Affinity capillary electrophoresis and density functional theory applied to binding constant determination and structure elucidation of hexaarylbenzene-based receptor complex with ammonium cation

Affinity capillary electrophoresis (ACE) and quantum mechanical density functional theory (DFT) calculations have been employed for the investigation of noncovalent interactions between hexaarylbenzene-based receptor (R) and ammonium cation NH(4)(+). Firstly, by means of ACE, the binding constant of the NH(4)R(+) complex in methanol was estimated from the dependence of the effective electrophoretic mobility of the receptor R (in advance corrected by our earlier developed procedure to a reference temperature of 25°C) on the concentration of ammonium ion in the background electrolyte using non-linear regression analysis. The logarithmic form of the apparent binding (stability) constant of NH(4)R(+) complex in the methanolic background electrolyte (25 mM Tris, 50 mM chloroacetate, pH(MeOH) 7.8) was evaluated as log K(NH(4)R) = 4.03 ± 0.15. Secondly, the structural characteristics of NH(4)R(+) complex were determined by DFT calculations.     

Affinity capillary electrophoresis employed for determination of stability constants of antamanide complexes with univalent and divalent cations in methanol

Affinity capillary electrophoresis (ACE) and pressure‐assisted ACE were employed to study the noncovalent molecular interactions of antamanide (AA), cyclic decapeptide from the deadly poisonous fungus Amanita phalloides, with univalent (Li⁺, Na⁺, K⁺, and NH₄⁺) and divalent (Mg²⁺ and Ca²⁺) cations in methanol. The strength of these interactions was quantified by the apparent stability constants of the appropriate AA‐cation complexes. The stability constants were calculated using the nonlinear regression analysis of the dependence of the effective electrophoretic mobility of AA on the concentration of the above ions in the BGE (methanolic solution of 20 mM chloroacetic acid, 10 mM Tris, pH_MeOH 7.8, containing 0–50 mM concentrations of the above ions added in the form of chlorides). Prior to stability constant calculation, the AA effective mobilities measured at actual temperature inside the capillary and at variable ionic strength of the BGEs were corrected to the values corresponding to the reference temperature of 25°C and to the constant ionic strength of 10 mM. From the above ions, sodium cation interacted with AA moderately strong with the stability constant 362 ± 16 L/mol. K⁺, Mg²⁺, and Ca²⁺ cations formed with AA weak complexes with stability constants in the range 37–31 L/mol decreasing in the order K⁺ > Ca²⁺ > Mg²⁺. No interactions were observed between AA and small Li⁺ and large NH₄⁺ cations.     

Affinity capillary electrophoresis and quantum mechanical calculations applied to the investigation of hexaarylbenzene-based receptor binding with lithium ion

In this study, two complementary approaches, affinity capillary electrophoresis (ACE) and quantum mechanical density functional theory (DFT) calculations, have been employed for quantitative characterization and structure elucidation of the complex between hexaarylbenzene (HAB)‐based receptor R and lithium ion Li⁺. First, by means of ACE, the apparent binding constant of Li R ⁺ complex (K) in methanol was determined from the dependence of the effective electrophoretic mobilities of Li R ⁺ complex on the concentration of lithium ions in the 25 mM Tris/50 mM chloroacetate background electrolyte (BGE) using non‐linear regression analysis. Prior to regression analysis, the effective electrophoretic mobilities of the Li R ⁺ complex were corrected to reference temperature 25°C and constant ionic strength 25 mM. The apparent binding constant of the Li R ⁺ complex in the above methanolic BGE was evaluated as logK = 1.15±0.09. Second, the most probable structures of nonhydrated Li R ⁺ and hydrated Li R ⁺·3H₂O complexes were derived by DFT calculations. The optimized structure of the hydrated Li R ⁺·3H₂O complex was found to be more realistic than the nonhydrated Li R ⁺ complex because of the considerably higher binding energy of Li R ⁺·3H₂O complex (500.4 kJ/mol) as compared with Li R ⁺ complex (427.5 kJ/mol).     

Application of affinity capillary electrophoresis and density functional theory to the investigation of benzo-18-crown-6-ether complex with ammonium cation

Affinity capillary electrophoresis (ACE) and quantum mechanical density functional theory (DFT) calculations have been employed for investigation of non-covalent interactions between macrocyclic ligand, benzo-18-crown-6-ether (B18C6) and ammonium cation, NH₄⁺. Firstly, by means of ACE, the strength of the B18C6-NH₄⁺ complex in mixed binary hydro-organic solvent system, methanol–water (50/50, v/v), was determined from the dependence of effective electrophoretic mobility of B18C6 (corrected to reference temperature 25 °C and constant ionic strength, 10 mM) on the concentration of ammonium ion in the background electrolyte (BGE) using non-linear regression analysis. The logarithmic form of the apparent binding (stability) constant (log K_b) of B18C6-NH₄⁺ complex in the above binary solvent system was found to be equal to log K_b = 1.63 ± 0.10. Secondly, the structural characteristics of B18C6-NH₄⁺ complex were described by quantum mechanical density functional theory (DFT) calculations. According to these calculations, in the energetically most favoured form of the B18C6-NH₄⁺ complex, three strong hydrogen bonds are formed between central ammonium ion and B18C6 ligand, one of them is directed to aryl-O-alkyl (Ar–O–CH₂) ethereal oxygen and the other two hydrogen bonds are oriented to alkyl-O-alkyl (CH₂–O–CH₂) ethereal oxygen atoms of the macrocyclic crown ligand.     

Affinity capillary electrophoresis method for investigation of bile salts complexation with sulfobutyl ether‐β‐cyclodextrin

Sulfobutyl ether‐β‐cyclodextrin (SBEβCD) is utilized in preformulation and drug formulation as an excipient for solubilization of drugs with poor aqueous solubility. Approximately seven negative charges of SBEβCD play a role with respect to solubilization and complexation, but also have an influence on the ionic strength of the background electrolyte when the cyclodextrin is used in capillary electrophoresis. Mobility‐shift affinity capillary methods for investigation of the complexation of taurocholate and taurochenodeoxycholate with the negatively charged cyclodextrin derivative applying constant power and ionic strength conditions as well as constant voltage and varying ionic strength were investigated. A new approach for the correction of background electrolyte ionic strength was developed. Mobility‐shift affinity capillary electrophoresis experiments obtained at constant voltage and constant power settings were compared and found to provide binding parameters that were in good agreement upon correction. The complexation of taurochenodeoxycholate with SBEβCD was significantly stronger than the corresponding interaction involving taurocholate. The obtained stability constants for the bile salts were in the same range as those previously reported for the interaction with neutral β‐cyclodextrins derivatives, i.e. the positions of the negative charges on SBEβCD and the bile salts within the complex did not lead to significant electrostatic repulsion.     

Structure and conformational equilibrium of thiacalix[4]arene by density functional theory

Density functional theory calculations for the structure and conformational equilibrium of thiacalix[4]arene are reported. The conformational equilibrium of thiacalix[4]arene, a heterocalixarene in which the phenol groups are bridged by sulphur atoms is compared to the conformational equilibrium of calix[4]arene. Thiacalix[4]arene conformational energies relative to the cone conformer (ΔE's) are reduced in comparison with calix[4]arene. This conformational change is in qualitative agreement with recent NMR spectroscopy measurements of the conformational equilibrium for a tetraethylether of thiacalix[4]arene in a CDCl₃ solution which indicates an enhanced chemical exchange of thiacalixarene conformers in comparison with similar methylene bridged structures. Density functional theory results for the structure of thiacalix[4]arene are in good agreement with recent X-ray diffraction measurements. The electrostatic potentials in the cone conformers of thiacalix[4]arene and calix[4]arene suggest that their complexation or recognition abilities can be significantly different. Dipole moments of the four thiacalix[4]arene conformers are in the order: cone>1,2-alternate>partial-cone>1,3-alternate.     

Ca2+‐complex stability of GAPAGPLIVPY peptide in gas and aqueous phase,investigated by affinity capillary electrophoresis and molecular dynamics simulations and compared to mass spectrometric results

Strong, sequence‐specific gas‐phase bindings between proline‐rich peptides and alkaline earth metal ions in nanoESI‐MS experiments were reported by Lehmann et al. (Rapid Commun. Mass Spectrom. 2006, 20, 2404–2410), however its relevance for physiological‐like aqueous phase is uncertain. Therefore, the complexes should also be studied in aqueous solution and the relevance of the MS method for binding studies be evaluated. A mobility shift ACE method was used for determining the binding between the small peptide GAPAGPLIVPY and various metal ions in aqueous solution. The findings were compared to the MS results and further explained using computational methods. While the MS data showed a strong alkaline earth ion binding, the ACE results showed nonsignificant binding. The proposed vacuum state complex also decomposed during a molecular dynamic simulation in aqueous solution. This study shows that the formed stable peptide–metal ion adducts in the gas phase by ESI‐MS does not imply the existence of analogous adducts in the aqueous phase. Comparing peptide–metal ion interaction under the gaseous MS and aqueous ACE conditions showed huge difference in binding behavior.     

Study of solvent effects on the stability constant and ionic mobility of the dibenzo‐18‐crown‐6 complex with potassium ion by affinity capillary electrophoresis

The effect of solvent on the strength of noncovalent interactions and ionic mobility of the dibenzo‐18‐crown‐6 complex with K⁺ in water/organic solvents was investigated by using affinity capillary electrophoresis. The proportion of organic solvent (methanol, ethanol, propan‐2‐ol, and acetonitrile) in the mixtures ranged from 0 to 100 vol.%. The stability constant, K_KL, and actual ionic mobility of the dibenzo‐18‐crown‐6‐K⁺ complex were determined by the nonlinear regression analysis of the dependence of the effective electrophoretic mobility of dibenzo‐18‐crown‐6 on the concentration of K⁺ (added as KCl) in the background electrolyte (25 mM lithium acetate, pH 5.5, in the above mixed hydro–organic solvents). Competitive interaction of the dibenzo‐18‐crown‐6 with Li⁺ was observed and quantified in mixtures containing more than 60 vol.% of the organic solvent. However, the stability constant of the dibenzo‐18‐crown‐6‐Li⁺ complex was in all cases lower than 0.5 % of K_KL. The log K_KL increased approximately linearly in the range 1.62–4.98 with the increasing molar fraction of organic solvent in the above mixed solvents and with similar slopes for all four organic solvents used in this study. The ionic mobilities of the dibenzo‐18‐crown‐6‐K⁺ complex were in the range (6.1–43.4) × 10^?9 m² V^?1 s^?1.     

A density functional theory study on diazotization and nitration of 3,5‐diamino‐1,2,4‐triazole

The reaction mechanism, thermodynamic and kinetic properties for diazotization and nitration of 3,5‐diamino‐1,2,4‐triazole were studied by a density functional theory. The geometries of the reactants, transition states, and intermediates were optimized at the B3LYP/6‐31G (d, p) level. Vibrational analysis was carried out to confirm the transition state structures, and the intrinsic reaction coordinate (IRC) method was used to explore the minimum energy path. The single‐point energies of all stagnation points were further calculated at the B3LYP (MP2)/6‐311+G (2d, p) level. The statistical thermodynamic method and Eyring transition state theory with Wigner correction were used to study the thermodynamic and kinetic characters of all reactions within 0–25°C. Two reaction channels are computed, including the diazotization and nitration of 3‐NH₂ or 5‐NH₂, and there are six steps in each channel. The reaction rate in each step is increased with temperature. The last step in each channel is the slowest step. The first, second, and fifth steps are exothermic reactions, and are favored at lower temperature in the thermodynamics. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Enantioseparation of dansyl amino acids by ligand‐exchange capillary electrophoresis with zinc(II)‐L‐phenylalaninamide complex

A novel method of chiral ligand‐exchange CE was developed with L ‐amino acylamides as a chiral ligand and zinc(II) as a central ion. It has been demonstrated that these chiral complexes, such as Zn(II)‐L ‐alaninamide, Zn(II)‐L ‐prolinamide, and Zn(II)‐L ‐phenylalaninamide, are suitable for use as chiral selectors for the enantioseparation of either individual pair of or mixed dansyl amino acids. The optimal separation running buffer consisted of 5 mM ammonium acetate, 100 mM boric acid, 4 mM ZnSO₄·7 H₂O, and 8 mM L ‐amino acylamides at pH 8.2. The experiments showed that apart from the effect of the concentration of the complexes on the resolution and the migration time, the buffer pH also had a sharp influence on resolution. The employed chiral ligands exhibited different enantioselectivities and enantiomer migration orders. D ‐Amino acids migrate faster than L ‐amino acids when Zn(II)‐L ‐alaninamide and Zn(II)‐L ‐phenylalaninamide are used as chiral selectors, but it was observed that the migration order is reversed when Zn(II)‐L ‐prolinamide is used as the chiral selector. Furthermore, the amount of dansylated amino acids is found to be highly dependent on the labeling temperature.     

Density functional theory study of calix[4]arene‐N‐azacrown‐5, calix[4]arene‐N‐phenyl‐azacrown‐5, and their complexes with alkali‐metal cations: Na+, K+, and Rb+

Theoretical studies of 1,3‐alternate‐25,27‐bis(1‐methoxyethyl)calix[4]arene‐azacrown‐5 ( L₁ ), 1,3‐alternate‐25,27‐bis(1‐methoxyethyl)calix[4]arene‐N‐phenyl‐azacrown‐5 ( L₂ ), and the corresponding complexes M⁺/ L of L₁ and L₂ with the alkali‐metal cations: Na⁺, K⁺, and Rb⁺ have been performed using density functional theory (DFT) at B3LYP/6‐31G* level. The optimized geometric structures obtained from DFT calculations are used to perform natural bond orbital (NBO) analysis. The two main types of driving force metal–ligand and cation–π interactions are investigated. The results indicate that intermolecular electrostatic interactions are dominant and the electron‐donating oxygen offer lone pair electrons to the contacting RY* (1‐center Rydberg) or LP* (1‐center valence antibond lone pair) orbitals of M⁺ (Na⁺, K⁺, and Rb⁺). What's more, the cation–π interactions between the metal ion and π‐orbitals of the two rotated benzene rings play a minor role. For all the structures, the most pronounced changes in geometric parameters upon interaction are observed in the calix[4]arene molecule. In addition, an extra pendant phenyl group attached to nitrogen can promote metal complexation by 3D encapsulation greatly. In addition, the enthalpies of complexation reaction and hydrated cation exchange reaction had been studied by the calculated thermodynamic data. The calculated results of hydrated cation exchange reaction are in a good agreement with the experimental data for the complexes. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Complex of [BMIm] PF6 with PEG1000: a high efficient and recycle system for palladium‐catalyzed Suzuki cross‐coupling and Heck reaction

A complex of [BMIm] PF₆ (1‐Butyl‐3‐methylimidazolium hexafluorophosphate) with PEG₁₀₀₀ has been successfully applied and found to be efficient for Suzuki cross‐coupling and Heck reaction using Pd(OAc)₂ as the catalyst under air atmosphere. Moreover, the isolation of the products is readily performed by the extraction of diethyl ether, and the catalytic system reported here can be recycled and reused several times. Copyright © 2012 John Wiley & Sons, Ltd.     

Electronic structure and spectroscopic properties of 6‐aminophenanthridine and its derivatives: Insights from density functional theory

6‐Aminophenanthridine (6AP) and its derivatives show important biological activities as antiprion compounds and inhibitors of the protein folding activity of the ribosome. Both of these activities depend on the RNA binding property of these compounds, which has been recently characterized by fluorescence spectroscopy. Hence, fundamental insights into the photophysical properties of 6AP compounds are highly important to understand their biological activities. In this work, we have calculated electronic structures and optical properties of 6AP and its three derivatives 6AP8CF₃, 6AP8Cl, and 6APi by density functional theory (DFT) and time‐dependent density functional theory (TDDFT). Our calculated spectra show a good agreement with the experimental absorption and fluorescence spectra, and thus, provide deep insights into the optical properties of the compounds. Furthermore, comparing the results obtained with four different hybrid functionals, we demonstrate that the accuracy of the functionals varies in the order B3LYP > PBE0 > M062X > M06HF. © 2015 Wiley Periodicals, Inc.     

Quantitative evaluation of lectin‐reactive glycoforms of α1‐acid glycoprotein using lectin affinity capillary electrophoresis with fluorescence detection

α₁‐Acid glycoprotein (AGP) was previously shown to be a marker candidate of disease progression and prognosis of patients with malignancies by analysis of its glycoforms via lectins. Herein, affinity capillary electrophoresis of fluorescein‐labeled AGP using lectins with the aid of laser‐induced fluorescence detection was developed for quantitative evaluation of the fractional ratios of concanavalin A‐reactive or Aleuria aurantia lectin‐reactive AGP. Labeled AGP was applied at the anodic end of a fused‐silica capillary (50 μm id, 360 μm od, 27 cm long) coated with linear polyacryloyl‐β‐alanyl‐β‐alanine, and electrophoresis was carried out for about 10 min in 60 mM 3‐morpholinopropane‐1‐sulfonic acid‐NaOH buffer (pH 7.35). Addition of the lectins to the anode buffer resulted in the separation of lectin‐reactive glycoform peaks from lectin‐non‐reactive glycoform peaks. Quantification of the peak area of each group revealed that the percent of lectin‐reactive AGP is independent of a labeling ratio ranging from 0.4 to 1.5 mol fluorescein/mol AGP, i.e. the standard deviation of 0.5% for an average of 59.9% (n=3). In combination with a facile procedure for micro‐purification of AGP from serum, the present procedure, marking the reactivity of AGP with lectins, should be useful in determining the prognosis for a large number of patients with malignancies.     

Formation and Molecular Structure of an Ion Pair Iron (II) Compound Derived from 2,6‐Bis‐ [1‐(2,6‐dibromophenylimino) ‐ethyl] pyridine and 2‐Acetyl‐6‐[1‐(2, 6‐dibromophenylimino)‐ethyl] pyridine

Two novel tridentate ligands of 2,6‐bis‐[l‐(2,6‐dibromophenylimino) ethyl] pyridine (L₁) and2‐acetyl‐6‐[1‐(2,6‐dibromophenylimino) ethyl] pyridine (L2) have been synthesized. The iron(II) complex of L₁ and L₂ has been characterized with the crystal structure of [Fe(L₁)(L₂)]²⁺ [FeCl₄]² CH₂Cl₂ [monoclinic, P2₁ (#11), a = 1.0562(4), b = 2.0928(4), c = 1.2914(2) nm, β = 100.12°, V = 2.810(1) nm³ D_c = 1.879 g/cm³ and Z = 2].     

Structure and stability of XeF6 isomers: density functional theory study and the maximum hardness principle

The geometrical configurations of the four possible isomers with C_3v, O_h, C_s and C_2v symmetry on the potential energy surface of the XeF₆ molecule are optimized by using DFT-LDA/NL. Their relative energies, vibration frequencies, electronic chemical potential and hardness have been calculated. It is found that the C_2v configuration has one imaginary frequency. The relative energies of the four isomers increase in order of C_3v, O_h, C_s and C_2v, and the hardness values in same order. The isomer stability obeys the maximum hardness principle (MHP), while their hardness values are very close to each other. It is quite evident that the very close hardness is the main reason for the structure fluxionality of XeF₆.     

Estimation of apparent binding constant of complexes of selected acyclic nucleoside phosphonates with β‐cyclodextrin by affinity capillary electrophoresis

Affinity capillary electrophoresis (ACE) has been applied to estimation of apparent binding constant of complexes of (R,S)‐enantiomers of selected acyclic nucleoside phosphonates (ANPs) with chiral selector β‐cyclodextrin (βCD) in aqueous alkaline medium. The noncovalent interactions of five pairs of (R,S)‐enantiomers of ANPs‐based antiviral drugs and their derivatives with βCD were investigated in the background electrolyte (BGE) composed of 35 or 50 mM sodium tetraborate, pH 10.0, and containing variable concentration (0–25 mM) of βCD. The apparent binding constants of the complexes of (R,S)‐enantiomers of ANPs with βCD were estimated from the dependence of effective electrophoretic mobilities of (R,S)‐enantiomers of ANPs (measured simultaneously by ACE at constant reference temperature 25°C inside the capillary) on the concentration of βCD in the BGE using different nonlinear and linear calculation methodologies. Nonlinear regression analysis provided more precise and accurate values of the binding constants and a higher correlation coefficient as compared to the regression analysis of the three linearized plots of the effective mobility dependence on βCD concentration in the BGE. The complexes of (R,S)‐enantiomers of ANPs with βCD have been found to be relatively weak – their apparent binding constants determined by the nonlinear regression analysis were in the range 13.3–46.4 L/mol whereas the values from the linearized plots spanned the interval 12.3–55.2 L/mol.     

Synthesis and Biological Activities of 7‐Arylazo‐7H‐pyrazolo[5,1‐c][1,2,4] Triazol‐6(5H)‐Ones and 7‐Arylhydrazono‐7H‐[1,2,4]triazolo[3,4‐b] [1,3,4]thiadiazines

Two series of 7‐arylazo‐7H‐3‐(2‐methyl‐1H‐indol‐3‐yl)pyrazolo[5,1‐c][1,2,4]triazol‐6(5H)‐ones 4 and 7‐arylhydrazono‐7H‐3‐(2‐methyl‐1H‐indol‐3‐yl)‐[1,2,4]triazolo[3,4‐b][1,3,4]thiadiazines 7 were prepared via reactions of 4‐amino‐3‐mercapto‐5‐(2‐methyl‐1H‐indol‐3‐yl)‐1,2,4‐triazole 1 with ethyl arylhydrazono‐chloroacetate 2 and N‐aryl‐2‐oxoalkanehydrazonoyl halides 5 , respectively. A possible mechanism is proposed to account for the formation of the products. The biological activity of some of these products was also evaluated.     

Regioselective Synthesis of 6‐[Chloro(difluoro)methyl]salicylates by [3+3] Cyclocondensations of 1,3‐Bis(trimethylsilyloxy)buta‐1,3‐dienes with 1‐Chloro‐1,1‐difluoro‐4‐(trimethylsilyloxy)pent‐3‐en‐2‐one

The TiCl₄‐mediated [3+3] cyclocondensation of various 1,3‐bis(trimethylsilyloxy)buta‐1,3‐dienes with 1‐chloro‐1,1‐difluoro‐4‐(trimethylsilyloxy)pent‐3‐en‐2‐one provides a regioselective access to novel 6‐(chlorodifluoromethyl)salicylates (=6‐(chlorodifluoromethyl)‐2‐hydroxybenzoates) with very good regioselectivity. For selected products, it was demonstrated that the CF₂Cl group can be transformed to CF₂H and CF₂(Allyl) by free‐radical reactions.     

Structure of AcMet-Gly and Its Interaction with Palladium（Ⅱ） Tetraaqua Complex Studied by Electrospray Mass Spectrometry and Density Functional Theory

The structure of dipepide AcMet‐Gly was determined by X‐ray crystallographic analysis. It possesses mono‐clinic, space group P2₁ (No. 4). with cell dimensions of α=0.8571(2) nm, b=0.5871(2) nm, c= 1.197(3) nm, β= 99.290(10)°. V=0.5944(15) nm³, Z=2, μ=2.74 cm^?1. Mononuclear chelates, described as [Pd(X)(S,N,O‐AcMet‐Gly)]^?, in which Pd(II) is coordinated by thioether, deprotonated amide nitrogen, carbonyl oxygen of me‐thionine and X (AcMetGly or other ligands present in aqueous solution or in mobile phase solution), were detected 5 min after mixing AcMet‐Gly with [Pd(H₂O)₄]^2‐ at room temperature using electrospray ionization mass spectrometry. The geometry of [Pd(H₂O)(S,N,O‐AcMet‐Gly)]^? is optimized at density functional B3LYP/LanL2DZ level. The fused five‐ and six‐membered chelate is responsible for cleavage of Met‐Gly bond. This is the first time to provide a direct evidence for Pd(II)‐mediated cleavage of dipeptides via external solvent attack.