Equilibrium Constants For Formation Of Complexes Research Articles

Structure, stability, and lability of copper(II) complexes with triglycine

Equilibrium constants of complex formation, rate constants of chemical exchange reactions, and characteristics of electronic absorption spectra for species detected in aqueous solution of copper(II) with triglycine were determined, and conclusions on the structure of the complexes were made. A possibility of H-bond formation between the ammonium group of the zwitter-ionic form of the ligand and the second peptide oxygen in the anionic form of an adjacent ligand was shown. Kinetics and mechanisms of ligand and proton exchanges in solutions of copper(II) bistripeptide complexes with the ligand containing a deprotonated peptide nitrogen atom were studied. A new mechanism was proposed for hydroxide-catalyzed substitution reactions in copper(II) complexes with tripeptides.

Hybridization of single-stranded DNA targets to immobilized complementary DNA probes: comparison of hairpin versus linear capture probes.

A microtiter-based assay system is described in which DNA hairpin probes with dangling ends and single-stranded, linear DNA probes were immobilized and compared based on their ability to capture single-strand target DNA. Hairpin probes consisted of a 16 bp duplex stem, linked by a T(2)-biotin.dT-T(2) loop. The third base was a biotinylated uracil (U(B)) necessary for coupling to avidin coated microtiter wells. The capture region of the hairpin was a 3' dangling end composed of either 16 or 32 bases. Fundamental parameters of the system, such as probe density and avidin adsorption capacity of the plates were characterized. The target DNA consisted of 65 bases whose 3' end was complementary to the dangling end of the hairpin or to the linear probe sequence. The assay system was employed to measure the time dependence and thermodynamic stability of target hybridization with hairpin and linear probes. Target molecules were labeled with either a 5'-FITC, or radiolabeled with [gamma-(33)P]ATP and captured by either linear or hairpin probes affixed to the solid support. Over the range of target concentrations from 10 to 640 pmol hybridization rates increased with increasing target concentration, but varied for the different probes examined. Hairpin probes displayed higher rates of hybridization and larger equilibrium amounts of captured targets than linear probes. At 25 and 45 degrees C, rates of hybridization were better than twice as great for the hairpin compared with the linear capture probes. Hairpin-target complexes were also more thermodynamically stable. Binding free energies were evaluated from the observed equilibrium constants for complex formation. Results showed the order of stability of the probes to be: hairpins with 32 base dangling ends > hairpin probes with l6 base dangling ends > 16 base linear probes > 32 base linear probes. The physical characteristics of hairpins could offer substantial advantages as nucleic acid capture moieties in solid support based hybridization systems.

Spectropotentiometric Study of Complexation Equilibrium in the System H2TAPBr4, H2TAPCl4-(CH3COO)2Cd-HClO4-DMSO at 298 K

A procedure was suggested for direct measurement of the equilibrium constant of complex formation of porphyrins MX2 + H2P ⇄ MP + 2HX, based on providing conditions under which the direct reaction is suppressed and the reverse reaction is facilitated. The procedure is applicable to complexation of Cd2+ with tetrachloro- and tetrabromotetraazaporphyrins in DMSO in the presence of HClO4.

Spectrophotometric investigations of the reaction between gold(III) and potassium iodide

The equilibria of the complex formation between AuCl4-n(OH)n - and I- was studied spectrophotometrically in aqueous solutions containing KI:Au(III)?2 in the acidity range from 1 M HCl to pH 7 at 25?C. From the hydrogen ion and Cl-ion dependence of the absorption spectra, it was found that AuCl2I2 - and Au(OH)2I2 - complexes were formed. The equilibrium constants of complex formation were determined.

Thermodynamic parameters of the interaction of cryptand[222] with amino acids in water at 298.15 K

The interaction of cryptand[222] with amino acids in water, which is weak for most amino acids and controlled by the solvent effect in the case of non-polar amino acids, was studied at 298.15 K by the calorimetric method. Cryptand[222] undergoes selective complex formation with some polar and aromatic amino acids. The thermodynamic functions and equilibrium constants of complex formation of the macrocyclic ligand withl-histidine,l-threonine, andl-glutamine were calculated.

Spectroscopic study of complex formation between aliphatic amines and chromogenic calix[4]arene derivatives

The complexation properties of two new calix[4]arene derivatives were studied by UV/Vis spectroscopy. They have been found to bind aliphatic amines, lithium and calcium ions in equilibrium reactions, particularly in polar solvents. The reaction is indicated through the emergence of a new band. The equilibrium constants of complex formation in the case of some ligand–substrate compositions have been determined.

Ground-state complex formation of perylene with pyromellitic dianhydride studied by static fluorescence quenching

The association of complex formation with static quenching in CT systems was investigated. Evaluation of the data made evident that the inner filter effect must be allowed for. Time-resolved and temperature-dependent stationary measurements of fluorescence led to the separation of dynamic and static quenching components. The static quenching constant is discussed with respect to the equilibrium constant of complex formation determined by absorption spectroscopy.

Redox Equilibria of Tin Ions in Alkali Borate Melts

Sn4+-Sn2+ redox in binary alkali silicate melts was studied by means of differential pulse voltammetry. The half-wave potential, E1/2, of the reduction of Sn4+ to Sn2+ shifts toward the negative with changing the alkali metal ions from Li+ through Na+, K+ and Cs+, at a fixed concentration for each alkali metal oxide. Conversely, E1/2 shifts to the positive with an increase in the content of each of the alkali oxide. This tendency for Sn4+-Sn2+ redox was different from those of Sb5+-Sb3+ and Cr6+-Cr3+ redox. The presence of Sb5+ and Cr6+ result in a strong acid, which leads to E1/2 shift to the negative with an increase in basicity. These results for Sn ions can be interpreted in terms of complex formations of Sn4+ and Sn2+, in which the coordination numbers of the oxide ion for both ions do not differ significantly and their equilibrium constants for complex formation depend largely on the alkali species.

Kinetic characteristics of the enzymatic conversion in presence of cyclodextrins: study of the oxidation of polyunsaturated fatty acids by lipoxygenase

The capability of cyclodextrins (CDs) to greatly enhance the solubility in water of poorly water-soluble substances makes them an ideal alternative system for studying the expression of enzyme activity with such substrates in aqueous solution. In order to evaluate the behaviour of the enzymes in presence of CDs a study of the lipoxygenase (LOX)-catalyzed oxidation of polyunsaturated fatty acids (PUFA) as model reaction has been carried out. This was done by using LOX from two different sources (soybean and potato tuber), at two pH values (6.3 and 9.0), with two substrates (linoleic acid and arachidonic acid) and three types of CD ( β-CD, methyl- β-CD and monoglucosyl- β-CD). PUFA have been shown to form inclusion complexes of 1:2 stoichiometry which are in equilibrium with free PUFA and free CD, thus complexation is governed by two equilibrium constants, K 1 and K 2 (J.M. López-Nicolás et al., Biochem. J. 308 (1995) 151–154; R. Bru et al., Colloids Surf. 97 (1995) 263–269). For the oxidation of PUFA by LOX in the presence of β-CD we propose a model in which free PUFA is the only effective substrate, thus the oxidation of the complexed substrate requires the previous dissociation of the complex. The equilibrium constants of complex formation are determined by both a physico-chemical and an enzymatic method. In spite of giving quite similar results, the second was proven to be more accurate so it was employed in further studies. CD was shown to slow down the reaction rate of LOX, specifically due to the increase of K m, V max remaining unchanged. That apparent inhibition is due to removal of effective (free) substrate in the form of inclusion complexes. This `sequestered' substrate can, however, be converted since it is in equilibrium with the free. The feasibility of realizing a CD-mediated accurate control over the conversion rate is demonstrated in the experiment called `cyclodextrin assay' in which the concentration of the free substrate is calculated by using the equilibrium constants of complex formation and setting the initial concentrations of total substrate and total CD. From the observation of the reaction progress curves in the conditions of the CD assay, we have studied some characteristic parameters of the oxidation of PUFA by LOX in this new medium, such as enzymatic activity, duration of linear product accumulation and the lag phase.

Reversed determination of the formation constants of 1-allyl terguride with mandelic acid optical isomers using capillary electrophoresis

A method is presented for the accurate determination of equilibrium constants of complex formation between mandelic acid enantiomers and 1-allyl terguride. In earlier studies this ergot alkaloid has proven to be a potential chiral selector for racemic acidic compounds. In these studies, the formation constant was determined by measuring the effective mobility of some racemic acidic analytes at varying concentrations of ergot alkaloid. For these experiments, the assumption was made that the complex mobility was zero. In order to validate these data, the experimental set-up was reversed. In this new set-up, the cationic chiral selector is injected as sample, while the background electrolyte (BGE) contained either of the two optically pure mandelic acid enantiomers, in varying concentrations. For accurate determination of the effective mobility, tetrabutylammonium was used as a mobility reference and the ionic strength of the BGE was kept constant. By performing these experiments at two different pH values, it was possible to determine complex-formation constants, and chiral selectivity towards both the dissociated and the non-dissociated mandelic acid enantiomers. Results show that only the dissociated acid interacts selectively with the ergot alkaloid confirming our earlier results. In our earlier experiments we made the assumption that the non-dissociated acid does not interact with the ergot alkaloid. The experimental data obtained by the current method however, show that, although this interaction is not enantioselective, it cannot be neglected. Optically pure mandelic acid proved to be a suitable chiral selector for the separation of terguride enantiomers.

Measurement of Equilibrium Constants for Complex Formation between Phenol and Hydrogen-Bond Acceptors by Kinetic Laser Flash Photolysis1

Measurement of Equilibrium Constants for Complex Formation between Phenol and Hydrogen-Bond Acceptors by Kinetic Laser Flash Photolysis¹

Measurement of Equilibrium Constants for Complex Formation between Phenol and Hydrogen-Bond Acceptors by Kinetic Laser Flash Photolysis J. Am. Chem. Soc. 1996, 118, 6790−6791

Measurement of Equilibrium Constants for Complex Formation between Phenol and Hydrogen-Bond Acceptors by Kinetic Laser Flash Photolysis <i>J</i>. <i>Am</i>. <i>Chem</i>. <i>Soc</i>. <b>1996</b>, <i>118</i>, 6790−6791

Styrene–maleic anhydride copolymerization through polar transition states in polar solvents

AbstractRadical copolymerization of styrene (M1) and maleic anhydride (M2) was performed in highly polar, oxygen lone‐pair donor solvents (N, N‐dimethylformamide, N‐methylfor‐mamide) at high temperatures (135 and 145°C) and in strong π‐electron donor solvents (1,3‐dimethoxybenzene and 1,2,4‐trimethoxybenzene) at 60‐145°C. The corresponding reactivity ratios were evaluated by Fineman–Ross, Kelen Tüdos, and error‐in‐variables model (RREVM) methods. The reactivity ratios obtained (r1r2 &lt; 1) show that alternating copolymers were produced. These results are best explained if the copolymerizations proceed through highly polarized radical transition states rather than styrene–maleic anhydride charge‐transfer complexes. Polar solvents stabilize highly polarized radical transition states thereby promoting alternating polymerization at higher temperatures than can be observed in nonpolar solvents. Donor solvents might also stabilize the electrophilic maleic anhydride radical more than the nucleophilic styryl radical thereby tending to depress k21 relative to k12. The copolymerizations were also studied in decalin (a nonpolar, nondonor solvent) at 60‐145°C. At lower temperatures alternating copolymers were combined. However, at temperatures &gt; 120°C, random copolymers were obtained in decalin. Thus, in decalin, styrene–maleic anhydride charge‐transfer complexes are present at lower temperatures where they might play a role in alternation. In decalin the solvation of the polar transition states (relative to the macroradicals and monomers) does not favor copolymerization through such dipolar structures. Thus, at low temperatures growing chains could add to charge‐transfer complexes. However, at temperatures above 120°C the concentration of charge‐transfer complexes becomes so small that they can no longer be kinetically important contributions to the copolymerization. Thus, random copolymers with high styrene mole ratios are produced in decalin. The equilibrium constants for complex formation between maleic anhydride and both 1,3‐dimethoxybenzene and 1,2,4‐trimethoxybenzene were de‐termined in 1 : 1 (v/v) CCl4/CDCl3 using a technique based on changes in the NMR chemical shifts of the maleic anhydride protons. © 1995 John Wiley &amp; Sons, Inc.

Metal-organic complexes in geochemical processes: Estimation of standard partial molal thermodynamic properties of aqueous complexes between metal cations and monovalent organic acid ligands at high pressures and temperatures

Regression of standard state equilibrium constants with the revised Helgeson-Kirkham-Flowers (HKF) equation of state allows evaluation of standard partial molal entropies ( S o) of aqueous metal-organic complexes involving monovalent organic acid ligands. These values of S o provide the basis for correlations that can be used, together with correlation algorithms among standard partial molal properties of aqueous complexes and equation-of-state parameters, to estimate thermodynamic properties including equilibrium constants for complexes between aqueous metals and several monovalent organic acid ligands at the elevated pressures and temperatures of many geochemical processes which involve aqueous solutions. Data, parameters, and estimates are given for 270 formate, propanoate, n-butanoate, n-pentanoate, glycolate, lactate, glycinate, and alanate complexes, and a consistent algorithm is provided for making other estimates. Standard partial molal entropies of association ( Δ − S r o ) for metal-monovalent organic acid ligand complexes fall into at least two groups dependent upon the type of functional groups present in the ligand. It is shown that isothermal correlations among equilibrium constants for complex formation are consistent with one another and with similar correlations for inorganic metal-ligand complexes. Additional correlations allow estimates of standard partial molal Gibbs free energies of association at 25°C and 1 bar which can be used in cases where no experimentally derived values are available.

On N-acetylcysteine. Part I. Experimental and theoretical approaches of the N-acetylcysteine/H2O2complexation

The complexation of N-acetylcysteine (RSH) with hydrogen peroxide has been studied experimentally and theoretically. Experimentally we have measured the evolution of RSH, H2O2, and RSSR (N-acetylcystine) as a function of time. Surprisingly, H2O2 decays by a biphasic process, which is not the case for RSH and RSSR. In the first stage of the kinetics, H2O2 disappears without oxidizing the thiol function of RSH. By analogy with glutathione (GSH), the formation of a complex between RSH and H2O2 has been proposed. The thermodynamic equilibrium constant of complex formation has been determined. Theoretical calculations were performed within the SIBFA method to pinpoint the sites of complexation in isolated and hydrated states. A mixed "discrete–continuum" model was used to evaluate the solvent effect. The two stable complexes found in isolated state have different behaviour under the influence of the solvent. Comparison with complexed GSH is discussed.

Interaction between tetracyanoethylene and hexamethylbenzene in polypropylene: effect of polymer medium

The formation of a charge-transfer complex between tetracyanoethylene (TCNE) and hexamethylbenzene (HMB) in isotactic polypropylene (PP) has been studied by using UV-VIS spectroscopy. The effective equilibrium constant of complex formation K e increases with the concentration of TCNE or HMB in the polymer. Hexadecane affects the K e in PP, while it has no influence on the complex in solution. The theory of non-uniform additives distribution in the polymer medium and the reactivity are discussed.

Time-resolved ESR spectra of the α-hydroxybenzyl-amine complex

Time-resolved ESR spectra of the α-hydroxybenzyl radical were measured in benzene and 2-propanol solutions by the photo-dissociation of benzoin. The hyperfine structure (hfs) of α-hydroxybenzyl depends on the solvents. In a benzene solution containing triethylamine, two species with different hyperfine structure appeared simultaneously. As the ratio of intensity for the two species depends on the concentration of triethylamine, one of them is assigned to the bare α-hydroxybenzyl and the other to the 1:1 complex of α-hydroxybenzyl and triethylamine. The equilibrium constant of complex formation was estimated to be about 450 M −1 from the analysis of CIDEP intensities.

Pulse voltammetric techniques applied to the anodization of copper microelectrodes

Pulse techniques at microelectrodes have been used to show their utility for the investigation of reactions important in corrosion and, specifically, to characterize the anodization of copper in ethylene glycol—water mixtures containing chloride. Pulse techniques render accessible the entire voltammetric wave and yield values of equilibrium constants for complex formation from transient measurements. The formal value of the formation constant for dichlorocuprate(I) in 50% v/v ethylene glycol—water is given by log β 2 = 5.8. At sufficiently high concentrations of chloride and low current density the reaction product is CuCl − 2, whereas in the limiting current region the product is the neutral monochloro complex.

Equilibrium constants of the formation of 1,3,5-trinitro-benzene complexes and their reaction rate constants with o-Ps

Equilibrium constants of complex formation between the strong electron acceptor 1,3,5-trinitro-benzene (TNB) and basic donors (n-butyl-amine and piperidine) were determined by positron annihilation lifetime technique. The reaction rate costants of the complexes with o-Ps were calculated too. The results were compared with our earlier investigation on TNB in methanolic solutions of NaOH /1/. All the complexes formed were so called Meisenheimer complexes /2/. The stability of the complexes and their reactivity towards o-Ps can be accounted for by the differences in their structure, in the size of the donor molecules and in the charge distribution on the complexes

Extraction of actinides with tropolone

The partition coefficient of tropolone in xylene, dichloroethane and chloroform was determined from 0.05M HCl medium. The values obtained were 5.6, 25.5 and 43, respectively. In the extraction of UO 2 2+ using tropolone (HT) in xylene from an aqueous medium of ionic strength 0.05, the species UO2T2·HT was established. The species extracted in presence of a neutral donor was found to be UO2T2·S[S+dioctyl sulfide (DOS), dibutylhexanamide (DBHA), tri-n-butyl phosphate (TBP), dioctyl sulfoxide (DOSO), and tri-n-octylphosphine oxide (TOPO)]. The equilibrium constant of complex formation between the self-adduct and the neutral donor was calculated and was found to follow the basicity order (DOS«DBHA<TBP<DOSO»TOPO). The thermodynamic parameters of the adduct UO2T2·DOSO were also calculated by the temperature coefficient method. The values of free energy, enthalpy and entropy changes obtained were −1.26 kJ·mol−1, −1.98 kJ/mol and −2.5 J·mol−1·deg−1, respectively.