G. A. Al'per

Molecular recognition of aromatic carboxylic acids by hydroxypropyl-γ-cyclodextrin: experimental and theoretical evidence

Inclusion complex formation of hydroxypropyl-γ-cyclodextrin with benzoic, nicotinic and isomeric aminobenzoic acids in water was studied by calorimetry, 1H NMR, densimetry and molecular modeling. It was observed that hydroxypropyl-γ-cyclodextrin selectively interacts with the considered acids forming stable inclusion complexes of 1 : 2 stoichiometry with benzoic and p-aminobenzoic acids, which exist in aqueous solution predominantly as neutral molecules. The binding affinity of hydroxypropyl-γ-cyclodextrin to m-aminobenzoic and nicotinic acids having the zwitterionic structure is considerably lower and a 1 : 1 inclusion complex is formed only with the former. The binding mode and thermodynamic parameters of complex formation were evaluated. It was shown that they strongly depend on the structure and ionization state of the acids. An efficient molecular modeling approach for simulating the encapsulation process for 1 : 1 and 1 : 2 stoichiometries was developed and implemented. A good agreement between experimental and theoretical results was demonstrated.

Thermodynamic characteristics of the formation of α- and β-cyclodextrin complexes with lumichrome, lumazine, and uracil in aqueous solution

Interactions of α- and β-cyclodextrins with lumichrome and its structural fragments, lumazine and uracil, were studied by means of solubility and 1H NMR spectroscopy. α-Cyclodextrin was found to have a weak complexing ability toward the studied compounds. It was established that β-cyclodextrin forms stable complexes with lumichrome and does not complex with lumazine and uracil. It was shown that only the benzene ring of lumichrome penetrates the β-cyclodextrin cavity, leading to a substantial increase in the solubility of lumichrome in water. We concluded that β-cyclodextrin complexation with lumichrome is highly exothermic due to the van der Waals interactions and hydrogen bonding between polar groups of the reagents.

Cyclodextrin–benzoic acid binding in salt solutions: Effects of biologically relevant anions

Inclusion complex formation of benzoic acid with α-, β- and γ-cyclodextrins in water and in 0.2 M solutions of inorganic salts (KCl, KBr, KH2PO4 and K2SO4) has been studied by means of 1H NMR at 298.15 K. Binding constants have been determined and role of biologically active inorganic anions in the inclusion complex formation has been revealed. It has been shown that effects of the anions are determined not only by changing the ionic strength. More pronounced influence of Br- and H2PO4- compared with Cl- and SO4(2-) is caused by specific ion-molecular interactions, occurrence of which depends on the physical-chemical properties of the anions as well as on the binding mode of cyclodextrins with benzoic acid. Competing interactions of cyclodextrin-anion were observed in the presence of KBr, while the ternary complex formation was detected upon addition of KH2PO4.

Solubility of caffeine in the supercritical CO2–methanol binary solvent

The caffeine–methanol association constant at 313 K has been determined by 1H NMR spectroscopy. The caffeine solubility in the supercritical carbon dioxide (SC-CO2)–methanol mixed solvent has been calculated using the association constant experimentally measured by NMR in the framework of the associated solution + lattice (ASL) model, which is based on the theory of molecular association and a simple lattice model. Individual contributions to the solubility have been determined, and the relative role of various factors determining the solubility of caffeine in the mixed solvent has been analyzed. The caffeine solubility as a function of the methanol content of the SC-CO2–methanol system is predicted to pass through a maximum.

Influence of molecular association on the solubility of methylxanthines in supercritical solvent CO2–methanol

Analysis of the experimental data on the solubility of methylxanthines (theophylline and theobromine) in supercritical (SC) solvent CO2–methanol at various concentrations of methanol within the framework of the ASL model (Associated Solution + Lattice) based on molecular association theory and the simple lattice model is presented. Hetero-association of methylxanthines with the molecules of methanol is studied by means of 1H NMR spectroscopy at 313 K. The contribution of molecular association to the solubility of methylxanthines in the mixed SC solvent CO2–methanol is analyzed. It is shown that the presence of NH group in the molecule of methylxanthine, when passing from caffeine to theophylline or theobromine, leads to an increase of the contribution of the component related to molecular association to solubility.

Viscosity of the Systems Methanol-n-Octane and Ethanol-n-Octane

The kinematic viscosity of binary solutions methanol-n-octane at 293.15-313.15 K and ethanol-n-octane at 263.15-298.15 K in the miscibility range (low concentrations of n-octane) was measured. In the methanol-octane system, the viscosity shows an abnormal behavior. As shown by molecular-dynamic calculations, this effect is due to formation of a specific type of associates, which results in a decrease in the molecular mobility as the n-octane concentration is increased (up to the phase separation point).

Comparative Analysis of the Methods for Calculating the Viscosity of Binary Nonelectrolyte Mixtures

The results of calculating the viscosity of binary nonelectrolyte systems by methods obviating fitting parameters were compared. A method was proposed for calculating the viscosity of n-alkanol-n-alkane solutions with account of the molecular association by the free-volume theory accurately to within 7.9%.

Molecular association in mixtures of acetonitrile with amides

The equilibrium constants of molecular association were calculated on the basis of the data from the PMR spectra in binary nonaqueous mixtures of acetonitrile with amides. The systems with formamide and N-methylformamide are comparatively strongly associated, whereas the association in mixtures of acetonitrile with DMF and HMPA is weak. The excess heat capacity of the systems was calculated by models of the associative equilibria and was compared with experiment. The developed approach to the description of molecular association makes it possible to obtain qualitative and quantitative data on the association processes in the investigated systems.