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 the 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 in calculation of binding constants leads to significant systematic and random errors, and the parameter a 1 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 a 1 significantly shifted. It was found that the permissible excess of analyte concentration over ligand concentration is approximately 10-35, provided that the parameter a 1 is 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 to use a wider range of ligand concentration leading to higher precision of determining the binding constants values. Kinetically labile 1:1 complexes between (2hydroxypropyl)-γ-cyclodextrin (HP-γ-CD) and betulin 3,28-diphthalate (DPhB) and betulin 3,28-disuccinate (DScB) were studied as an example. The binding constants 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.

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.

The interaction between (2-hydroxypropyl)-β-cyclodextrin (HP-β-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 affinity capillary electrophoresis. It was found that 1:1 and 1:2 complexes were formed. The stability constants of the complexes in the temperature range of 293.15-318.15 K were determined. The values obtained are sufficiently large; log K (1:1) and log K (1:2) are 4.25-5.02 and 6.08-7.59, respectively. This phenomenon can be explained by the presence of broad hydrophobic regions in the molecules of the compound studied. The stability constants decrease with increasing temperature. The stability constants for ASB complexes are slightly higher as compared to the constants for DSB complexes. The thermodynamic parameters for the complexation were calculated from the van't Hoff plots. The complexation was found to be controlled by the enthalpy change. The obtained values of stability constants at 298 K were compared with values for the β-CD complexes of the compounds under study and for the HP-β-CD an d β-CD complexes of water-insoluble betulin derivatives. It was found that water-soluble betulin derivatives form more stable complexes with CDs as compared to water-insoluble derivatives (betulonic and betulinic acids), and the HP-β-CD complexes are more stable than the β-CD complexes.

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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-β- and γ-cyclodextrins (HP-β-CD and HP-γ-CD) were studied. It was found that solubility of the acids studied in HP-β-CD solutions did not differ from their solubility in pure water. That is, the HP-β-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-β-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 (A 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.

The system peaks that often appear on electropherograms in anion separation by CE with indirect spectrophotometric detection, negative voltage polarity and cathodic EOF are studied. The system peaks are shown to correspond to the zones with the changed concentration of the BGE constituents; they appear while the zone of each analyte anion passes through the outlet end of the capillary and are transported to the detector by EOF. An equation is suggested for predicting migration times of the system peaks with an error of 1%. The ratios of the system peak area to the analyte peak area are found to amount to 20%. It is shown that it is possible to avoid overlapping of the system peaks and analyte peaks by controlling the EOF velocity owing to hydrodynamic pressure. Using the mathematical simulation of CE shows that the system peaks and baseline shift can result from changing the transference numbers of the BGE ions and analyte ions at the capillary edge. The cases when the system peak may be incorrectly identified as the peak of analyte ion are considered. In order to avoid such errors, some practical recommendations are given.

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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.