-aqueous water (1) + DMSO (2) solution mixtures can fundamentally elucidated as substantial from the nonaqueous solutions of DMSO/water is much basic than water only explaining why higher pKa constants values were determined in aqueous solutions compared to non-aqueous one (Table 3). The errors persuaded in the measurements of pKa values are reflected in Gibbs free energy changes (ΔGo) values calculated using DFT quantum chemical calculations (Table 4). Consequently an estimation of error is
Table 3 Acid dissociation constants (pKa values) of 2,3-dihydroxybenzoic acid in different Water (1) + DMSO (2) Mixture at T = 298.15 K and I = 0.16 mol·dm−3 NaNO3. Functional group
COOH meta OH ortho OH
necessary to show how dependable are these results. The key alterations between the different procedures for the study of solution equilibria in water solutions and in non-aqueous solutions are owing to the activity coefficients. As in all equilibria in non-aqueous solutions, a background electrolyte is added to keep constant ionic strength fluctuating from about (0.1 to 3.0) mol·dm−3 NaNO3. This is permitted in some nonaqueous solution mixtures, nevertheless not in organic solvents of low dielectric constants wherever the solubility of electrolytes is very small. Negative values of ΔG° (Table 4) for dissociation of ligands showing spontaneous and exothermic behaviors suggesting that lower temperature is better for the protonation equilibria processes. Absorbance ratio analysis of the systems at different pH can give a brief estimation of pKa values. This analysis also can confirm the formation of metal ligand complexes, where the formation is indicated by the inflections in the curve. This analysis can be done by observing the ratio between the hypochromic (lower absorbance) to hyperchromic (higher absorbance) band. Some of the systems below are analyzed as representative, specifically, at acidic pH (Fig. 2), the inflection of DA was indicated
pKa 100 wDMSO (%) 20%
40%
60%
80%
2.58 ± 0.03 9.94 ± 0.04 12.89 ± 0.02
2.39 ± 0.04 9.73 ± 0.04 12.60 ± 0.06
2.11 ± 0.03 9.62 ± 0.05 12.17 ± 0.08
2.02 ± 0.06 9.51 ± 0.04 11.83 ± 0.06
All the values were calculated with the programs Hyperquad 2008 and HypSpec from the pH-potentiometric, and spectrophotometric titration data measurements, respectively. The experimental pressure is (101.3 ± 10.0) kPa. Standard uncertainty u is u (T) 0.15 K. The reported uncertainties are combined expanded uncertainties at 0.95 level of confidence (k = 2). Uncertainties are the maximum deviation of one measurement from the average of three independent measurements. Uncertainties were estimated by propagating the errors in all of the parameters in the appropriate equation assuming that they were independent.
Table 4 Gibbs free energy change ΔG° for the dissociation processes of 2,3-dihydroxybenzoic acid in different Water (1) + DMSO (2) Mixture at T = 298.15 K and I = 0.16 mol·dm−3 NaNO3 obtained from optimization and frequency calculation by using Gaussian 09 program. Functional group
COOH meta OH ortho OH
ΔG° (kJ mol−1)a 0%
20%
40%
60%
80%
15,524.92 57,248.15 74,199.10
14,725.85 56,734.46 73,572.50
13,641.38 55,535.84 71,916.92
12,043.22 54,908.00 69,462.61
11,529.54 54,280.15 67,522.00
a The calculation was done using density functional theory (DFT)-B3LYP method combined with 6–31 + G(d) as a basis set.
Fig. 2. UV–visible spectrum of the ligand DA.
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A.E. Fazary et al. / Journal of Molecular Liquids 300 (2020) 112349
at pH around 2.0–3.0, suggesting that the pKa1 value of DA at acidic pH is approximately 2.0 N pKa1 N 3.0, where precisely the value is 2.63 as determined by the equilibrium constant program (Fig. 1). Meanwhile, at basic pH, the inflection occurs around pH 9.0–10.0 and the determined pKa2 value from the program is 9.98 (Figs. 3 and 4). Subsequently, at highly basic pH, the first inflection is attributed to pKa2. As shown, pKa3 values is indicated by the second inflection point with the pH value around 13.0 (Fig. 4).
4. Concluding remarks The learning of the protonation equilibria of DA could be a significant data in the development of quantification and biological systems in aqueous and aqua-organic media. To differentiate which formula of a drug species be real in which ratio in a persuaded aqua or aqua-organic solvent solutions is certainly of excessive usage in choosing the accurate biological technique through the quantification processes. Finally, DA appears to be a probable decent metal chelating triprotic bioligand drug as the Gaussian calculations showed three sites for binding to metal ions.
Fig. 3. UV–visible spectrum of the ligand DA at highly basic pH N 10.0.
Fig. 4. Absorbance ratio of ligand DA at lower, medium and high basic pH values.
A.E. Fazary et al. / Journal of Molecular Liquids 300 (2020) 112349
Author's statement All authors read and approved the final revised manuscript. Professor Ahmed E. Fazary was a major contributor in writing the manuscript and processing analytical data, designed and led this research. Hala A. Ibrahium, and Ali A. Shati designed and performed the experiments. Nasser S Awwad via Research Center for Advanced Materials (RCAMS) at King Khalid University supported this work. Professor YiHsu Ju made final editing and proofreading of the manuscript. Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. All the values were calculated with the programs Hyperquad 2008 and HypSpec from the pH-potentiometric, and spectrophotometric titration data measurements, respectively. The experimental pressure is (101.3 ± 10.0) kPa. Standard uncertainty u is u (T) 0.15 K. The reported uncertainties are combined expanded uncertainties at 0.95 level of confidence (k = 2). Uncertainties are the maximum deviation of one measurement from the average of three independent measurements. Uncertainties were estimated by propagating the errors in all of the parameters in the appropriate equation assuming that they were independent. Acknowledgement The authors extend their appreciation to the Research Center for Advanced Materials (RCAMS) at King Khalid University, Saudi Arabia for supporting this work through research groups program under grant number RCAMS/KKU/006-19. References [1] S. George, P.J. Benny, S. Kuriakose, C. George, S. Gopalakrishnan, Antiprotozoal activity of 2,3-dihydroxybenzoic acid isolated from the fruit extracts of Flacourtia inermis Robx, Asian J. Pharm. Clin. Res. 3 (2011) 237–241. [2] M. Sousa, J. Ousingsawat, R. Seitz, S. Puntheeranurak, A. Regalado, A. Schmidt, T. Grego, C. Jansakul, M.D. Amaral, R. Schreiber, K. Kunzelmann, An extract from the medicinal plant Phyllanthus acidus and its isolated compounds induce airway chloride secretion: a potential treatment for cystic fibrosis, Mol. Pharmacol. 71 (2006) 366–376. [3] M. Grootveld, B. Halliwell, 2,3-Dihydroxybenzoic acid is a product of human aspirin metabolism, Biochem. Pharmacol. 37 (1988) 271–280. [4] T. Cam, G. Irez, R. Aydin, Determination of stability constants of mixed ligand complexes of the lanthanum(III) ion and identification of structures, J. Chem. Eng. Data 56 (2011) 1813–1820. [5] B.Y. Hirpaye, G.N. Rao, Chemical speciation of 2,3-dihydroxybenzoic acid complexes with some biologically essential metal ions in 1,2-propanediol-water mixtures, Chem. Spec. Biolavailab. 25 (2013) 179–186. [6] T. Kiss, H. Kozlowski, G. Micera, L.S. Erre, Copper(II) complexes of 2,3dihydroxybenzoic acid, J. Coord. Chem. 20 (1989) 49–56. [7] N. Turkel, M. Berker, U. Ozer, Potentiometric and spectroscopic studies on aluminium(III) complexes of some catechol derivatives, Chem. Pharm. Bull. 52 (8) (2004) 929–934. [8] A.E. Fazary, A.S. Al-Shihri, K.A. Saleh, M.Y. Alfaifi, M.A. Alshehri, S.E.I. Elbehairi, Diand tri-valent metal ions interactions with four biodegradable hydroxamate and cataecholate siderophores: new insights into their complexation equilibria, J. Solut. Chem. 45 (5) (2016) 732–749. [9] A.E. Fazary, A.M. Ramadan, Stability constants and complex formation equilibria between iron, calcium, and zinc metal ions with vitamin B9 and glycine, Complex Met 1 (1) (2014) 139–148. [10] A.Y. Rajhi, Y.-H. Ju, A.E. Angkawijaya, A.E. Fazary, Complex formation equilibria and molecular structure of divalent metal ions-vitamin B3-glycine oligopeptides systems, J. Sol. Chem. 42 (12) (2013) 2409–2442. [11] A.E. Angkawijaya, A.E. Fazary, S. Ismadji, Y.-H. Ju, Cu(II), Co(II), and Ni(II)-antioxidative phenolate-glycine peptide systems: an insight into its equilibrium solution study, J. Chem. Eng. Data 57 (12) (2012) 3443–3451. [12] A.E. Angkawijaya, A.E. Fazary, E. Hernowo, S. Ismadji, Y.-H. Ju, Nickel and cobalt complexes of non-protein l-norvaline and antioxidant ferulic acid: potentiometric and spectrophotometric studies, J. Sol. Chem. 41 (7) (2012) 1156–1164. [13] E. Hernowo, A.E. Angkawijaya, A.E. Fazary, S. Ismadji, Y.-H. Ju, Complex stability and molecular structure studies of divalent metal ion with L-norleucine and vitamin b3, J. Chem. Eng. Data 56 (12) (2011) 4549–4555.
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Influence of DMSO organic liquid media on the solution equilibria of 2,3-dihydroxybenzic acid Ahmed E. Fazary a,⁎, Nasser S. Awwad b, Hala A. Ibrahium b, Ali A. Shati c, Yi-Hsu Ju d,e,f a
Applied Research Department, Research and Development Sector, Egyptian Organization for Biological Products and Vaccines (VACSERA Holding Company), 51 Wezaret El-Zeraa St., Agouza, Giza, Egypt Research Centre for Advanced Materials Science (RCAMS), King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia c Biology Department, Faculty of Science, King Khalid University, Abha 9004, Saudi Arabia d Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, 43 Section 4, Keelung Road, Taipei 10607, Taiwan e Department of Chemical Engineering, National Taiwan University of Science and Technology, 43 Section 4 Keelung Road, Taipei 10607, Taiwan f Taiwan Building Technology Center, National Taiwan University of Science and Technology, 43 Section 4 Keelung Road, Taipei 10607, Taiwan b
a r t i c l e
i n f o
Article history: Received 28 October 2019 Received in revised form 13 December 2019 Accepted 18 December 2019 Available online 20 December 2019 Keywords: 2,3-Dihydroxybenzic acid DMSO Protonation constants Potentiometry Spectrophotometry
a b s t r a c t The protonation equilibrium constants of 2,3-dihydroxybenzic acid (DA) in aqueous solutions was achieved by the HYPERQUAD 2008 software estimation from the pH-potentiometric and spectrophotometeric titration data, which present a multiplicity of statistics arrangements. Aa assessment of the influence of the organic solvent dimethylsulfoxide (DMSO) on the association and protonation processes was also testified and elucidated. The solution equilibria of 2,3-dihydroxybenzic acid was studied at 298.15 K in a water + DMSO liquid mixtures [100 wDMSO = 20%, 40%, 60%, and 80%] with an ionic strength of 0.16 mol·dm−3. The effect of DMSO on the protonation processes was measured and explained. The spectrophotometry evaluation was also done for 2,3-dihydroxybenzic acid bioligand at different pH stages in aqueous solutions and its absorbance ratio was also detected. © 2019 Elsevier B.V. All rights reserved.
1. Introduction 2,3-Dihydroxybenzic acid (DA) is a phenolic bioactive compound which can be found in natural sources such as aquatic fern (Salvinia molesta), lobi-lobi fruit (Flacourtia inermis), and gooseberry (Phyllanthus acidus) [1,2]. DA is also found in human blood plasma and urine as the product of aspirin metabolism [3]. Study by George et al. showed that DA has antimicrobial activity against microorganisms such as Serratia marcescens, Escherichia coli, and Staphylococcus aureus [1]. DA is also known for its ability to form stable chelate complexes with metal ions such as La3+, Al3+, Cr3+, Zn2+, Cu2+, Ni2+, and Co2+ [4–7]. The water and qua-DMSO organic liquid equilibria studies of 2,3dihydroxybenzic acid have attracted our attention to give details about their chemical structure-function association, from which the information of the protonation equilibrium constants and diverse ionization and dissociation states of 2,3-dihydroxybenzic acid, would be precious in drug research and development to understand DA bioligand pharmacokinetics and pharmacodynamics to manage its solubility, absorption, and delivery in the human body, and how much DMSO organic solvent influence the drug biofunction and stability. Throughout the last ten years, there is a many publications concerned in an incessant research ⁎ Corresponding author. E-mail address: [email protected] (A.E. Fazary).
https://doi.org/10.1016/j.molliq.2019.112349 0167-7322/© 2019 Elsevier B.V. All rights reserved.
work leaning towards the study of the aqueous and non-aqueous solution equilibria of many bimolecules with diverse chelating ligand property [8–17]. Consequently, the solution equilibria of 2,3-dihydroxybenzic acid is of particular attention in the present work presented in this paper. 2. Experimental section 2.1. Materials, chemicals, and solutions All chemicals and solvents were of analytical grade and used as provided by the suppliers without further purification (Table 1). The primary material used in this study was 2,3-dihydroxybenzoic acid (DA, C7H6O4, 99% purity) was obtained from Sigma Aldrich (St. Louis, MO, USA). The organic solvent DMSO was of high quality and analytical GC/Spectro grade and supplied from Sigma Aldrich, UK. Sodium hydroxide (NaOH, 96% purity, Yakuri Pure Chemical, Kyoto, Japan) used in titration was first standardized by using potassium hydrogen phthalate (KHP, 99.95% purity, Sigma Aldrich St. Louis, MO). Hydrochloric acid (37.6% purity, Fischer Scientific, Bridgewater, NJ) used to acidify the solution was standardized against NaOH. Ionic strength of the solutions was maintained by using sodium chloride (NaNO3, 99.5% purity, Showa, Tokyo, Japan). All solutions were prepared freshly before use, using ultra-pure water obtained from an ultrapure water system with a resistance of 18.3 MΩ·cm−1.
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Table 1 Supplier, purities and pretreatments of the chemicals used in this work. Chemical substance
Supplier
Purity % (mass fraction)
Pretreatment
2,3-Dihydroxybenzoic acid (DA, C7H6O4) Dimethyl sulfoxide (DMSO) Hydrochloric acid Sodium nitrate Sodium hydroxide Standard pH buffering solutions Potassium hydrogen phthalate Nitric acid
Sigma Aldrich, St. Louis, MO, USA Sigma Aldrich, UK Fischer Scientific, Bridgewater, NJ, USA Showa, Tokyo, Japan Yakuri Pure Chemical, Kyoto, Japan Across Organics, USA Sigma Aldrich, USA Panreac, Spain
≥99% ≥99.95% ≥37.6% ≥99.50% ≥96% ~98.00% ≥98.00% ~65.00%
Storage in vacuum desiccator at room temperature Storage in room temperature Storage in room temperature Storage in vacuum desiccator at room temperature Storage in vacuum desiccator at room temperature Storage at room temperature Storage in vacuum desiccator at room temperature Storage at room temperature
2.2. Potentiometry and spectrophotometry measurements As done in our previously published solution and molecular liquid studies [8–17], pH-Potentiometric measurements were done using an autotitrator Metrohm 888 Titrando, supported by a 805 Dosimat, ion selective electrodes “Ecotrode Plus” and a 802 rod stirrer with 804 Ti stand [18]. As well, spectrophotometry titrations and measurements were achieved by using a Jasco V-550 spectrophotometer. The tested solutions were located in a standard 10 mm path length quartz glass cuvettes and the spectrum were measured in the wavelength range of 200–400 nm. For the measurements of protonation equilibrium constants, the solutions of bioligand DA were prepared. pH-potentiometric titrations were carried out in a 150 cm3 double walled glass vessel connected with a refrigerated circulating bath to uphold constant temperature. The tested solutions contained an appropriate proportion (w/w) of the DMSO organic solvent studied. Each solution was set in a volume of 50 mL and was major acidify to pH ≈ 2.5 by adding drops of 3 × 10−2 mol·L−1 HCl acid. The ionic strength (I) was preserved at 0.16 mol·L−1 by using NaNO3 solution. The solution was left to stand at 25 °C for 5 min before titration. Free carbonate sodium hydroxide was prepared at 1 × 10−1 mol·L−1 as the titrant to alter the pH until 11.0. In pHpotentiometric technique, the volume increase injected from microsyringe was sustained at 5 μL to obtain the titration data points up to 100 point with an addition of 5 s. Most of the titration measurements were monitored and recorded by TiNet 2.4 titration software [18]. Calibration of the glass electrode was done before and after each runs of the measurements by using three buffer solutions; pH 4.01 (0.05 mol·L−1 potassium hydrogen phthalate), 6.87 (0.05 mol dm−3 disodium hydrogen phosphate + 0.025 mol dm−3 potassium dihydrogen phosphate) and 9.18 (0.01 mol·dm−3 sodium tetraborate decahydrate) as known to IUPAC reference and by the aid of Glee Software [19]. All organizing buffer solutions were prepared in similar ionic strength as in the reputable solutions (0.16 mol·L−1 NaNO3). Triplicate titrations were done for each process with a reproducibility ± 0.02 in pH unit. The equilibrium constants from pH-potentiometric methods were developed by using the Hyperquad 2008 program [20]. As well, HypSpec software overtakes both Hyperquad 2006 and pHab software's for the calculations of equilibrium constants (pKa values) from spectrophotometric titration data as it combines and improves the functionality of both programs [21].
2.3. Quantum chemical calculations The alteration of protonation equilibrium constants of DA by Hyperquad 2006 are a non-linear process. The constraints in these programs that should be applied to obtain a good refinement result are: (1) The goodness of fitting is stated as standard deviation value obtained from the difference between theoretical calculated and experimental data, in which the value should be b10% to obtain a valid log10β; (2) For Hypequad 2008, the electrode calibration always obeys the Nernst Law: E = E0 + f (RT/F) log10[H+], where E is the standard potential, f is slope factor, F is Faraday constant. To interpretation for the alterations in acidity, basicity, dielectric constant, and ion activities for
moderately aqua-organic media comparative to the pure aqueous ones, pH values of each former solutions were modified by gathering the usage of the procedure designated by Douheret [22,23]. Computational density functional theory and quantum calculations were done using Gaussian 09 program to study the dissociation processes behavior of DA molecule in aqueous and non-aqueous solutions [22–32].
3. Results and discussions DA is a triprotic ligand with three binding oxygen donor functional groups starts to dissociate at the carboxyl (\\COO) group with pKa1 of 2.72, followed by two hydroxyl (OH\\) groups at meta and ortho positions with pKa2 value of 10.03 and pKa3 value of 13.00, respectively (Table 2). This dissociation process occurred stepwise with increasing pH, and can be expressed by the following equations: H3DA ↔ H2DA− + H+; [H+][H2DA−] = Ka1[H3DA]
H2DA− ↔ HDA2− + H+; [H+][HDA2−] = Ka2[H2DA−]
HDA2− ↔ DA3− + H+; [H+][DA3−] = Ka3[HDA2−] The dissociation process of DA could be schematically depicted in Fig. 1. From these results, we stated that protonation equilibrium constants are precise by the electronic effects of the substituent groups. The first and second proton dissociation constants (pKa1, and pKa2) values compared with those of previous research in aqueous solutions displayed that, the present results agreed within a very rational range. The maximum deviation in all values (Table 2) was found to be 0.19 units. Such deviations between the results could be attributed to the variety of experimental conditions [6,33,34].As shown in Table 2, the first dissociation occurs at lower value (pKa1) than the second dissociation value (pKa2). While for ortho\\O, the pKa3 occurs at highly basicenvironment and cannot be obtained accurately by potentiometric method due to accuracy limitation. The ortho OH group tends to be more nucleophilic due to benzene substituent effect and not easily dissociated [6,33–35].
Table 2 Acid dissociation constants (pKa's values) of 2,3-dihydroxybenzoic acid. Functional group
COOH meta OH ortho OH a b c d e
pKa Our work
Ref. [6]c
Ref. [34]d
Ref. [35]e
2.72 ± 0.03a 10.03 ± 0.02a 13.00 ± 0.02b
2.66 9.80 N14
2.70 9.76 13.0
2.70 10.06 13.1
Potentiometric, I = 0.16 mol∙dm−3 NaNO3, T = 298 K. Spectrophotometric, I = 0.16 mol∙dm−3 NaNO3, T = 298 K. Potentiometric, I = 0.2 mol∙dm−3 KCl, T = 25 °C. Potentiometric, I = 1.0 mol∙dm−3 NaClO4, T = 25 °C. Potentiometric, I = 0.02–0.13 mol∙dm−3 NaCl, T = 25 °C.
A.E. Fazary et al. / Journal of Molecular Liquids 300 (2020) 112349
HO
3
OH
HO
O
Fig. 1. Molecular Structure of 2,3-Dihydroxybenzic Acid (DA), its Computer-Generated Model, and its Protonation Equilibria.
Thus, spectrophotometric method was chosen instead of potentiometric to determine the pKa3 value which was found to be 13.00. The effect of DMSO organic solvent on the pKa values of DA can be explained by the solvation chromic values of Kamlet–Taft hydrogen bond and dipolarity polarizability π⁎ of DMSO organic solvent [36–38]. The investigational reliability in dissociation constants values (pKa 's) for DA in a variety of non-aqueous water (1) + DMSO (2) solution mixtures can fundamentally elucidated as substantial from the nonaqueous solutions of DMSO/water is much basic than water only explaining why higher pKa constants values were determined in aqueous solutions compared to non-aqueous one (Table 3). The errors persuaded in the measurements of pKa values are reflected in Gibbs free energy changes (ΔGo) values calculated using DFT quantum chemical calculations (Table 4). Consequently an estimation of error is
Table 3 Acid dissociation constants (pKa values) of 2,3-dihydroxybenzoic acid in different Water (1) + DMSO (2) Mixture at T = 298.15 K and I = 0.16 mol·dm−3 NaNO3. Functional group
COOH meta OH ortho OH
necessary to show how dependable are these results. The key alterations between the different procedures for the study of solution equilibria in water solutions and in non-aqueous solutions are owing to the activity coefficients. As in all equilibria in non-aqueous solutions, a background electrolyte is added to keep constant ionic strength fluctuating from about (0.1 to 3.0) mol·dm−3 NaNO3. This is permitted in some nonaqueous solution mixtures, nevertheless not in organic solvents of low dielectric constants wherever the solubility of electrolytes is very small. Negative values of ΔG° (Table 4) for dissociation of ligands showing spontaneous and exothermic behaviors suggesting that lower temperature is better for the protonation equilibria processes. Absorbance ratio analysis of the systems at different pH can give a brief estimation of pKa values. This analysis also can confirm the formation of metal ligand complexes, where the formation is indicated by the inflections in the curve. This analysis can be done by observing the ratio between the hypochromic (lower absorbance) to hyperchromic (higher absorbance) band. Some of the systems below are analyzed as representative, specifically, at acidic pH (Fig. 2), the inflection of DA was indicated
pKa 100 wDMSO (%) 20%
40%
60%
80%
2.58 ± 0.03 9.94 ± 0.04 12.89 ± 0.02
2.39 ± 0.04 9.73 ± 0.04 12.60 ± 0.06
2.11 ± 0.03 9.62 ± 0.05 12.17 ± 0.08
2.02 ± 0.06 9.51 ± 0.04 11.83 ± 0.06
All the values were calculated with the programs Hyperquad 2008 and HypSpec from the pH-potentiometric, and spectrophotometric titration data measurements, respectively. The experimental pressure is (101.3 ± 10.0) kPa. Standard uncertainty u is u (T) 0.15 K. The reported uncertainties are combined expanded uncertainties at 0.95 level of confidence (k = 2). Uncertainties are the maximum deviation of one measurement from the average of three independent measurements. Uncertainties were estimated by propagating the errors in all of the parameters in the appropriate equation assuming that they were independent.
Table 4 Gibbs free energy change ΔG° for the dissociation processes of 2,3-dihydroxybenzoic acid in different Water (1) + DMSO (2) Mixture at T = 298.15 K and I = 0.16 mol·dm−3 NaNO3 obtained from optimization and frequency calculation by using Gaussian 09 program. Functional group
COOH meta OH ortho OH
ΔG° (kJ mol−1)a 0%
20%
40%
60%
80%
15,524.92 57,248.15 74,199.10
14,725.85 56,734.46 73,572.50
13,641.38 55,535.84 71,916.92
12,043.22 54,908.00 69,462.61
11,529.54 54,280.15 67,522.00
a The calculation was done using density functional theory (DFT)-B3LYP method combined with 6–31 + G(d) as a basis set.
Fig. 2. UV–visible spectrum of the ligand DA.
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at pH around 2.0–3.0, suggesting that the pKa1 value of DA at acidic pH is approximately 2.0 N pKa1 N 3.0, where precisely the value is 2.63 as determined by the equilibrium constant program (Fig. 1). Meanwhile, at basic pH, the inflection occurs around pH 9.0–10.0 and the determined pKa2 value from the program is 9.98 (Figs. 3 and 4). Subsequently, at highly basic pH, the first inflection is attributed to pKa2. As shown, pKa3 values is indicated by the second inflection point with the pH value around 13.0 (Fig. 4).
4. Concluding remarks The learning of the protonation equilibria of DA could be a significant data in the development of quantification and biological systems in aqueous and aqua-organic media. To differentiate which formula of a drug species be real in which ratio in a persuaded aqua or aqua-organic solvent solutions is certainly of excessive usage in choosing the accurate biological technique through the quantification processes. Finally, DA appears to be a probable decent metal chelating triprotic bioligand drug as the Gaussian calculations showed three sites for binding to metal ions.
Fig. 3. UV–visible spectrum of the ligand DA at highly basic pH N 10.0.
Fig. 4. Absorbance ratio of ligand DA at lower, medium and high basic pH values.
A.E. Fazary et al. / Journal of Molecular Liquids 300 (2020) 112349
Author's statement All authors read and approved the final revised manuscript. Professor Ahmed E. Fazary was a major contributor in writing the manuscript and processing analytical data, designed and led this research. Hala A. Ibrahium, and Ali A. Shati designed and performed the experiments. Nasser S Awwad via Research Center for Advanced Materials (RCAMS) at King Khalid University supported this work. Professor YiHsu Ju made final editing and proofreading of the manuscript. Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. All the values were calculated with the programs Hyperquad 2008 and HypSpec from the pH-potentiometric, and spectrophotometric titration data measurements, respectively. The experimental pressure is (101.3 ± 10.0) kPa. Standard uncertainty u is u (T) 0.15 K. The reported uncertainties are combined expanded uncertainties at 0.95 level of confidence (k = 2). Uncertainties are the maximum deviation of one measurement from the average of three independent measurements. Uncertainties were estimated by propagating the errors in all of the parameters in the appropriate equation assuming that they were independent. Acknowledgement The authors extend their appreciation to the Research Center for Advanced Materials (RCAMS) at King Khalid University, Saudi Arabia for supporting this work through research groups program under grant number RCAMS/KKU/006-19. References [1] S. George, P.J. Benny, S. Kuriakose, C. George, S. Gopalakrishnan, Antiprotozoal activity of 2,3-dihydroxybenzoic acid isolated from the fruit extracts of Flacourtia inermis Robx, Asian J. Pharm. Clin. Res. 3 (2011) 237–241. [2] M. Sousa, J. Ousingsawat, R. Seitz, S. Puntheeranurak, A. Regalado, A. Schmidt, T. Grego, C. Jansakul, M.D. Amaral, R. Schreiber, K. Kunzelmann, An extract from the medicinal plant Phyllanthus acidus and its isolated compounds induce airway chloride secretion: a potential treatment for cystic fibrosis, Mol. Pharmacol. 71 (2006) 366–376. [3] M. Grootveld, B. Halliwell, 2,3-Dihydroxybenzoic acid is a product of human aspirin metabolism, Biochem. Pharmacol. 37 (1988) 271–280. [4] T. Cam, G. Irez, R. Aydin, Determination of stability constants of mixed ligand complexes of the lanthanum(III) ion and identification of structures, J. Chem. Eng. Data 56 (2011) 1813–1820. [5] B.Y. Hirpaye, G.N. Rao, Chemical speciation of 2,3-dihydroxybenzoic acid complexes with some biologically essential metal ions in 1,2-propanediol-water mixtures, Chem. Spec. Biolavailab. 25 (2013) 179–186. [6] T. Kiss, H. Kozlowski, G. Micera, L.S. Erre, Copper(II) complexes of 2,3dihydroxybenzoic acid, J. Coord. Chem. 20 (1989) 49–56. [7] N. Turkel, M. Berker, U. Ozer, Potentiometric and spectroscopic studies on aluminium(III) complexes of some catechol derivatives, Chem. Pharm. Bull. 52 (8) (2004) 929–934. [8] A.E. Fazary, A.S. Al-Shihri, K.A. Saleh, M.Y. Alfaifi, M.A. Alshehri, S.E.I. Elbehairi, Diand tri-valent metal ions interactions with four biodegradable hydroxamate and cataecholate siderophores: new insights into their complexation equilibria, J. Solut. Chem. 45 (5) (2016) 732–749. [9] A.E. Fazary, A.M. Ramadan, Stability constants and complex formation equilibria between iron, calcium, and zinc metal ions with vitamin B9 and glycine, Complex Met 1 (1) (2014) 139–148. [10] A.Y. Rajhi, Y.-H. Ju, A.E. Angkawijaya, A.E. Fazary, Complex formation equilibria and molecular structure of divalent metal ions-vitamin B3-glycine oligopeptides systems, J. Sol. Chem. 42 (12) (2013) 2409–2442. [11] A.E. Angkawijaya, A.E. Fazary, S. Ismadji, Y.-H. Ju, Cu(II), Co(II), and Ni(II)-antioxidative phenolate-glycine peptide systems: an insight into its equilibrium solution study, J. Chem. Eng. Data 57 (12) (2012) 3443–3451. [12] A.E. Angkawijaya, A.E. Fazary, E. Hernowo, S. Ismadji, Y.-H. Ju, Nickel and cobalt complexes of non-protein l-norvaline and antioxidant ferulic acid: potentiometric and spectrophotometric studies, J. Sol. Chem. 41 (7) (2012) 1156–1164. [13] E. Hernowo, A.E. Angkawijaya, A.E. Fazary, S. Ismadji, Y.-H. Ju, Complex stability and molecular structure studies of divalent metal ion with L-norleucine and vitamin b3, J. Chem. Eng. Data 56 (12) (2011) 4549–4555.
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