A Comparative Study of Electrochemical Reduction of Levulinic Acid on Various Electrodes in Organic Solvents.

Studies on the electrochemical hydrogenation (ECH) of levulinic acid (LA) to valeric acid (VA) or γ-valerolactone (GVL) have mainly focused on the electrochemical reduction of LA in acidic aqueous solutions. However, the narrow range of applied potentials has hindered understanding of some mechanistic aspects of LA electrochemical conversion. Earlier, we discovered that employing proton-deficient non-aqueous reaction media provides more comprehensive insights into the mechanism of LA electrochemical reduction. Here, we conducted further investigations into the LA electroreduction process using cyclic voltammetry in various organic solvents on a Pt electrode and on various electrode materials in acetonitrile, both with and without the addition of proton donors. The products of the ECH processes were identified using HPLC. The solvent nature, the presence of proton donors, the electrode material, and the applied potential strongly influence the LA electroreduction process. This study reveals that LA, in the presence proton donors, can undergo electrochemical reduction through different pathways, depending on the difference (ΔE1/2) between the reduction half-wave potential of protons and LA on a certain electrode. When the difference is large, the LA reduction is incomplete and the formation of GVL is observed. Under the close reduction potentials of protons and LA, LA can be completely reduced to VA.

Electrophoretic mobility of ester betulin derivatives and their complexation with γ-cyclodextrin studied by capillary electrophoresis in aqueous solutions at different pH values.

In this article, capillary electrophoresis was used to measure the effective electrophoretic mobility of ester betulin derivatives as a pH function and to study their complexation with γ-cyclodextrin (γ-CD). The electrophoretic mobility of betulin 3,28-diphthalate (DPhB) and 3,28-disuccinate (DScB) changed unusually with decreasing pH: instead of decreasing, it first increased and then decreased. This fact as well as the turbidity of sample solutions at pH from 2.5 to 6, broadening of electrophoretic peaks and a decrease in the surface tension of the solutions indicates that these betulin derivatives, being amphiphilic compounds and weak acids, exist as micelles in aqueous solutions at pH 6 and below. The inclusion complexation of betulin derivatives with γ-CD at pH 9.18 and 4.5 was studied by mobility shift affinity capillary electrophoresis. At pH 9.18, the apparent binding (stability) constant logarithms for 1:1 γ-CD complexes of DPhB, betulin 3,28-disulfate (DSB) and DScB with 95% confidence interval limits were equal to 7.44 ± 0.02, 7.09 (7.01-7.19), and 6.97 (6.87-7.08) at 25°C, respectively. At pH 4.5, the binding constant for the DSB complex was slightly lower, while the micelle formation did not allow determining the exact values of the constants for the DPhB and DScB complexes.

Evaluation of the effect of background electrolyte composition and independence of parameters in determining binding constants of betulin derivatives to ß- and dimethyl-ß-cyclodextrins by affinity capillary electrophoresis.

The values of the apparent binding constants for ß-cyclodextrin complexes of betulin derivatives determined by mobility shift affinity capillary electrophoresis were found to be independent of the composition of the two background electrolytes used (tetraborate buffer, pH 9.18, and phosphate buffer, pH 8.00, both of them with 20 mM ionic strength). It has been found that if there is not a constant plateau on the binding curve then four independent parameters can be determined: binding constants (also referred to as stability, association, or formation constants) and ionic mobilities of 1:1 and 1:2 complexes. However, at least 10-12 data points in the binding curve should be used to reliably estimate the parameters. For the first time, the apparent binding constants for complexes of ester betulin derivatives with dimethyl-ß-cyclodextrin have been determined by mobility shift affinity capillary electrophoresis. The logarithms of the constants for 1:1 and 1:2 complexes at 25°C for betulin 3,28-diphthalate with a 95% confidence interval are 4.98 (4.95-5.01) and 7.52 (7.26-7.68); for betulin 3,28-disulfate, the values are 4.97 (4.89-5.03) and 8.24 (6.82-8.52). It has been found that betulin 3,28-disuccinate forms only a 1:1 complex and the binding constant logarithm is 5.25 ± 0.02.

Determining binding constants for 1:1 and 1:2 inclusion complexes of ester betulin derivatives with (2-hydroxypropyl)-ß-cyclodextrin by affinity capillary electrophoresis.

The complexation of ester betulin derivatives with (2-hydroxypropyl)-ß-cyclodextrin (HP-ß-CD) was studied by mobility shift affinity CE. Electrophoretic mobility for triangular peaks was calculated using the parameter a1 of the Haarhoff-Van der Linde function instead of the peak top time. Dependences of the viscosity corrected electrophoretic mobility on HP-ß-CD concentration were not described on the basis of only complexes with 1:1 stoichiometry due to the fact that these binding curves did not reach a plateau. However, the dependences were well described taking into account both 1:1 and 1:2 complexes. The presence of higher order equilibria was also revealed by x-reciprocal plots. The values of apparent binding constant logarithm, obtained for the first time, for 1:1 and 1:2 HP-ß-CD complexes of betulin 3,28-diphthalate and betulin 3,28-disuccinate with 95% confidence interval limits in brackets are the same within error and are equal to 4.85 (4.73-4.95), 8.56 (7.75-8.82), 4.92 (4.86-4.97), and 8.54 (8.23-8.72) at 25°C, respectively. These values for 1:1 and 1:2 HP-ß-CD complexes of betulin 3,28-disulfate at 25°C are 4.61 (4.57-4.64) and 7.11 (6.57-7.34), respectively. The binding constants for betulin 3,28-disulfate agree with the previously obtained results from the separation in the thermostatted capillary segment.

Boundary values of binding constants determined by affinity capillary electrophoresis.

This study shows that the upper limit of binding (stability) constants determined by mobility shift affinity capillary electrophoresis can be increased from 104 to 106 -109 L/mol if the lowest possible analyte concentration in samples is used (for example, the concentration that gives electrophoretic peaks with a signal-to-noise ratio of 10) and the effective electrophoretic mobility of the analyte is calculated via the parameter a1 of the Haarhoff-Van der Linde function. The equation to calculate the boundary values of binding constants for 1:1 complexes was derived for the case when the constants cannot be calculated in the usual way. These values are obtained from the inequality: the difference between the ionic mobility of the analyte-ligand complex and the effective electrophoretic mobility of the analyte determined at the lowest ligand concentration in the background electrolyte at which the analyte appears as an undistorted peak in electropherograms is less than or equal to the absolute error in mobility measurements. The application of the equation was illustrated by the example of electrophoretic data for a complex between betulin 3,28-diphthalate and (2-hydroxypropyl)-γ-cyclodextrin. An algorithm to determine the binding constants for strong complexation by mobility shift affinity capillary electrophoresis was suggested.

Strong complexation of water-soluble betulin derivatives with (2-hydroxypropyl)-γ-cyclodextrin studied by affinity capillary electrophoresis.

The complexation between (2-hydroxypropyl)-γ-cyclodextrin (HP-γ-CD) and water-soluble betulin derivatives, betulin 3,28-disulfate (DSB) and betulin 3-acetate-28-sulfate (ASB), belonging to the class of pentacyclic lupane triterpenoids, was studied using mobility shift ACE (ms ACE). It was found that the complexation is a high-affinity interaction. In this case, a very low amount of HP-γ-CD should be added to the BGE, and triangular peaks are observed as a result of ligand deficiency in the sample zone. Le Saux et al. showed in 2005 that using the parameter a1 of the Haarhoff-Van der Linde (HVL) function instead of the migration time measured at the peak apex eliminates the effect of ligand deficiency on effective electrophoretic mobility. Therefore, the electrophoretic mobilities of asymmetrical peaks of DSB and ASB were calculated in this way. The obtained experimental data correspond to 1:1 complexes. The calculated values of binding constants logarithms at 25°C are 6.70 ± 0.05 and 7.03 ± 0.10 for the HP-γ-CD complexes of DSB and ASB, respectively.

Determination of binding constants for strong complexation by affinity capillary electrophoresis: the example of complexes of ester betulin derivatives with (2-hydroxypropyl)-γ-cyclodextrin.

Complexation plays an important role in many biological phenomena, the analysis of different samples, optimization of separation processes, and increasing the pharmacological activity of drugs. This paper discusses the features of using mobility shift affinity capillary electrophoresis for studying strong complexation. Electrophoretic peaks for this case are often triangular. It was shown that the use of electrophoretic mobility obtained from the peak apex time to calculate binding constants leads to significant systematic and random errors, and the parameter a1 of the Haarhoff-Van der Linde function should be used instead of the apex time. Distorted triangular peaks with dips were shown to be observed at too high a ratio of analyte concentration in the sample to ligand concentration in the background electrolyte, and the peaks and parameter a1 significantly shifted. It was found that the permissible excess of analyte concentration over ligand concentration was approximately 10-35, provided that the parameter a1 was used, but the peak shape should be used as a landmark, and only triangular peaks without dips should be fitted with the function. The lowest possible analyte concentration should be utilized, which allows the use of a wider range of ligand concentration leading to higher precision of determining the binding constants values. Kinetically labile 1:1 complexes between (2-hydroxypropyl)-γ-cyclodextrin (HP-γ-CD) and betulin 3,28-diphthalate (DPhB) and betulin 3,28-disuccinate (DScB) were studied as an example. The binding constant logarithms at 25 °C are 7.23 ± 0.03 and 7.13 ± 0.10 for the HP-γ-CD complexes of DPhB and DScB, respectively. Graphical Abstract.

Thermodynamic parameters for the complexation of water-insoluble betulin derivatives with (2-hydroxypropyl)-γ-cyclodextrin determined by phase-solubility technique combined with capillary zone electrophoresis.

The complexation of betulinic and betulonic acids (BIA and BOA) (pentacyclic lupane triterpenoids) with (2-hydroxypropyl)-γ-cyclodextrin (HP-γ-CD) was studied at different temperatures using the method combining phase-solubility technique and CZE. In contrast to mobility shift ACE utilizing the electrophoretic mobility, in this approach, the peak areas are used. The apparent binding (stability, formation) constants are obtained by the Higuchi and Connors method from the linear segment of compound solubility diagrams in CD solutions. It was found that the apparent binding constants of the HP-γ-CD complexes of BIA and BOA decrease with increasing temperature. The binding constants of BOA complexes are slightly higher than those of BIA complexes; this can be explained by difference in the hydration degrees of carbonyl and hydroxyl groups. On the basis of the binding constants obtained and their temperature dependences (van't Hoff plot), the enthalpy as well as entropy changes and Gibbs free energies were calculated. It was found that the complexation was characterized by negative changes of enthalpy and entropy, that is, it was controlled by enthalpy changes. The results obtained can be used for the optimization of microcapsulation processes of BOA and BIA with the HP-γ-CD application in order to increase solubility and bioavailability of these compounds.

Stability constants of adducts of succinate copper(II) complexes with ß-cyclodextrin determined by capillary electrophoresis.

Cyclodextrins (CD) form inclusion complexes with different "guests" owing to the fact that the shape of the CD molecule is a truncated cone with a hydrophobic cavity. The adducts of CD with metal complexes remain scantily explored. In this study, the stability constants of the adducts between succinate copper(II) complexes and ß-CD was determined using capillary electrophoresis. The ß-CD concentration in background electrolytes (BGE) was found to influence on the effective electrophoretic mobility of the copper(II) complexes in succinate BGEs. It was shown that succinic acid and its anions and copper(II) ions did not form a significant amount of the inclusion complexes with ß-CD and the mobility change was caused by the adduct formation between succinate copper(II) complexes and ß-CD. The stability constants of these adducts were determined at 25°Ð¡ and ionic strength of 0.100 M: log ß(CuL22- /ß-CD) = 1.76 ± 0.06, log ß(CuL0 /ß-CD) = 0.98 ± 0.09. The [CuHL]+ and [CuHL2 ]- species were found to do not form adducts with ß-CD.

Strategy for non-target ionic analysis by capillary electrophoresis with ultraviolet detection.

A strategy for non-target analysis of samples with unknown composition by capillary electrophoresis (CE) with ultraviolet (UV) detection is suggested. The strategy is based on the preliminary identification of analytes and further optimization of the conditions for their separation using the developed computational tool set ElphoSeparation. It is shown that, in order to record electrophoretic peaks with the mobilities from the maximum to minimum possible values, the positive and negative voltage polarity and hydrodynamic pressure should be used. To choose the optimal separation conditions, dynamic maps of electrophoretic separation (DMES) are suggested. DMES is a bar chart with theoretical resolutions of adjacent peaks presented in ascending order of the migration time. The resolution is calculated as the division of the difference of the effective electrophoretic mobilities of adjacent analytes by their average peak width in terms of electrophoretic mobility. The suggested strategy is tested by the example of the analysis of herbal medicine (Holosas) on the basis of rose hips. The approach should be used to analyze samples with not very complex composition, such as environmental water and precipitation samples, process liquors, some vegetable extracts, biological fluids, food, and other samples for the determination of widespread compounds capable of forming ionic species. For samples with complex composition, the approach used together with other techniques may produce advantageous information due to specificity of the method, particularly it can be useful for determination of compounds suffering from low volatility or thermal stability, and for analysis of samples with difficult matrices. Graphical Abstract The scheme of performing the non-target ionic analysis by capillary electrophoresis with ultraviolet detection.

New peak broadening parameter for the characterization of separation capability in capillary electrophoresis.

The influence of separation conditions on peak broadening is usually estimated by the number of theoretical plates. Using the data available in literature and experimental data, it is shown that in pressure-assisted capillary electrophoresis the plate number is not directly related to the separation capability of conditions used. The experiments at different electrolyte flow velocities demonstrate that a higher plate number (the best separation efficiency) can be obtained with a lower peak resolution. Since ions are separated by electrophoresis due to the difference in electrophoretic mobilities, the peak width in terms of electrophoretic mobility is suggested as a new peak broadening parameter describing the separation capability of the conditions used. The parameter can be calculated using the tailing factor and the temporal peak width at 5% of the peak height. A simple equation for the resolution calculation is derived using the parameter. The advantage of the peak width in terms of mobility over other parameters is shown. The new parameter is recommended to be used not only in pressure-assisted capillary electrophoresis but also in general capillary electrophoresis when in a number of runs the virtual separative migration distance and separation capability of the conditions used change widely.

Trinuclear ReFePt clusters with a µ3-phenylvinylidene ligand: synthetic approaches, rearrangement of vinylidene, and redox-induced transformations.

A series of trinuclear µ3-vinylidene ReFePt clusters were synthesized by the application of two approaches: (i) reactions of the binuclear RePt µ-vinylidene complexes with Fe2(CO)9; (ii) ligand substitution or exchange reactions at the Pt atom in the synthesized ReFePt clusters. The molecular structures of CpReFePt(µ3-CCHPh)(CO)5[P(OEt)3]L [L = CO; P(OEt)3] were determined by an X-ray diffraction study. The obtained compounds were studied by IR and 1H, 13C and 31P NMR spectroscopy. The spectroscopic study revealed that the clusters CpReFePt(µ3-CCHPh)(CO)5[P(OEt)3]L [L = CO; P(OEt)3] and CpReFePt(µ3-CCHPh)(CO)6[P(OPri)3] undergo isomerization upon dissolution, resulting in three isomers with different positions of the µ3-vinylidene ligand over the ReFePt core. The redox properties of the clusters were studied by electrochemical methods. The relatively stable cation-radicals obtained by chemical oxidation of CpReFePt(µ3-CCHPh)(CO)6[P(OPri)3] and CpReFePt(µ3-CCHPh)(CO)5[P(OEt)3]2 with ferrocenium tetrafluoroborate were characterized by EPR spectroscopy.

Combination of phase-solubility method and capillary zone electrophoresis to determine binding constants of cyclodextrins with practically water-insoluble compounds.

The combined method based on phase-solubility technique and capillary zone electrophoresis (PS-CZE) was suggested for the determination of binding (stability) constants of cyclodextrins (CD) complexes with water-insoluble organic compounds that have no or weak UV chromophores. In this method, the insoluble compounds are agitated at the desired temperature in CD solutions with different concentration up to the attainment of equilibrium and then CZE is used to determine the concentration of the compounds that have passed into the solutions. To avoid precipitation and complex dissociation, the background electrolyte (BGE) for CZE should contain ethanol and, if necessary, cyclodextrin. The samples should be diluted with the BGE without CD so that the CD concentrations in BGE and samples were equal to preclude a baseline shift. Using the suggested approach, the inclusion complexes between betulinic and betulonic acids, pentacyclic lupane-type triterpenes, and hydroxypropyl-ß- and γ-cyclodextrins (HP-ß-CD and HP-γ-CD) were studied. It was found that solubility of the acids studied in HP-ß-CD solutions did not differ from their solubility in pure water. That is, the HP-ß-CD complexes of the acids studied were not formed in noticeable amount. At the same time, the acids formed inclusion complexes with HP-γ-CD, what possibly was caused by the greater size of the HP-γ-CD molecule as compared to HP-ß-CD. To determine binding constants by Higuchi and Connors method, the acids solubility was determined by CZE after their agitation in the solutions of HP-γ-CD (with 0.6 molar substitution) at 25 °C for 3 days. The dependences of acids solubility on HP-γ-CD concentration deviated from straight line in the range of high concentration (AN mode). This can be explained by a self-association of HP-γ-CD molecules. Using the linear segment of the solubility dependences on CD concentration, the binding constants were determined. Their logarithms for the HP-γ-CD complexes with betulonic and betulinic acids were 3.88 ± 0.14 and 3.82 ± 0.12, respectively.

Influence of Analyte Concentration on Stability Constant Values Determined by Capillary Electrophoresis.

The influence of analyte concentration when compared with the concentration of a charged ligand in background electrolyte (BGE) on the measured values of electrophoretic mobilities and stability constants (association, binding or formation constants) is studied using capillary electrophoresis (CE) and a dynamic mathematical simulator of CE. The study is performed using labile complexes (with fast kinetics) of iron (III) and 5-sulfosalicylate ions (ISC) as an example. It is shown that because the ligand concentration in the analyte zone is not equal to that in BGE, considerable changes in the migration times and electrophoretic mobilities are observed, resulting in systematic errors in the stability constant values. Of crucial significance is the slope of the dependence of the electrophoretic mobility decrease on the ligand equilibrium concentration. Without prior information on this dependence to accurately evaluate the stability constants for similar systems, the total ligand concentration must be at least >50-100 times higher than the total concentration of analyte. Experimental ISC peak fronting and the difference between the direction of the experimental pH dependence of the electrophoretic mobility decrease and the mathematical simulation allow assuming the presence of capillary wall interaction.