Sille Ehala

Determination of stability constants of valinomycin complexes with ammonium and alkali metal ions by capillary affinity electrophoresis

Capillary affinity electrophoresis (CAE) has been employed to investigate quantitatively the interactions of valinomycin, macrocyclic depsipeptide antibiotic ionophore, with univalent cations, ammonium and alkali metal ions, K+, Cs+, Na+, and Li+, in methanol. The study involved measuring the change in effective electrophoretic mobility of valinomycin while the cation concentrations in the BGE were increased. The corresponding apparent stability (binding) constants of the valinomycin–univalent cation complexes were obtained from the dependence of valinomycin effective mobility on the cation concentration in BGE using a nonlinear regression analysis. The calculated apparent stability constants of the above‐mentioned complexes show the substantially higher selectivity of valinomycin for K+ and Cs+ ions over Li+, Na+, and NH4+ ions. CAE proved to be a suitable method for the investigation of both weak and strong interactions of valinomycin with small ions.

ACE applied to the quantitative characterization of benzo-18-crown-6-ether binding with alkali metal ions in a methanol-water solvent system

ACE was applied to the quantitative evaluation of noncovalent binding interactions between benzo‐18‐crown‐6‐ether (B18C6) and several alkali metal ions, Li+, Na+, K+, Rb+ and Cs+, in a mixed binary solvent system, methanol–water (50/50 v/v). The apparent binding (stability) constants (Kb) of B18C6–alkali metal ion complexes in the hydro‐organic medium above were determined from the dependence of the effective electrophoretic mobility of B18C6 on the concentration of alkali metal ions in the BGE using a nonlinear regression analysis. Before regression analysis, the mobilities measured by ACE at ambient temperature and variable ionic strength of the BGE were corrected by a new procedure to the reference temperature, 25°C, and the constant ionic strength, 10 mM. In the 50% v/v methanol–water solvent system, like in pure methanol, B18C6 formed the strongest complex with potassium ion (log Kb=2.89±0.17), the weakest complex with cesium ion (log Kb=2.04±0.20), and no complexation was observed between B18C6 and the lithium ion. In the mixed methanol–water solvent system, the binding constants of the complexes above were found to be about two orders lower than in methanol and about one order higher than in water.

Determination of acid–base dissociation constants of azahelicenes by capillary zone electrophoresis

CZE was employed to determine acid-base dissociation constants (pK(a)) of ionogenic groups of azahelicenes in methanol (MeOH). Azahelicenes are unique 3-D aromatic systems, which consist of ortho-fused benzene/pyridine units and exhibit helical chirality. The pK(a) values of pyridinium groups of the studied azahelicenes were determined from the dependence of their effective electrophoretic mobility on pH by a nonlinear regression analysis. The effective mobilities of azahelicenes were determined by CZE at pH range between 2.1 and 10.5. Thermodynamic pK(a) values of monobasic 1-aza[6]helicene and 2-aza[6]helicene in MeOH were determined to be 4.94 +/- 0.05 and 5.68 +/- 0.05, respectively, and pK(a) values of dibasic 1,14-diaza[5]helicene were found to be equal to 7.56 +/- 0.38 and 8.85 +/- 0.26. From these values, the aqueous pK(a) of these compounds was estimated.

Application of bare gold nanoparticles in open-tubular CEC separations of polyaromatic hydrocarbons and peptides.

In this study, bare gold nanoparticles (GNPs) immobilized in the sol-gel-pretreated fused-silica (FS) capillary as a stationary phase for open-tubular capillary electrochromatography (OT-CEC) are for the first time shown to be able to separate both hydrophobic polyaromatic hydrocarbons (PAHs) as well as hydrophilic cationic antimicrobial peptides. Model mixture of four PAHs, naphthalene, fluorene, phenanthrene, and anthracene, was resolved by OT-CEC in the GNP-modified FS capillaries using the hydro-organic background electrolyte (BGE) composed of 20 mmol/L sodium phosphate buffer, pH 7, modified with ACN at 8:2 v/v ratio. On the other hand, three synthetic analogues of an antimicrobial peptide mastoparan PDD-B, basic tetradecapeptides INWKKLGKKILGAL-NH(2), INSLKLGKKILGAL-NH(2) and NWLRLGRRILGAL-NH(2), were separated in aqueous acidic BGEs, pH 2.1-3.1, composed of weak acids (formic and acetic) or amphoteric amino or imino acids (aspartic or iminodiacetic), utilizing the advantage of a slow reversed (anodic) EOF and slightly positive charge of the GNP-modified FS capillary suppressing the adsorption of cationic peptides on the inner capillary wall and improving their resolution.

Capillary affinity electrophoresis and ab initio calculation studies of valinomycin complexation with Na+ ion

In a combined experimental and theoretical approach, the interactions of valinomycin (Val), macrocyclic depsipeptide antibiotic ionophore, with sodium cation Na(+ )have been investigated. The strength of the Val-Na(+ )complex was evaluated experimentally by means of capillary affinity electrophoresis. From the dependence of valinomycin effective electrophoretic mobility on the sodium ion concentration in the BGE (methanolic solution of 20 mM chloroacetic acid, 10 mM Tris, 0-40 mM NaCl), the apparent binding (stability) constant (K(b)) of the Val-Na(+ )complex in methanol was evaluated as log K(b) = 1.71 +/- 0.16. Besides, using quantum mechanical density functional theory (DFT) calculations, the most probable structures of the nonhydrated Val-Na(+) as well as hydrated Val-Na(+).H(2)O complex species were proposed. Compared to Val-Na(+), the optimized structure of Val-Na(+).H(2)O complex appears to be more realistic as follows from the substantially higher binding energy (118.4 kcal/mol) of the hydrated complex than that of the nonhydrated complex (102.8 kcal/mol). In the hydrated complex, the central Na(+) cation is bound by strong bonds to one oxygen atom of the respective water molecule and to four oxygens of the corresponding C=O groups of the parent valinomycin ligand.

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, NH(4)(+). Firstly, by means of ACE, the strength of the B18C6-NH(4)(+) 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 degrees C and constant ionic strength, 10mM) 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 (logK(b)) of B18C6-NH(4)(+) complex in the above binary solvent system was found to be equal to logK(b)=1.63+/-0.10. Secondly, the structural characteristics of B18C6-NH(4)(+) complex were described by quantum mechanical density functional theory (DFT) calculations. According to these calculations, in the energetically most favoured form of the B18C6-NH(4)(+) 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-CH(2)) ethereal oxygen and the other two hydrogen bonds are oriented to alkyl-O-alkyl (CH(2)-O-CH(2)) ethereal oxygen atoms of the macrocyclic crown ligand.

Affinity capillary electrophoresis and density functional theory employed for the characterization of hexaarylbenzene‐based receptor complexation with alkali metal ions

In this study, affinity capillary electrophoresis (ACE) and quantum mechanical density functional theory (DFT) calculations were combined to investigate non‐covalent binding interactions between the hexaarylbenzene‐based receptor (R) and alkali metal ions, Rb+ and Cs+, in methanol. The apparent binding (stability) constants (Kb) of the complexes of receptor R with alkali metal ions in the methanolic medium were determined by ACE from the dependence of effective electrophoretic mobility of the receptor R on the concentration of Rb+ and Cs+ ions in the BGE using a non‐linear regression analysis. The receptor R formed relatively strong complexes both with rubidium (log Kb=4.04±0.21) and cesium ions (log Kb=3.72±0.22). The structural characteristics of the above alkali metal ion complexes with the receptor R were described by ab initio density functional theory calculations. These calculations have shown that the studied cations bind to the receptor R because they synergistically interact with the polar ethereal fence and with the central benzene ring via cation‐π interaction.

Application of capillary affinity electrophoresis and density functional theory to the investigation of valinomycin-lithium complex.

Capillary affinity electrophoresis (CAE) and quantum mechanical density functional theory (DFT) have been applied to the investigation of interactions of valinomycin (Val), a macrocyclic dodecadepsipeptide antibiotic ionophore, with lithium cation Li(+). Firstly, from the dependence of effective electrophoretic mobility of Val on the Li(+) ion concentration in the background electrolyte (BGE) (methanolic solution of 50mM chloroacetic acid, 25 mM Tris, pH(MeOH) 7.8, 0-40 mM LiCl), the apparent binding (stability) constant (K(b)) of Val-Li(+) complex in methanol was evaluated as logK(b)=1.50+/-0.24. The employed CAE method include correction of the effective mobilities measured at ambient temperature, at different input power (Joule heating) and at variable ionic strength of the BGEs to the mobilities related to the reference temperature 25 degrees C and to the constant ionic strength 25 mM. Secondly, using DFT calculations, the most probable structures of the non-hydrated Val-Li(+) and hydrated Val-Li(+) x 3H(2)O complex species were predicted.

Capillary electrophoretic and computational study of the complexation of valinomycin with rubidium cation

This study is focused on the characterization of interactions of valinomycin (Val), a macrocyclic dodecadepsipeptide antibiotic ionophore, with rubidium cation, Rb+. Capillary affinity electrophoresis was employed for the experimental evaluation of the strength of the Val–Rb+ complex. The study involved the measurement of the change of effective electrophoretic mobility of Val at increasing concentration of Rb+ cation in the BGE. From the dependence of Val effective electrophoretic mobility on the Rb+ cation concentration in the BGE (methanolic solution of 100 mM Tris, 50 mM acetic acid, 0−1 mM RbCl), the apparent binding (stability) constant (Kb) of the Val–Rb+ complex in methanol was evaluated as log Kb=4.63±0.27. According to the quantum mechanical density functional theory calculations employed to predict the most probable structure of Val–Rb+ complex, Val is stabilized by strong non‐covalent bond interactions of Rb+ with six ester carbonyl oxygen atoms so that the position of the “central” Rb+ cation in the Val cage is symmetric.

Theoretical and experimental study of the complexation of valinomycin with ammonium cation

The interactions of valinomycin, macrocyclic depsipeptide antibiotic ionophore, with ammonium cation NH4+ have been investigated. Using quantum mechanical density functional theory (DFT) calculations, the most probable structure of the valinomycin-NH4+ complex species was predicted. In this complex, the ammonium cation is bound partly by three strong hydrogen bonds to three ester carbonyl oxygen atoms of valinomycin and partly by somewhat weaker hydrogen bonds to the remaining three ester carbonyl groups of the valinomycin ligand. The strength of the valinomycin-NH4+ complex was evaluated experimentally by capillary affinity electrophoresis. From the dependence of valinomycin effective electrophoretic mobility on the ammonium ion concentration in the background electrolyte, the apparent binding (association, stability) constant (Kb) of the valinomycin-NH4+ complex in methanol was evaluated as log Kb = 1.52 +/- 0.22.