Molecular complexation: β-cyclodextrin and steroid hormones inclusion complexes studied by high performance liquid chromatography

A Review on Cyclodextrins/Estrogens Inclusion Complexes

This review focuses on the methods of preparation and biological, physiochemical, and theoretical analysis of the inclusion complexes formed between estrogens and cyclodextrins (CDs). Because estrogens have a low polarity, they can interact with some cyclodextrins' hydrophobic cavities to create inclusion complexes, if their geometric properties are compatible. For the last forty years, estrogen-CD complexes have been widely applied in several fields for various objectives. For example, CDs have been used as estrogen solubilizers and absorption boosters in pharmaceutical formulations, as well as in chromatographic and electrophoretic procedures for their separation and quantification. Other applications include the removal of the endocrine disruptors from environmental materials, the preparation of the samples for mass spectrometric analysis, or solid-phase extractions based on complex formation with CDs. The aim of this review is to gather the most important outcomes from the works related to this topic, presenting the results of synthesis, in silico, in vitro, and in vivo analysis.

HPLC and solubility study of the interaction between pindolol and cyclodextrins

The complexation with beta-cyclodextrin (beta-CD) has been investigated using reversed-phase liquid chromatography. The compounds tested have been pindolol and, for comparison purposes, indole and 4-methoxyindole. The retention behaviour has been analysed on a Kromasil 100 C18 column and the mobile phase used was methanol-pH 6 phosphate buffer (15/85v/v) in which beta-CD was incorporated as a mobile phase additive. The decrease in the retention times with increasing concentrations of beta-CD enables the determination of the apparent stability constants of the complexes. In addition, the low solubility of pindolol, a weak base, in pH 12 aqueous solution has been improved by complexation with different cyclodextrins. The solubility enhancements with 1.4 x 10(-2) M beta-, hydroxypropyl-beta, and gamma-CD have been 1.9, 1.8 and 1.4-fold, respectively, with 2.4 x 10(-2) M methyl-beta-CD it was 2.8-fold whilst no effect was observed with alpha-CD. The stability constants of the complexes at pH 12 have been determined from the solubility isotherms.

Determination of second-order association constants by global analysis of 1H and 13C NMR chemical shifts. Application to the complexation of sodium fusidate and potassium helvolate by beta- and gamma-cyclodextrin

The host-guest interaction between the steroid antibiotics sodium fusidate and potassium helvolate as guests and the hosts beta- and gamma-cyclodextrin was studied by 13C and 1H NMR techniques. The analysis of chemical shifts of individual nuclei leads to inconsistent values of the association constants and fails generally in the case of mixtures of 1:1 and 1:2 stoichiometries. The problem of parameter correlation is identified and the global analysis of two or more nuclei is proposed as a very effective method for the detection of complexes of higher stoichiometries and for the precise determination of the involved association constants. A matrix formulation of global analysis and the determination of confidence intervals is described. An analytical solution of the cubic equation, necessary for the description of higher order complexes, is presented in detail and its use together with commercial fitting software is compared with dedicated implementations. gamma-Cyclodextrin forms with both studied steroids, sodium fusidate and potassium helvolate, 1:1 complexes with high values of the association constants, K(1)=(60+/-24)x10(3)lmol(-1), and K(2)=(22+/-9)x10(3)lmol(-1), respectively. To the contrary, beta-cyclodextrin forms 1:1 and 1:2 (guest:host) complexes with both steroids, with moderate K(1) and low K(2) values (K(1)=(0.74+/-0.13)x10(3)lmol(-1), K(2)=(0.210+/-0.075)x10(3)lmol(-1)), and (K(1)=(2.42+/-0.87)x10(3)lmol(-1), K(2)=(0.06+/-0.09)x10(3)lmol(-1)), respectively.

Vancomycin dimerization and chiral recognition studied by high-performance liquid chromatography

The retention and separation of D,L-dansylvaline enantiomers (used as test solutes) were investigated using silica gel as stationary phase and vancomycin as chiral mobile-phase additive. A retention model was developed to describe the mechanistic aspects of the interaction between solute and vancomycin in the chromatographic system. It considered the formation of vancomycin dimers both "free" in the mobile phase and adsorbed on silica. By fitting the model equation to experimental data, it appeared clearly that the approach taking into account the vancomycin dimerization described accurately the retention behavior of the compounds. The examination of the model equation parameters showed that the glycopeptide dimerization increased the enantioselectivity by a factor of approximately 3.7. This study demonstrated the preponderant role of the vancomycin dimerization on the chiral recognition process of D,L-dansylvaline. Also, an additional analysis on a vancomycin chiral stationary phase indicated that the addition of vancomycin in the mobile phase promoted a greater enantioselectivity mediated by the formation of dimers in the stationary phase.

Characterization of inclusion complexes of betamethasone-related steroids with cyclodextrins using high-performance liquid chromatography

HPLC was used to study the inclusion complexes formed between various beta- and gamma-cyclodextrins and a series of corticosteroids related to betamethasone. Apparent association constants were measured in acetonitrile-water for a set of 13 steroids. An increase in the stability of the steroid-cyclodextrin complex is observed at lower concentrations of acetonitrile. The effects of the nature of the halide at the 9-position, the location of a double bond within the C-ring, substitution at the 9- and 11-positions, and modification of the D-ring of the steroid backbone were studied. The 11- and 17-positions were found to be critically involved in the inclusion process. Larger apparent association constants were obtained with gamma-cyclodextrin (gamma-CD) than with beta-cyclodextrin (beta-CD) due to the increased diameter of the gamma-CD cavity. Van't Hoff plots were constructed to examine the thermodynamic properties of the inclusion process. Plots constructed using retention factors were found to be nonlinear when gamma-CD was present in the mobile phase. This is due to an increase in the strength of the inclusion complex as temperature decreases. Plots constructed using apparent association constants were linear, indicating that the mechanism of inclusion does not change over the range of temperatures studied (10 to 80 degrees C). Enthalpy-entropy compensation was observed for 11 of the 13 steroids studied. The usefulness of cyclodextrins to achieve the separation of steroids in HPLC is discussed and a practical application for the analysis of a steroid and three potential impurities is described.

Symmetry breaking during the formation of beta-cyclodextrin-imidazole inclusion compounds: capillary electrophoresis study

A mathematical model was developed for the estimation of binding constants by capillary electrophoresis. The effective electrophoretic mobility in a solute mixture is dependent on the cyclodextrin concentration in the background electrolyte (BGE) as well as the stoichiometry and the binding constant of the guest-cyclodextrin complex. As well, a determination of the degree of complexation, nc (the percent of complexed guest) could be carried out. The model was applied to a series of imidazole derivatives. Thermodynamic data for the solute complexation mechanism were calculated. Different van't Hoff plot shapes of the degree of complexation were observed with different BGE pH values, indicating a change in the solute complexation mechanism. Enthalpy-entropy compensation revealed that the solute complexation mechanism was independent of the imidazole derivative molecular structure, the same at pH = 4.5, 5.0, 5.5, 6.0, and 6.5 but changed at pH = 7.0 and 7.5. Topological defects formed during a symmetry-breaking transition could be responsible for the modification of the structure of the cyclodextrin cavity and explained the nc variations in relation to pH and temperature.