Studies on kinetics of complexation of lanthanide α-hydroxycarboxylates with 1,10-phenanthroline

Kinetics of the coordination reaction of lanthanide (La^III, Eu^III) α-hydroxycarboxylates [LnL₃(H₂O)₂] with 1,10-phenanthroline (phen) in methanol-water (v/v, 3:2) were studied at 25°C by calorimetric titration. A one-step reaction process in accordance with the rate law has been suggested. The reaction is found to be first order for both lanthanide α-hydroxycarboxylates and phen. We have evaluated rate constants of the reactions. It is found that a linear free energy relationship exists between the stability constants of the lanthanide-α-hydroxycarboxylate-phen ternary complex and the rate constants. It is also found that a linear free energy relationship exists between the rate constants of La-hydroxycarboxylate with phen and the acid strength of α-hydroxy-acid as primary ligand, but the linear free energy relationship does not exist in the Eu-α-hydroxycarboxylate-phen ternary complex. The influence of other factors upon the reaction rate constants was also discussed.     

Mechanism of Exciplex Decay: The Quantum Yields and the Rate Constants of Radical Ion Formation from Exciplexes with Partial Charge Transfer

The dynamics of exciplex and radical ion formation was studied in donor–acceptor systems with G ^* _et > –0.1 eV. It was shown that the quenching of excited singlet states of aromatic molecules by electron donors in polar solvents led to the formation of radical ions via exciplex dissociation resulting to complete charge separation. Intersystem crossing and internal conversion into the ground state (back electron transfer) compete with this process. The quantum yields and the rate constants of the radical ion formation were measured.     

Synthesis of novel N-protected β-amino nitriles: study of their hydrolysis involving a nitrilase-catalyzed step

Several commercially available nitrilases were investigated with regard to their potential to hydrolyze N-protected β³-amino nitriles into their corresponding N-protected β³-amino acids.The biotransformations were obtained in different proportions depending on the nitrilase involved. The best hydrolysis results were achieved for the N-Cbz-β³-amino nitrile from l-alanine using the NIT-107, in a phosphate buffer at 0.05 M. However, no biotransformation into the corresponding acids was observed for the N-sulfonylamide β³-amino nitriles. Two simple and efficient procedures to prepare the β³-amino nitriles from their analogous α-amino acids are described. Thirty four new substances were synthesized and characterized over the course of this work.     

Analysis of Bioactive Triterpenes from Rubus chingii by Cyclodextrin-Modified Capillary Electrophoresis

A capillary electrophoretic (CE) method with β-cyclodextrin (CD) as modifier has been developed to enable separation, for the first time, of bioactive pentacyclic triterpene acids from the fruits of Rubus chingii. The effects of conditions such as the concentration of the running buffer, amounts of added β-cyclodextrin and organic modifier, applied voltage, and column temperature were systematically investigated to optimize the separation conditions. Baseline separation was achieved for the seven triterpenes by use of background electrolyte consisting of 200 mmol L⁻¹ disodium tetraborate, 15 mmol L⁻¹ β-cyclodextrin, and 12.5% (v/v) methanol. Binding constants between β-cyclodextrin and the triterpenes in the capillary electrophoresis buffer were calculated from the order of migration to elucidate the separation mechanism. The amounts of the triterpenes in the fruits of R. chingii were also determined by use of the method after a relatively simple extraction procedure.Revised: 5 January and 22 April 2005     

Effect of the acyl group and the dodecylsulfonate chain on the inclusion of pyrene inside the cavity of β-cyclodextrin

The solubilization of pyrene in aqueous solution of β-cyclodextrin (β-CD) or its derivatives such as β-CD-hexanoyl, β-CD-benzoyl and β-CD-dodecylsulfonate was investigated by spectrophotometry. Linear and non-linear regression methods were used to estimate the association constants (K₁). A 1:1 stoichiometric ratio and different effects of the hexanoyl, benzoyl and dodecylsulfonate groups on the association constant were observed for the binary inclusion complex between pyrene and β-CD. The formation constant was shown to decrease when β-CD was modified by a dodecylsulfonate chain. The value of K₁ was 190 ± 10 L mol⁻¹ for the [pyrene/β-CD] complex and 145 L mol⁻¹ for the [pyrene/β-CD-dodecylsulfonate] complex. Partitioning of the pyrene molecules between the dodecylsulfonate chains and cyclodextrin cavities can explain the decrease in the association constant value. In the cases of β-CD-hexanoyl and β-CD-benzoyl derivatives, no association constants were detected. Results suggest that the high hydrophobicity of the hexanoyl and benzoyl groups prevents the inclusion of pyrene molecules inside the cyclodextrin cavity.     

Base Equilibria of Substituted Pyridines in Nitromethane

In the framework of our studies on acid=nbase equilibria in systems comprisingsubstituted pyridines and nonaqueous solvents, acid dissociation constants havebeen determined potentiometrically for a variety of cationic acids conjugatedwith pyridine and its derivatives in the polar protophobic aprotic solvent nitromethane. The potentiometric method enabled a check as to whether and to whatextent cationic homoconjugation equilibria of the BH⁺/B type, as well as cationicheteroconjugation equilibria in BH⁺/B₁ systems without proton transfer, are setup in nitromethane. The equilibrium constants were compared with thosedetermined in water and two other polar protophobic aprotic solvents, propylenecarbonate and acetonitrile. The pK _a values of acids conjugate to the N-bases innitromethane fall in the pK _a range of 5.84 to 17.67, i.e., 6 to 7 pK _a units, onaverage, higher than in water, 1 to 2 units higher than in propylene carbonate,and less than 1 unit lower than in acetonitrile. This means that the basicity ofthe pyridine derivatives increases on going from propylene carbonate throughnitromethane to acetonitrile. Further, it was found that the sequence of the pK _achanges of the protonated amines was consistent in all three media, thus providingthe basis for establishing linear correlations among these values. In the majorityof the BH⁺/B systems in nitromethane, cationic homoconjugation equilibria havebeen established. The cationic homoconjugation constants, log K _BHB+, arerelatively low, falling in the range 1.60–2.89. A comparison of the homoconjugationconstants in nitromethane with those in propylene carbonate and acetonitrile showsthat nitromethane is a more favorable solvent for the cationic homoconjugationequilibria than the other two solvents. Moreover, results of the potentiometricmeasurements revealed that cationic heteroconjugation equilibria were not presentin the majority of the BH⁺/B₁ systems in nitromethane. The heteroconjugationconstant could be determined in one system only, with logdiK _BHB1 + = 2.56.     

Equilibrium Studies of Schiff Bases and Their Complexes with Ni(II), Cu(II) and Zn(II) derived from Salicylaldehyde and Some α-Amino Acids

The acid-base equilibria of Schiff bases derived from salicylaldehyde, glycine, alanine, serine, tyrosine, and phenylalanine, and their Ni(II), Cu(II) and Zn(II) complex formation equilibria were investigated by a potentiometric method in aqueous solution (t = 25^∘C, μ = 0.1 M, KCl). The data from the potentiometric titrations were evaluated by means of the BEST computer program. The order of the formation constant values of the Schiff bases was Sal-Ala > Sal-Gly > Sal-Ser > Sal-Phe > Sal-Tyr, which is the same order as the increasing log K₁ values of amino acids (and the log K₂ values of tyrosine) with the exception of an inversion between serine and phenylalanine. Also, it was seen that the stability constants, log β₁ and log β₂, of Schiff base–metal complexes vary for all the metal ions investigated, viz., Sal-Gly > Sal-Ala > Sal-Ser > Sal-Tyr > Sal-Phe with the exception of Sal-Gly in the copper complex. The effect of the nature of the amino acids on their formation, protonation and stability constants was also discussed.     

Factors controlling the dissociation of aliphatic carboxylic acids in 80 % methylcellosolve - 20 % water. A case for the importance of polar effects

The dissociation constants of a wide range of aliphatic carboxylic acids in 80 % methylcellosolve- 20 % water have been investigated. The pK variation between the limits defined by the acids MeCO₂H and i-Pr₃CCO₂H is controlled principally by polar effects as defined by σ^*.     

Investigations of Cobalt, Nickel, and Copper Diethyldithiocarbamates Using Thermal Lens Spectrometry

Thermal lens spectrometry was used to study the dissociation kinetics of diethyldithiocarbamate complexes of copper(II), cobalt(III), and nickel(II) as a function of pH in the presence of chloride and sulfate ions. It is shown that, as distinct from conventional spectrophotometric and potentiometric measurements, the reversible dissociation of the test complexes and the irreversible oxidation of the ligand can be studied separately (at a level of n × 10^–8–n × 10^–6 M) using thermal lens spectrometry. Because of work in more dilute solutions and due account of the kinetic features of the systems in question, thermal lens spectrometry provides a higher accuracy of the determination of stability constants for diethyldithiocarbamate complexes of copper(II), cobalt(III), and nickel(II). The adsorption of the diethyldithiocarbamate complexes in question from water–ethanol solutions (1 : 3) on Silasorb C₁₈ silica is studied, and the adsorption constants are determined. The limits of detection of copper(II), cobalt(III), and nickel(II) diethyldithiocarbamates obtained in extraction–thermal-lens determination are n × 10^–8 M.     

Chemical bond inside a fullerene cage: is it possible?

The geometries, electronic structures, and hyperfine coupling constants of azafullerene C₅₉N (a π-electron radical) and its derivatives, C₅₉NH and endofullerene H@C₅₉N, were calculated at the B3LYP level of the density functional theory. Analysis of calculated potential energy profiles along trajectories of the motion of encapsulated hydrogen atom from the center of the fullerene sphere toward different atoms of C₅₉N revealed formation of a chemical bond between the H atom and a carbon atom that is involved in the 6,6-bond with the N atom and bears the most part of the π-electron spin density. The C—H endo-bond length is 1.12 Å, the bond dissociation energy being equal to 26.4 kcal mol⁻¹. The C—H exo-bond involving the same carbon atom is 0.02 Å shorter than the endo-bond, the bond dissociation energy being much higher (78.4 kcal mol⁻¹).__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 51–54, January, 2005.     

Experimental and calculated complex formation curves for mixed, dynamic and semi-dynamic, metal–ligand systems.: A differential pulse polarographic study of Cu–sarcosine–OH and Cd–MIDA–OH systems at a fixed ligand to metal ratio and varied pH

The Cu–sarcosine–OH and Cd–MIDA–OH systems have been studied by differential pulse polarography (DPP) at a fixed total ligand to total metal concentration ratio and varied pH at 298 K and μ=0.5 mol dm⁻³ in the background of NaNO₃. Both the metal–ligand systems show initially dynamic (labile), followed by semi-dynamic behaviour on the DPP time scale. It has been shown that the experimental and calculated DPP complex formation curves used previously only for labile metal–ligand systems can be employed for the modelling of all species formed in a solution and optimisation of their stability constants. The stability constants of ML and ML₂ complexes as log β were estimated for Cu^II and Cd^II as 7.75±0.02, 14.49±0.01 and 6.67 ±0.02, 12.00±0.02, respectively (all known hydroxide species of copper and cadmium, including polynuclear species, were incorporated into the metal–ligand–OH systems). The formation of the complex CuL₂(OH) is suggested also and its stability constant as log β has been estimated to be 16.2±0.2. Results reported here seem to be reasonable when compared with the literature data reported at 298 K and different ionic strengths.     

Isotachophoretic determination of stability constants of Ho and Y complexes with diethylenetriaminepentaacetic acid and 1,4,7,10-tetraazadodecane-N,N′,N″,N-tetraacetic acid

The method of capillary isotachophoresis with conductivity detection was applied for the determination of the physico-chemical characteristics (conditional stability constants log β′) of holmium and yttrium complexes with DTPA (diethylenetriaminepentaacetic acid) and DOTA (1,4,7,10-tetraazadodecane-N,N′,N″,N-tetraacetic acid). The log β′ determination is based on the linear relation between the stability constants of lanthanide–DTPA (lanthanide–DOTA) complexes and the reduction of the zone of the complex owing to the bleeding phenomena (liberating free metal ion). The stability constants calculated using this relationship are comparable with the literary data obtained by other methods for both holmium (log β′_Ho–DTPA=21.9, log β′_Ho–DOTA=24.5) and yttrium complexes (log β′_Y–DTPA=21.2, log β′_Y–DOTA=24.4). Capillary isotachophoresis was applied for the determination of the optimum composition of the reaction mixture (metal:ligand ratio) as well.     

Investigation of the interactions between ginsenosides and amino acids by mass spectrometry and theoretical chemistry

In order to evaluate the essence of the interactions of ginsenosides and proteins which are composed by α-amino acids, electrospray ionization mass spectrometry was employed to study the noncovalent interactions between ginsenosides (Rb₂, Rb₃, Re, Rg₁ and Rh₁) and 18 kinds of α-amino acids (Asp, Glu, Asn, Phe, Gln, Thr, Ser, Met, Trp, Val, Gly, Ile, Ala, Leu, Pro, His, Lys and Arg). The 1:1 and 2:1 noncovalent complexes of ginsenosides and amino acids were observed in the mass spectra. The dissociation constants for the noncovalent complexes were directly calculated based on peak intensities of ginsenosides and the noncovalent complexes in the mass spectra. Based on the dissociation constants, it can be concluded that the acidic and the basic amino acids, Asp, Glu, Lys and Arg, bound to ginsenosides more strongly than other amino acids. The experimental results were verified by theoretical calculations of parameters of noncovalent interaction between ginsenoside Re and Arg which served as a representative example. Two kinds of binding forms, “head–tail” (“H–T”) and “head–head” (“H–H”), were proposed to explain the interaction between ginsenosides and amino acids. And the interaction in “H–T” form was stronger than that in “H–H” form.     

Nonaqueous Solution Studies on the Protonation Equilibria of some Phenolic Acids

The protonation equilibria for some phenolic acids in nonaqueous solutions have been studied by pH-potentiometry. The dissociation constants, pK _a, of these phenolic acids and the thermodynamic functions, ΔG ^o,ΔH ^o and ΔS ^o, for the successive and overall protonation processes of these phenolic acids have been derived at different temperatures in three different mixtures of water and dioxane (mole fractions of dioxane were 0.083, 0.174 and 0.33). Titrations were also carried out in (water + dioxane) with ionic strengths of 0.15, 0.20 and 0.25 mol⋅dm⁻³ NaNO₃, and the resulting dissociation constants are reported. A detailed thermodynamic analysis of the effects of organic solvent, dioxane, temperature and ionic strength on the protonation processes of phenolic acids is presented and discussed to determine the factors which control these processes. Ahmed E. Fazary; previous address: Egyptian Organization for Biological Products and Vaccines, 51 Wezaret El-Zeraa Street, Agouza, Giza, Egypt. Tel. +2010-3017357.     

Thermodynamic Properties of Alanylpeptide Buffer Systems and Their Potential as Standards in Biological Applications

The second dissociation constants pK ₂of the NH₃ ⁺charge center of the alanylpeptides, alanylglutamine (Ala–Gln), alanylleucine (Ala–Leu), alanylglycine (Ala–Gly), and DL-alanyl–DL-methionine (DL-Ala–DL-Met) were determined at ten temperatures in the range, 5–50°C. These pK ₂values were calculated from the emf of cells containing buffer solutions of these dipeptides. A cell of the type described by Harned and Ehlers,⁽¹⁾utilizing hydrogen and silver–silver bromide electrodes was used. The thermodynamic quantities, H^o, S^o, and C_p ^owere derived from the temperature coefficients of the dissociation constants. The pK ₂values at 25°C, 8.2105 ( Ala–Gln), 8.2668 ( Ala–Leu), 8.2940 ( Ala–Gly), and 8.3054 ( DL-Ala–DL-Met). These values show that different substituent groups on the -carbon atom (which include polar and nonpolar groups), have a small effect on the dissociation of the NH₃ ⁺charge center. These compounds were also found to be suitable as buffers in the pH range(7–9). The thermodynamics of the solute–solvent interaction is interpreted in terms of the mixture model.⁽²⁾     

Surface dissociation constants for solid oxide/aqueous solution systems

Summary An equation for the surface potential ₀ was used to define the surface dissociation constant of surface hydroxyls at a solid oxide/aqueous solution interface.Using the measurements of the surface charge, the Gouy-Chapman theory and crystallo-chemical data for oxides, the calculations of the surface dissociation constants have been carried out. The values of the acidic surface dissociation constants (in minus logarithmic scale) fall in range 8.7±0.8 at ionic strength 1 M and in the range 7.2±0.7 at 10^–3 M KNO₃ These constants exceed by 2 to 5 orders of magnitude the dissociation constants of M(OH) _naq species in solution.With 1 table     

Effect of β-Cyclodextrin on the Photoinduced Charge Transfer in Sodium 1-Anilino-8-naphthalene Sulfate (ANS)/CdS Colloidal System

The S-center radical (ANS^·) of sodium 1-anilino-8-naphthalene sulfate (ANS) generated by photoinduced charge transfer in ANS/CdS and ANS/CdS/β-cyclodextrin(β-CD) systems has been studied by using spin trapping electron spin resonance techniques, UV-visible spectroscopic methods, and fluorescence spectroscopic methods. It was found that the S-centered radical (ANS^·) was produced by the charge transfer reaction between the ground state ANS and the positive hole h⁺(CdS) from the valence band of CdS colloids, by the charge transfer from the excited singlet state ¹ANS* to the conduction band of CdS colloids, or by both in the ANS/CdS and ANS/CdS/β-CD systems. The ESR signal intensity of the spin adduct (5,5′-dimethyl-1-pyrroline-N-oxide (DMPO)–ANS)^·, which is formed from ANS^· trapped by DMPO, in the latter system is 15 times stronger than that in the former system. The apparent association constants between ANS and CdS colloids in the absence and presence of β-CD determined from fluorescence quenching experiments are 1097 and 1606 M⁻¹, respectively. From ESR and fluorescence results, it is estimated that the efficiency of photoinduced charge transfer from ANS to CdS colloids in the ANS/CdS/β-CD system is 12.5 times that in the ANS/CdS system.     

Investigation of the chromatographic process by the correlation between the RM-values in adsorption and partition chromatography

Summary Investigations were carried out on the correlations between the R_M-values measured in both adsorption and partition chromatography for some heterocyclic bases, 1-naphthylamine, 1-naphthol and phenol. Silica gel G and aluminium oxide G were used as the adsorbents while paraffin oil served as the liquid stationary phase in partition chromatography. Methanol, ethanol, isopropyl alcohol and acetone were used as the mobile phases. It was found that, similarly to the previous papers [1–3], the relationship for all substances is linear described by the equation . However, in contrast to the previously investigated chromatographic systems [1–3], the constants a and b take negative values. This is due to the fact that in the adsorbent — mobile phase and liquid stationary phase — mobile phase interphase layers, contrasting types of molecular interactions prevail. In the polar adsorbent — apolar liquid stationary phase systems the values of the constant a for the substances belonging to the same compound class were more differentiated than those in the polar adsorbent — polar liquid stationary phase systems. The constants b are characterized by a smaller differentiation. The results of the present examinations on the correlations between the R_M-values in adsorption and partition chromatography show that these may be used in investigating the mechanism of the chromatographic processes.     

The Complexes of Holmium with Methylenediphosphonate and 1-Hydroxyethylidenephosphonate

The composition and stability of holmium methylenediphosphonate (MDP) and 1-hydroxyethylidenephosphonate (HEDP) complexes were studied by potentiometric titration methods in 0.1M NaCl at 25 °C. It was found that besides L^4– anions the protonated H _n L^(4–n)– species (n = 1–3 for MDP and n = 1–4 for HEDP) are present in the pH region 3 to 10. The presence of the undissociated acids (H₄L) has not been unambiguously proved for MDP. The complexes of the composition HoH _n L (n varies from 1 to –2 for MDP and from 1 to –1 for HEDP) have been found if the concentration of the ligand is higher than the concentration of holmium. The protonation constants of both acids and the stability constants of the complexes discussed were determined and the comparison with literature data of analogical complexes of other lanthanides was performed.