Values Of Stability Constants Research Articles

In vitro, in vivo and in silico drugdrug interaction study between Vildagliptin and Bisoprolol fumarate

Aim of the present study is to evaluate the in vitro, in vivo and in silico drug-drug interaction between Vildagliptin and Bisoprolol fumarate. Interaction between Vildagliptin and Bisoprolol fumarate has been studied in a system of water at a fixed 370C at pH 1.0, 3.0 and 6.8 by using spectral observation, Job’s method of continuous variation, Ardon’s method. In silico complexation was measured by NMR prediction and DDI-CPI analysis. From spectrophotometric study, Vildagliptin and Bisoprolol fumarate give different spectra when Vildagliptin mixed with Bisoprolol fumarate in 1:1 mixture, intensity of spectra of Vildagliptin transform surprisingly due to interaction. The jobs plot was attained by plotting absorbance difference (D) against the mole fraction of the each drug at pH 1, 3, and 6.8. Vildagliptin moderate for 1:1 mixture with Bisoprolol fumarate and slightly lower spectra indicate the formation of 1:1 mixture of Vildagliptin with Bisoprolol fumarate. The value of stability constant for the drug-drug system at pH 1.0, 3.0 and 6.8 are 1.875 0.7778 and 1.2000 respectively. From the IR report it also proved that Vildagliptin and Bisoprolol fumarate produced interaction. In this OGTT test at 1:1, 1:2 and 2:1 complex blood glucose level decrease compared to Vildagliptin. Because when drug-drug interaction happened, then drug cannot exhibit its main activity. That’s why in this test complex showed different activity. In in silico methods, results showed that the drugs interact with each other. So, a careful concern is desired during simultaneous administration of Vildagliptin with Bisoprolol fumarate.IIUC Studies Vol.14(1) June 2017: 29-44

Extraction of trivalent europium and americium by the synergistic mixture of bis-1,2-dicarbollylcobaltate and N,N,N′,N′-tetraoctyl diglycolamide in highly polar 3-nitro-α,α,α-trifluorotoluene solvent

The extraction of trivalent europium and americium in the system water—HNO3—3-nitro-α,α,α-trifluorotoluene (F-3)—bis-1,2-dicarbollylcobaltate—N,N,N′,N′-tetraoctyl diglycolamide (L) has been investigated. The experimental data were treated by curve-fitting method using the LETAGROP program. It has been found that the complexes $${\text{HL}}_{{}}^{ + }$$ , $${\text{HL}}_{2}^{ + }$$ , $${\text{ML}}_{ 2}^{3 + }$$ , $${\text{ML}}_{ 3}^{3 + }$$ and $${\text{ML}}_{ 4}^{3 + }$$ (M3+ = Eu3+, Am3+) are extracted into the organic phase. The values of extraction and stability constants of the cationic complex species in F-3 saturated with water have been determined. Extraction in this system can be used for separation of lanthanides and minor actinides from PUREX process raffinate.

Steric Effects on the Binding of Phosphate and Polyphosphate Anions by Zinc(II) and Copper(II) Dinuclear Complexes of m-Xylyl-bis-cyclen.

The triethylbenzene-bis-cyclen (cyclen = 1,4,7,10-tetraazacyclododecane) compound (tbmce) was designed with an imposed structural rigidity at the m-xylyl spacer to be compared to a less restrained and known parent compound (bmce). The framework of both compounds differs only in the substituents of the m-xylyl spacer. The study was centered in the differences observed in the acid-base reactions of both compounds, their copper(II) and zinc(II) complexation behaviors, as well as in the uptake of phosphate and polyphosphate anions (HPPi3-, ATP4-, ADP3-, AMP2-, PhPO42-, and HPO42-). On the one hand, the acid-base reactions showed lower values for the third and fourth protonation constants of tbmce than for bmce, suggesting that the ethyl groups of the spacer in tbmce force the two cyclen units to more conformational restricted positions. On the other hand, the stability constant values for copper(II) and zinc(II) complexes revealed that bmce is a better chelator than tbmce pointing out to additional conformational restraints imposed by the triethylbenzene spacer. The binding studies of phosphates by the dinuclear copper(II) and zinc(II) complexes showed much smaller effective association constants for the dicopper complexes. Single-crystal X-ray and computational (density functional theory) studies suggest that anion binding promotes the formation of tetranuclear entities in which anions are bridging the metal centers. Our studies also revealed the dinuclear zinc(II) complex of bmce as a promising receptor for phosphate anions, with the largest effective association constant of 5.94 log units being observed for the formation of [Zn2bmce(HPPi)]+. Accordingly, a colorimetric study via an indicator displacement assay to detect phosphates in aqueous solution found that the [Zn2bmce]4+ complex acts as the best receptor for pyrophosphate displaying a detection limit of 2.5 nM by changes visible to naked eye.

Solid State Analysis and In-Vitro Dissolution Behavior of MeloxicamHydroxy Propyl Beta Cyclodextrin-Ethanolamines Ternary Complexes

The present study was aimed to improve the aqueous solubility and dissolution rate of an NSAID meloxicam by hydroxy propyl β-cyclodextrin ternary complexes employing ethanolamines. Initially, meloxicam (MLX) binary complexes with Hydroxy propyl β-Cyclodextrin (HPβCD) were formulated by kneading and solvent evaporation techniques which was followed by ternary complex preparation of selected MLX-HPβCD binary complex employing different ethanolamines by solvent evaporation method. The solvent evaporationwas used in preparing ternary complexes of MLX, because it was proved to be the best method comparatively in yielding promising binary complexes of meloxicam in the initial stage of this study. MLX formed 1:1 M stoichiometric binary and ternary inclusion complexes as demonstrated by the AL-type of phase solubility curve. An increment in the stability constant value (Kc) of MLX- HPβCD complex in the presence of ethanolamines conceded higher complexation efficiency. Solid state analysis (FTIR, TGA, and SEM studies) of ternary compounds evidenced the perfect inclusion complex formation. Ternary complexes showed significant improvement in drug dissolution compared topure MLX and MLX-HPβCD binary complex. The ternary complex containing 1:1:1 molar ratio of MLX-HPβCD-DEA exhibited 86.91% drug dissolution in 1 hour, which was significantly high in relation to ternary complexes containing mono and tri ethanolamines, and it was found to follow imperatively matrix order release mechanism. On aging studied complexes showed no significant change in physical appearance, drug content and drug dissolution attributes, which clearly shows high in-vitro stability of the complexes.

Thermodynamic Study on the Protonation and Na+, Ca2+, Mg2+-Complexation of a Biodegradable Chelant (HEIDA) at Different Ionic Strengths and Temperatures

A potentiometric method has been used for the determination of the protonation constants of N-(2-hydroxyethyl)iminodiacetic acid (HEIDA or L) at various temperatures 283.15 ≤ T/K ≤ 383.15 and different ionic strengths of NaCl(aq), 0.12 ≤ I/mol·kg−1 ≤ 4.84. Ionic strength dependence parameters were calculated using a Debye–Huckel type equation, Specific Ion Interaction Theory and Pitzer equations. Protonation constants at infinite dilution calculated by the SIT model are $$ \log_{10} \left( {{}^{T}K_{1}^{\text{H}} } \right) = 8.998 \pm 0.008 $$ (amino group), $$ \log_{10} \left( {{}^{T}K_{2}^{\text{H}} } \right) = 2.515 \pm 0.009 $$ and $$ \log_{10} \left( {{}^{T}K_{3}^{\text{H}} } \right) = 1.06 \pm 0.002 $$ (carboxylic groups). The formation constants of HEIDA complexes with sodium, calcium and magnesium were determined. In the first case, the formation of a weak complex species, NaL, was found and the stability constant value at infinite dilution is log10KNaL = 0.78 ± 0.23. For Ca2+ and Mg2+, the CaL, CaHL, CaL2 and MgL species were found, respectively. The calculated stability constants for the calcium complexes at T = 298.15 K and I = 0.150 mol·dm−3 are: log10βCaL = 4.92 ± 0.01, log10βCaHL = 11.11 ± 0.02 and $$ \log_{10} \beta_{\text{Ca{L}}_{2}} $$ = 7.84 ± 0.03, while for the magnesium complex (at I = 0.176 mol·dm−3): log10βMgL = 2.928 ± 0.006. Protonation thermodynamic functions have also been calculated and interpreted.

Host-guest interaction study of Efavirenz with hydroxypropyl‑β‑cyclodextrin and l‑arginine by computational simulation studies: Preparation and characterization of supramolecular complexes

The present study demonstrates the solubility and dissolution enhancement of Efavirenz (EFV) through preparation of supramolecular inclusion complexes. The supramolecular system comprising of EFV, hydroxypropyl‑β‑cyclodextrin (HP-β-CD) and l‑Arginine was prepared by physical mixing and spray drying methods. The complexes were characterized by powder X-ray diffraction, differential scanning calorimetry, 1H NMR spectroscopy and FT-IR spectroscopy to study alteration in crystallinity of drug. Intrinsic stability constant values (Kc) and complexation efficiency (CE) of HP-β-CD was substantially enhanced due to addition of l‑Arginine. Improved solubility and dissolution was evidenced in the supramolecular system than binary system of EFV and HP-β-CD. Computational modelling and molecular dynamics study revealed that l‑Arginine increased the stability of the complex by bridging between the EFV and the HP-β-CD. It also confirmed that l‑Arginine was positioned on the outer surface of complex and improved polar surface of inclusion complex resulting in increased solubility of the complex. Thus, supramolecular inclusion complexes of EFV with HP-β-CD and l‑Arginine could contribute as an innovative outcome in formulation development of EFV through solubility and dissolution enhancement.

A Constrained Tetrapeptide as a Model of Cu(I) Binding Sites Involving Cu4S6 Clusters in Proteins.

Peptide design is an efficient strategy to create relevant models of natural metal binding sites found in proteins. The two short tetrapeptides Ac-Cys-dPro-Pro-Cys-NH2 (CdPPC) and Ac-Cys-Pro-Gly-Cys-NH2 (CPGC) were synthesized and studied as mimics of Cu(I) binding sites involved in Cu homeostasis. Both sequences contain β turn inducing motifs to rigidify the peptide backbone structure and thereby preorganize the metal-binding side chains. The more constrained structure of the peptide CdPPC with respect to CPGC was evidenced by the measurements of the temperature coefficients of the amide protons by 1H NMR, which suggest a solvent-shielded intramolecular hydrogen bond in CdPPC, and no H-bond in CPGC. The Cu(I) complexes were studied by UV, circular dichroism (CD), and NMR spectroscopies as well as electrospray ionization mass spectrometry (ESI-MS) experiments in aqueous solution at physiological pH. The complexes formed with CPGC showed a complicated speciation with the possible formation of many polymetallic species. By contrast, the better preorganization in CdPPC leads to the formation of a unique Cu4L3 complex involving a Cu4S6 core. The formation of this specific cluster was confirmed by ESI-MS and by diffusion-ordered NMR spectroscopy in solution. The affinity of CdPPC for Cu(I) (β11pH7.4 = 1017.5 calculated for a CuL complex) is more than 1 order of magnitude larger than the affinity measured for the less constrained peptide CPGC. Besides, this stability constant value is very similar to those reported with proteins. Therefore, the Cu(I) complex formed with the simple tetrapeptide CdPPC in water at physiological pH represents a very good model of Cu(I)-thiolate clusters found in proteins. The extremely large selectivity (1011) in favor of Cu(I) with respect to Zn(II), an abundant competitor in cells, makes it a promising candidate to be targeted to the liver cells for the localized treatment of Cu overload in Wilson's disease.

Chlorofullerene C60Cl6: A Precursor for Straightforward Preparation of Highly Water‐Soluble Poly‐hydroxypyridinone Fullerene Derivatives as Potential Radionuclide Chelators

AbstractRadionuclide internal contamination can induce chemical and radiological intoxication and produce harmful free radicals in vivo. Conventional studies on chelating agents have disregarded the damage of free radicals to the body. Therefore, novel poly‐hydroxypyridinone fullerene derivatives were designed and prepared by using chlorofullerene as precursor. The poor water solubility of fullerene was ameliorated by chemically modifying of hydrophilic hydroxypyridinone, and obtaining high Water‐soluble fullerene derivatives. The radical scavenging reaction kinetics of target products were studied. They all exhibited much higher rate constants than other fullerene polymer system. The CeIII ion was used as a simulated radionuclide for evaluating the binding ability of target products, and the values of conditional stability constants were about 5.15‐6.11. Thus, the bifunctional, Water‐soluble fullerene derivatives were successfully generated. The results provided basis for further study on the potential application of poly‐hydroxypyridinone fullerene derivatives as radionuclide chelators.

Origin of the unusually strong and selective binding of vanadium by polyamidoximes in seawater

Amidoxime-functionalized polymeric adsorbents are the current state-of-the-art materials for collecting uranium (U) from seawater. However, marine tests show that vanadium (V) is preferentially extracted over U and many other cations. Herein, we report a complementary and comprehensive investigation integrating ab initio simulations with thermochemical titrations and XAFS spectroscopy to understand the unusually strong and selective binding of V by polyamidoximes. While the open-chain amidoxime functionalities do not bind V, the cyclic imide-dioxime group of the adsorbent forms a peculiar non-oxido V5+ complex, exhibiting the highest stability constant value ever observed for the V5+ species. XAFS analysis of adsorbents following deployment in environmental seawater confirms V binding solely by the imide-dioximes. Our fundamental findings offer not only guidance for future optimization of selectivity in amidoxime-based sorbent materials, but may also afford insight to understanding the extensive accumulation of V in some marine organisms.

Preparation and characterization of artemether inclusion complexes with hydroxypropyl-β-cyclodextrin

Purpose: To investigate experimentally the inclusion of artemether into the cavity of hydroxypropyl-β-cyclodextrin and examine its effect on the solubility and dissolution rate of the drug.Methods: Inclusion complexes of artemether with hydroxypropyl-β-cyclodextrin of molar ratios 1:1, 1:2 and 1:3 were prepared using the kneading method. Phase solubility analysis and in vitro dissolution studies were utilized in evaluating the influence of inclusion complex formation on the solubility and dissolution rate of the drug. The complexes were characterized using differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR). The inclusion complex containing equimolar concentrations of artemether and hydroxypropyl-β-cyclodextrin was then formulated into tablets via direct compression and evaluated for various pharmaceutical characteristics including hardness, friability, absolute drug content and comparative in vitro dissolution profiles with some commercially available brands of artemether.Results: The phase solubility diagram for the formed complexes in water at 37 oC indicated a linear curve soluble complex system (referred to as the AL system), and a stability constant (KC) value of 143 M-1. Evidence consistent with inclusion complex formation was obtained using FT-IR and DSC. The formulated inclusion complex tablets exhibited a higher rate of dissolution than the pure drug and commercial brands, showing 3.9-, 1.8- and 1.6-fold increases, respectively, over a period of 15 min.Conclusion: Inclusion complexation of artemether with hydroxypropyl-β-cyclodextrin is a promising approach to enhance the solubility and dissolution rate of the drug.Keywords: Artemether, 2-Hydroxypropyl-β-cyclodextrin, Dissolution, Solubility enhancement, Inclusion complex

P-NO2-Bn-H4neunpa and H4neunpa-Trastuzumab: Bifunctional Chelator for Radiometalpharmaceuticals and 111In Immuno-Single Photon Emission Computed Tomography Imaging.

Potentially nonadentate (N5O4) bifunctional chelator p-SCN-Bn-H4neunpa and its immunoconjugate H4neunpa-trastuzumab for 111In radiolabeling are synthesized. The ability of p-SCN-Bn-H4neunpa and H4neunpa-trastuzumab to quantitatively radiolabel 111InCl3 at an ambient temperature within 15 or 30 min, respectively, is presented. Thermodynamic stability determination with In3+, Bi3+, and La3+ resulted in high conditional stability constant (pM) values. In vitro human serum stability assays have demonstrated both 111In complexes to have high stability over 5 days. Mouse biodistribution of [111In][In(p-NO2-Bn-neunpa)]-, compared to that of [111In][In(p-NH2-Bn-CHX-A″-diethylenetriamine pentaacetic acid (DTPA))]2-, at 1, 4, and 24 h shows fast clearance of both complexes from the mice within 24 h. In a second mouse biodistribution study, the immunoconjugates 111In-neunpa-trastuzumab and 111In-CHX-A″-DTPA-trastuzumab demonstrate a similar distribution profile but with slightly lower tumor uptake of 111In-neunpa-trastuzumab compared to that of 111In-CHX-A″-DTPA-trastuzumab. These results were also confirmed by immuno-single photon emission computed tomography (immuno-SPECT) imaging in vivo. These initial investigations reveal the acyclic bifunctional chelator p-SCN-Bn-H4neunpa to be a promising chelator for 111In (and other radiometals) with high in vitro stability and also show H4neunpa-trastuzumab to be an excellent 111In chelator with promising biodistribution in mice.

PHYSICOCHEMICAL CHARACTERIZATION AND EVALUATION OF TELMISARTAN: HYDROXYPROPYL-β-CYCLODEXTRIN: TWEEN 80 INCLUSION COMPLEX

Objective: The present work was aimed to study the effect of the ternary component on complexation efficiency of cyclodextrins towards telmisartan which is a poorly soluble anti-hypertensive agent.Methods: The elucidation of inclusion complexation of telmisartan (TEL) with hydroxypropyl-β-cyclodextrin (HP-β-CD) in the presence and absence of tween 80 was done by investigating their interactions in solid and solution state. The solid state characterization was performed using differential scanning calorimetry (DSC), powder X-ray diffraction studies (PXRD) fourier transform infra-red spectroscopy (FTIR) studies. The host guest stoichiometry was confirmed in solution state by proton nuclear magnetic resonance (1HNMR) and solution calorimetry studies. The improvement in solubility was evaluated through dissolution studies, which was further confirmed by in vivo studies.Results: In solution state, the phase solubility studies indicated 1:1 stoichiometry between TEL and HP-β-CD both in presence and absence of tween 80. The NMR and molecular modelling studies indicated the insertion of N-methyl benzimidazole and biphenyl carboxylate regions of TEL into HP-β-CD cavity suggesting the coexistence of two 1:1 complexes in equilibrium with each other. The stability constants, K1 (imidazole region of TEL-CD) and K2 (biphenyl acetic acid region of TEL-CD), were enhanced in the presence of tween 80 as evident by the higher value of stability constants. Efficacy of ternary complex was established by a significant decrease in the systolic blood pressure of deoxycorticosterone acetate (DOCA) induced hypertensive rats.Conclusion: It can be concluded that solubility of TEL was increased by encapsulation with HP-β-CD. Tween 80 further increased the complexation efficiency and decreased the bulk of cyclodextrin.

β-Cyclodextrin-based ternary complexes of haloperidol and organic acids: the effect of organic acids on the drug solubility enhancement

Haloperidol (HALO) is a weak base with very low aqueous solubility that is used as an antipsychotic drug. This study aimed to improve its solubility by forming HALO/β-cyclodextrin (β-CD)-based ternary complexes with organic acids. The solubility of HALO/β-CD binary and HALO/β-CD/organic acid ternary complexes in different media (i.e. citrate buffer pH 3 and 6) was explored. The stoichiometric ratio between the drug and β-CD was 1:1 in all complexes formed. The solubility of HALO/β-CD binary complexes significantly increased in citrate buffer pH 3 compared with citrate buffer pH 6. For the ternary complexes, HALO/β-CD/tartaric acid and HALO/β-CD/lactic acid in citrate buffer pH 3 increased HALO solubility compared with HALO/β-CD/succinic acid due to their higher unionized species. The highest stability constant and complexation efficiency values in citrate buffer pH 3 were shown by the ternary complexes with lactic acid followed by tartaric acid and succinic acid, respectively. Results indicated that lactic acid provided the greatest binding strength and solubilization efficiency for the complex.

Removal of di- and tri-alkyltin(IV) compounds by polyphosphonate ligand: A speciation perspective

The potential of employing diethylenetriamine-N,N,N′,N″,N″-pentakis(methylenephosphonic acid), DTPMP, to remove (CH3)3Sn+, (C2H5)3Sn+, (C3H7)3Sn+, (CH3)2Sn2+, (C2H5)2Sn2+ cations from aqueous solution was evaluated on the basis of speciation modeling. For this purpose, the interactions of DTPMP towards alkyltin(IV) cations in NaCl at I=0.1molL−1 (for (CH3)2Sn2+, also in the ionic strength range 0.1≤I≤1molL−1) was studied. Potentiometric measurements evidenced the formation of MLHj species, with 0≤j≤5, and stability constant values that follow the trend (CH3)3Sn+>(C2H5)3Sn+>(C3H7)3Sn+, for trialkyltin(IV) cations and (CH3)2Sn2+>(C2H5)2Sn2+, for dialkyl ones. 1H NMR investigations in solution allowed to confirm all the models proposed on the basis of potentiometric findings and, in some cases, to gain information about the coordination arrangement around the tin. Once assessed the most reliable speciation models, the sequestering ability of the polyphosphonic ligand was evaluated by pL0.5 empiric parameter (ligand concentration required to sequester 50% of the metal cation present in traces).

METAL-LIGAND STABILITY CONSTANTS OF Pr(III) , Nd(III) AND Sm(III) METAL IONS COMPLEXES WITH SOME SUBSTITUTED ISOXAZOLINE pH-METRICALLY

The stepwise stability constant values of substituted isoxazoline with Pr(III), Nd(III), and Sm(III) metal ion have been studied at 0.1M ionic strength pH-Metrically in 70% ethanol-water mixture by Bjerrum method as adopted by Calvin Wilson. The values of proton-ligand stability constant (pK) and metal-ligand stability constant (log K) are calculated and from the data obtained it is observed that metal ions formed 1:1 complexes with ligands L1 and L2 Â using half integral & pointwise calculation method

Thermodynamic Parameters for the Interaction of Amoxicillin and Ampicillin with Magnesium in NaCl Aqueous Solution, at Different Ionic Strengths and Temperatures

In this paper, the sequestering ability of amoxicillin and ampicillin toward Mg2+ in NaCl aqueous solutions at different ionic strengths I = (0 to 1.0) mol·kg–1 and temperatures of T = (288.15 to 318.15) K was investigated by potentiometry (ISE-H+, glass electrode). The complex formation constants determined at different ionic strengths and temperatures were modeled by means of the Debye–Hückel equation and the Specific ion Interaction Theory (SIT). From the results, a weak ability of the two penicillins to bind the metal ion can be observed; in fact, the stability constants of the ML species (M = Mg2+ and L = amoxicillin or ampicillin) are log β = 4.348 and 3.242 at infinite dilution and T = 298.15 K, respectively. The dependence of the formation constants on the temperature was modeled by means of a van’t Hoff equation, which allowed us to calculate the enthalpy and entropy change values of formation of each species. The sequestering ability of amoxicillin and ampicillin toward Mg2+ in the different experimental conditions (pH, ionic strength, temperature) was quantified by means of a sigmoid equation and of the pL0.5 parameter. The pL0.5 values reflect the low stability constant values of the species; as an example at I = 0.15 mol·kg–1, pH = 7.4, and T = 298.15 K, we have pL0.5 = 2.52 and 2.78, for Mg2+/amox2− and Mg2+/amp− systems, respectively.

PH-Metric study of picolinic acid with Cu (II), Co (II) and Fe(II)ions

pH-Metric measurements of picolinic acid were studied with Cu(II), Co (II) and Fe(II)metal ions at 0.2 M ionic strength in water by Irving-Rossotti titration technique for determination of stability constants. 1: 1 complexes were formed in present investigation in between picolinic acid (PA) and Cu (II), Co (II)and Fe(II) metal ions. Values of proton-ligand stability constant (pK) and metal-ligand stability constants log k were evaluate and compared from resultant data.

Thermodynamic stability of mercury(II) complexes formed with environmentally relevant low-molecular-mass thiols studied by competing ligand exchange and density functional theory

Environmental contextThe chemical speciation of mercury (Hg) largely controls its biogeochemical cycling and exposure to biota. Here, we investigate the thermodynamic stabilities of complexes formed between inorganic divalent Hg (HgII) and 15 biogeochemically relevant low-molecular-mass (LMM) thiol ligands. This information is critical for accurate modelling of the chemical speciation of HgII and to clarify the role of HgII–LMM thiol complexes in the cycling of Hg in the environment. AbstractInorganic divalent mercury (HgII) has a very high affinity for reduced sulfur functional groups. Reports from laboratory experiments suggest that HgII complexes with specific low-molecular-mass (LMM) thiol (RSH) ligands control rates of HgII transformation reactions. Because of methodological limitations for precise determination of the highly stable HgII complexes with LMM thiol ligands, constants reported in the literature remain inconsistent. This uncertainty impedes accurate modelling of the chemical speciation of HgII and the possibility to elucidate the role of HgII complexes with LMM thiols for Hg transformation reactions. Here, we report values of thermodynamic stability constants for 15 monodentate, two-coordinated HgII complexes, Hg(SR)2, formed with biogeochemically relevant LMM thiol ligands. The constants were determined by a two-step ligand-exchange procedure where the specific Hg(SR)2 complexes were quantified by liquid chromatography–inductively coupled plasma mass spectrometry. Thermodynamic stability constants (log β2) determined for the Hg(SR)2 complexes ranged from 34.6, N-cysteinylglycine, to 42.1, 3-mercaptopropionic acid, for the general reaction Hg2++2RS– ⇌ Hg(SR)2. Density functional theory (DFT) calculations showed that electron-donating carboxyl and carbonyl groups have a stabilising effect on the HgII–LMM thiol complexes, whereas electron-withdrawing protonated primary amino groups have a destabilising effect. Experimental results and DFT calculations demonstrated that the presence of such functional groups in the vicinity of the RSH group caused significant differences in the stability of Hg(SR)2 complexes. These differences are expected to be important for the chemical speciation of HgII and its transformation reactions in environments where a multitude of LMM thiol compounds are present.

Composition and stability constants of copper(II) complexes with succinic acid determined by capillary electrophoresis

The advantage of capillary electrophoresis was demonstrated for studying a complicated system owing to the dependence of direction and velocity of the electrophoretic movement on the charge of complex species. The stability constants of copper(II) complexes with ions of succinic acid were determined by capillary electrophoresis, including the 1 : 2 metal to ligand complexes which are rarely mentioned. The measurements were carried out at 25 °C and ionic strength of 0.1, obtained by mixing the solutions of succinic acid and lithium hydroxide up to pH 4.2–6.2. It was shown that while pH was more than 4.5 the zone of copper(II) complexes with succinate moves as an anion. It is impossible to treat this fact using only the complexes with a metal-ligand ratio of 1 : 1 (CuL0, CuHL+). The following values of stability constants were obtained: log β(CuL) = 2.89 ± 0.02, log β(CuHL+) = 5.4 ± 0.5, log β(CuL22−) = 3.88 ± 0.05, log β(CuHL2−) = 7.2 ± 0.3.

Ionic liquid vs tri-block copolymer in a new aqueous biphasic system for extraction of Zn-cholesterol complex

Abstract A new aqueous biphasic system (ABS) was designed by mixing aqueous solutions of tri-block copolymer polypropylene glycol-block-polyethylene glycol-block-polypropylene glycol (PPG-PEG-PPG) with a nontoxic ionic liquid tris (2-hydroxyethyl) methyl-ammonium methyl sulfate. Well defined phase diagram validated with Merchuk equation along with suitable tie-lines was obtained by turbidometric titration method at 298 K. The tie-line lengths and slope of tie lines were calculated and the results were validated using Othmer-Tobias and Bancroft equations. The newly designed ABS has been used to perform the extraction studies of cholesterol, zinc and Zn-cholesterol complex at different pHs of the solution. Zn-cholesterol complex was found to be formed at an acidic pH (1.5). The stability constant value of Zn-cholesterol complex was obtained in ethanolic medium, pristine polymer medium and in the ABS extracted polymer rich phase using Benesi-Hildebrand equation. The aggregation number of PPG-PEG-PPG solution obtained after extraction of cholesterol and Zn-cholesterol complex in ABS was found to be greater in comparison to the pristine copolymer solution. Kinetic spectrophotometric studies were also supporting higher stability of Zn-cholesterol complex in the ABS extracted polymer rich phase. Zn-cholesterol complex formation in acidic micellar medium (pH 1.5) also falls in the pH range of normal human stomach and may offer a suitable method of zinc supplementation by absorption via cell membrane that resembles with the polymeric micellar aggregation.