Entropy Effects in Intermolecular Associations of Crown-Ethers and Cyclodextrins with Amino Acids in Aqueous and in Non-Aqueous Media.

The analysis of the ratios of entropy and enthalpy characteristics and their contributions to the change in the Gibbs energy of intermolecular interactions of crown ethers and cyclodextrins with amino acids is carried out. Two different types of macrocycles were chosen for examination: crown ethers with a hydrophilic interior and cyclodextrins with a hydrophobic inner cavity and a hydrophilic exterior. The thermodynamics of complex formation of crown ethers and cyclodextrins with amino acids in water and aqueous-organic solvents of variable composition was examined. The contributions of the entropy solvation of complexes of 18-crown-6 with glycine, alanine, phenylalanine to the change in the entropy of complexation in water-ethanol and water-dimethyl sulfoxide solvents was calculated and analyzed. It was found that the ratios of the entropy and enthalpy solvation of the reagents for these systems have similar trends when moving from water to aqueous-organic mixtures. The relationship between the thermodynamic characteristics and structural features of the complexation processes between cyclodextrins and amino acids has been established. The thermodynamic enthalpy-entropy compensation effect was revealed, and its features for complexation of cyclodextrins and 18-crown-6 were considered. It was concluded that, based on the thermodynamic parameters of molecular complexation, one could judge the mode of the formation of complexes, the main driving forces of the interactions, and the degree of desolvation.

Selective Binding of Cyclodextrins with Leflunomide and Its Pharmacologically Active Metabolite Teriflunomide.

The selectivity of encapsulation of leflunomide and teriflunomide by native α-, ß- and γ-cyclodextrins was investigated through 1H NMR and molecular modeling. Thermodynamic analysis revealed the main driving forces involved in the binding. For α-cyclodextrin, the partial encapsulation was obtained while deep penetration was characterized for the other two cyclodextrins, where the remaining polar fragment of the molecule is located outside the macrocyclic cavity. The interactions via hydrogen bonding are responsible for high negative enthalpy and entropy changes accompanying the complexation of cyclodextrins with teriflunomide. These results were in agreement with the molecular modeling calculations, which provide a clearer picture of the involved interactions at the atomic level.

Effects of Biorelevant Media Components on Dissolution Behaviour of 1,2,4-Thiadiazole Derivative Designed for Alzheimer's Disease Prevention.

In this study, dissolution behaviour of 1,2,4-thiadiazole derivative (1-[5-(3-chloro-phenylamino)-1,2,4-thiadiazol-3-yl]-propan-2-ol) displaying an anti-Alzheimer activity was examined in biorelevant media such as Simulated Gastric Fluid (SGF, pH 1.2), Fasted State Simulated Gastric Fluid (FaSSGF, pH 1.6) and Fasted State Simulated Intestinal Fluid (FaSSIF, pH 6.5). It was found that solubility and dissolution rate of 1,2,4-thiadiazole derivative under consideration are not strongly dependent on pH, whereas these parameters are significantly affected by the buffer composition. Dissolution was found to be more effective in buffers composed of the surfactant micelles. It was demonstrated that considerable increase in solubility and dissolution rate in SGF is achieved through the interaction of 1,2,4-thiadiazole derivative with the micelles of sodium dodecyl sulfate. On the contrary, CMC of sodium taurochalate was shifted in the presence of 1,2,4-thiadiazole derivative, therefore, dissolution process is not so efficient in FaSSIF. Interactions occurring between 1,2,4-thiadiazole derivative and the components of biorelevant media were investigated in detail by means of UV/VIS spectroscopy, 1 H-NMR and phase solubility methods.

Design of Dosage Forms with Improved Biopharmaceutical Properties.

The development of a pharmaceutical product consists of giving a drug an optimal dosage form (a certain state of aggregation, consistency, structural, mechanical, physicochemical, and functional properties), which ensure stability, the possibility of accurate dosage, the required pharmacological effect, and ease of administration with minimal side effects [...].

ß-Cyclodextrin Modified Hydrogels of Kappa-Carrageenan for Methotrexate Delivery.

This work is aimed at developing a kappa-carrageenan (kCR) gel with increased methotrexate (MTX) content. ß-Cyclodextrin (ßCD), which is able to inclusion complex formation with MTX, has been used to increase the drug concentration in the hydrogel. The rheological behavior of the designed gels was investigated and the influence of MTX and ßCD on the viscoelastic properties of kCR gel was studied in detail. The effect of ßCD and its concentration on the MTX-releasing rate from the kCR gels was examined. The properties of kappa- and iota-carrageenans loaded with MTX were compared and the differences observed were explained in terms of different binding affinities of MTX to these polymers. The obtained gels provided desirable viscoelastic properties useful for topical application.

Regularities of Encapsulation of Tolfenamic Acid and Some Other Non-Steroidal Anti-Inflammatory Drugs in Metal-Organic Framework Based on γ-Cyclodextrin.

Metal-organic frameworks based on cyclodextrins (CDs) have been proposed as promising drug delivery systems due to their large surface area, variable pore size, and biocompatibility. In the current work, we investigated an incorporation of tolfenamic acid (TA), a representative of non-steroidal anti-inflammatory drugs (NSAIDs), in a metal-organic framework based on γ-cyclodextrin and potassium cations (γCD-MOF). Composites γCD-MOF/TA obtained by absorption and co-crystallization methods were characterized using powder X-ray diffraction, low temperature nitrogen adsorption/desorption, scanning electron microscopy, and FTIR spectroscopy. It was demonstrated that TA loaded in γCD-MOF has an improved dissolution profile. However, the inclusion of TA in γ-CD reduces the membrane permeability of the drug. A comparative analysis of the encapsulation of different NSAIDs in γCD-MOF was performed. The impact of NSAID structure on the loading capacity was considered for the first time. It was revealed that the presence of heterocycles in the structure and drug lipophilicity influence the loading efficiency of NSAIDs in γCD-MOF.

Improvement of pharmacologically relevant properties of methotrexate by solid dispersion with Pluronic F127.

Solid dispersion with Pluronic F127 was proposed as alternative approach to modify the pharmacologically relevant properties of methotrexate (MTX). Solid dispersion of MTX with Pluronic F127 was prepared by fusion method and characterized by powder X-ray diffraction, thermal analysis, scanning electron microscopy and FTIR spectroscopy with the aim to elucidate the physical state of the dispersed MTX and the nature of the interactions occurring between MTX and the carrier. Effect of Pluronic F127 on solubility, dissolution rate, membrane permeability, and pharmacokinetic parameters was revealed in vitro and in vivo. It was found that physical interactions of MTX with Pluronic F127 are predominant in the solid dispersion. The effect of Pluronic F127 on the MTX solubility and release rate of MTX from the solid dispersion is pH dependent. Apparent solubility of MTX released from the solid dispersion is increased in the acidic medium and remains unchanged in the alkaline medium. In comparison with the pristine MTX, the release of MTX from the solid dispersion is faster in the acidic medium and slower in the alkaline medium. Influence of Pluronic F127 on the membrane permeability of MTX is insignificant. Bioavailability of orally administrated solid dispersion in increased. Results from in vitro and in vivo studies suggested that the pharmacokinetic properties of MTX can be improved by solid dispersion with Pluronic F127.

Investigation of the pH-dependent complex formation between beta-cyclodextrin and dipeptide enantiomers by capillary electrophoresis and calorimetry.

The effect of pH on complex formation between beta-CD and the enantiomers of the dipeptides Ala-Phe, Ala-Tyr and Asp-PheOMe was investigated at 298.15 K by CE and calorimetry. Beta-CD displayed a higher enantioselectivity toward the protonated peptides compared to their zwitterionic forms. While stronger binding of the DD-enantiomers than the LL-stereoisomers were found by calorimetry regardless of the ionization state of the peptides, essentially equal complexation constants of the enantiomers were determined by CE for the zwitterionic species of the peptides. The reversal of the enantiomer migration order observed in CE was attributed primarily to a stereoselective complexation-induced pK(a) shift. In calorimetry, complexation of the protonated DD-enantiomers by beta-CD was accompanied by higher enthalpy and entropy changes resulting in more stable complexes compared to the LL-peptides. The enthalpy and entropy of complexation was affected by pH and peptide structure.

Methotrexate-loaded metal-organic frameworks on the basis of γ-cyclodextrin: Design, characterization, in vitro and in vivo investigation.

In this work, metal-organic frameworks on the basis of γ-cyclodextrin (γCD-MOF) were proposed as carriers for methotrexate (MTX) which is widely used as chemotherapy agent and immune system suppressant. The synthesized γCD-MOF was loaded with MTX by impregnation and co-crystallization. The obtained composites were characterized using powder X-ray diffraction, N2 adsorption/desorption, FTIR spectroscopy, solid-state 13C MAS CP/TOSS NMR and scanning electron microscopy. Pharmaceutically relevant properties of MTX alone and loaded in γCD-MOF were investigated in vitro and in vivo. The faster dissolution of MTX incorporated in γCD-MOF was demonstrated in blank buffers and biorelevant media (FaSSGF, FaSSIF) simulating the gastrointestinal fluids. Inclusion complex formation of MTX with γ-CD enhances the drug dissolution rate and, at the same time, slightly decreases the drug permeability through the lipophilic membrane. The in vivo experiments showed the improved pharmacokinetic parameters of MTX loaded in γCD-MOF.

Calorimetric and X-ray studies of clathrate hydrates of tetraisoamylammonium polyacrylates.

The structure of clathrate hydrates with tetraisoamylammonium polyacrylate salt incorporated as guest has been studied in this work. Also, quantitative studies on the stability changes of the clathrate hydrates with different degrees of cross-linking of the guest polymer (varied from 0 to 3%) have been conducted. A single crystal X-ray diffraction study of a crystal of the hydrate with linear (uncross-linked) tetraisoamylammonium polyacrylate as guest reveals a hexagonal structure (space group P6m2, a = 12.15 A, c =12.58 A at 100 K) with 39 host framework water molecules per one guest monomeric unit. Powder X-ray diffraction analyses confirm the identity of the above crystal structure of the hydrate with linear guest polymer and the crystal structure of the hydrates with cross-linked guest (hexagonal, a = 12.25 A, c =12.72 A at 276 K). In order to quantitatively determine the stability differences of the hydrates with the included guests having various degrees of cross-linking of the anionic chain, a series of differential scanning calorimetry measurements of the fusion enthalpy of the hydrate samples has been carried out. On the basis of the results obtained, a structural model describing the decrease in the stability of the clathrate hydrates with tetraisoamylammonium polyacrylate guest as a function of the degree of cross-linking of the guest polymer has been suggested.

γ-Cyclodextrin-metal organic frameworks as efficient microcontainers for encapsulation of leflunomide and acceleration of its transformation into teriflunomide.

γ-Cyclodextrin-based metal-organic framework (γCD-MOF) crystals were successfully synthesized using a vapor diffusion method. An applicability of γCD-MOF for encapsulation of immunosuppressive disease-modifying antirheumatic drug leflunomide (LEF) was examined. Loading of LEF in γCD-MOF was performed by impregnation and co-crystallization. The empty and loaded γCD-MOFs were characterized using X-ray powder diffraction, N2 adsorption/desorption, thermogravimetric analysis, 1H NMR and FTIR spectroscopy. It was shown that in the presence of γCD-MOF leflunomide is transformed into its pharmacologically active form - teriflunomide that can be also applied alone in the treatment of multiple sclerosis. It was demonstrated that teriflunomide released from γCD-MOF has improved pharmacologically relevant properties such as solubility, dissolution rate and membrane permeability. It can be proposed that γCD-MOF can be considered as novel strategy for delivery of leflunomide.

Interactions of beta- and hydroxypropyl-beta-cyclodextrin with some purine alkaloids: thermodynamic study.

The effect of native and hydroxypropylated beta-cyclodextrin on the solubility and activity of some purine alkaloids was examined. For this purpose, the solubility of purine alkaloids in pure water and in aqueous solutions of mentioned beta-cyclodextrins was determined at 298.15 K. Stability constants of inclusion complexes and their stoichiometry were obtained from solubility diagrams. Enthalpic characteristics of interactions occurring between beta-cyclodextrins and purine alkaloids in aqueous solution were calculated from the direct calorimetric measurements. It was found, that beta-cyclodextrin forms with purine alkaloids weak complexes which are stabilized only by the entropy term. Due to very low complexing affinity of both beta-cyclodextrins to studied purine alkaloids their solubilizing effect is insignificant. The influence of structure of purine alkaloids and beta-cyclodextrin on the thermodynamic parameters of interaction was discussed.

Improved Biopharmaceutical Properties of Oral Formulations of 1,2,4-Thiadiazole Derivative with Cyclodextrins: in Vitro and in Vivo Evaluation.

The synthesized 1,2,4-thiadiazole derivative displaying biological activity has low aqueous solubility and dissolution rate. Novel oral formulations of thiadiazole with ß- and hydroxypropyl-ß-cyclodextrins were obtained by grinding and freeze-drying methods with the purpose to improve the aqueous solubility. Complex formation of 1,2,4-thiadiazole derivative with cyclodextrins was confirmed by means of solid-state 13C MAS CP/TOSS NMR. Solubility, dissolution rate and permeability of the solid inclusion complexes were evaluated in different biorelevant media (SGF, FaSSGF, FaSSIF) simulating the conditions in the gastrointestinal tract. It was demonstrated that the content of biorelevant media affects the properties of the inclusion complexes. In particular, solubilizing effect of cyclodextrins became less pronounced when the micelles of taurocholic acid and lecithin are formed in the dissolution media. The inclusion of thiadiazole into cyclodextrin cavity is in competition with its partitioning into the micelles and this should be taken into account when the in vivo behavior is predicted. The results of in vitro and in vivo experiments were found to be in agreement and showed the highest solubility, dissolution rate and bioavailability of the freeze-dried complexes of thiadiazole with hydroxypropyl-ß-cyclodextrin. These complexes can be proposed as more effective dosage forms for oral administration.

Study on complex formation of biologically active pyridine derivatives with cyclodextrins by capillary electrophoresis.

The capillary electrophoretic separation of the pyridine derivatives pyridoxine, pyridoxal, nicotinamide, nicotinic acid and isonicotinic acid in phosphate buffer using cyclodextrins as buffer additives was studied at pH 2.0 and 3.5. Superior separation was achieved at pH 2.0. Addition of alpha- and beta-cyclodextrin and the respective 2-hydroxypropyl derivatives as well as carboxymethyl-alpha-cyclodextrin to the running buffer did not significantly improve the resolution of the compounds. The interactions of alpha- and beta-cyclodextrin as well as their hydroxypropyl derivatives with the pyridine derivatives were investigated by capillary electrophoresis at pH 2.0. No complex formation was observed between the cyclodextrins and pyridoxine, pyridoxal and nicotinamide. alpha-Cyclodextrin and 2-hydroxypropyl-alpha-cyclodextrin form weak 1:1 complexes with nicotinic and isonicotinic acids in aqueous media at 298.15K, while beta-cyclodextrin and its hydroxypropyl derivative did not form complexes. The apparent stability constants (K) of the complexes calculated from the electrophoretic mobility data ranged between 3 and 33 kg/mol. The negative values of enthalpy and entropy of complex formation obtained from the graphical plot of the van't Hoff equation indicate an important role of van der Waals and electrostatic interactions in the binding of nicotinic acid with alpha-cyclodextrin.

Cocrystal formation, crystal structure, solubility and permeability studies for novel 1,2,4-thiadiazole derivative as a potent neuroprotector.

The cocrystallization approach has been applied to modify the poor solubility profile of the biologically active 1,2,4-thiadiazole derivative (TDZ). Extensive cocrystal screening with a library of coformers resulted in formation of a new solid form of TDZ with vanillic acid in a 1:1 molar ratio. The cocrystalline phase was identified and characterized by thermal and diffraction analyses including single-crystal X-ray diffraction. The energies of intermolecular interactions in the crystal were calculated by solid-state DFT and PIXEL methods. Both calculation schemes show good consistency in terms of total energy of the intermolecular interactions and suggest that the cocrystal is mainly stabilized via hydrogen bonds, which provide ca. 44% of the lattice energy. Since the cocrystal contained the hydroxybenzoic acid derivative as a coformer, the solubility profile of the cocrystal was investigated at different pHs using eutectic concentrations of the components. Furthermore, the influence of the cocrystallization on the permeability performance of the 1,2,4-thiadiazole through an artificial regenerated cellulose membrane was also evaluated. In addition, the thermodynamic functions of the cocrystal formation were estimated from the solubility of the cocrystal and the corresponding solubility of the pure compounds at various temperatures. The cocrystal formation process was found to have a relatively small value of the driving force (-5.3kJ·mol-1). The most significant contribution to the Gibbs energy was provided by the exothermic enthalpy of formation.

Thermostability of photosystem I trimers and monomers from the cyanobacterium Thermosynechococcus elongatus.

The performance of solar energy conversion into alternative energy sources in artificial systems highly depends on the thermostability of photosystem I (PSI) complexes Terasaki et al. (2007), Iwuchukwu et al. (2010), Kothe et al. (2013) . To assess the thermostability of PSI complexes from the thermophilic cyanobacterium Thermosynechococcus elongatus heating induced perturbations on the level of secondary structure of the proteins were studied. Changes were monitored by Fourier transform infrared (FT-IR) spectra in the mid-IR region upon slow heating (1°C per minute) of samples in D2O phosphate buffer (pD 7.4) from 20°C to 100°C. These spectra showed distinct changes in the Amide I region of PSI complexes as a function of the rising temperature. Absorbance at the Amide I maximum of PSI monomers (centered around 1653cm-1), gradually dropped in two temperature intervals, i.e. 60-75 and 80-90°C. In contrast, absorbance at the Amide I maximum of PSI trimers (around 1656cm-1) dropped only in one temperature interval 80-95°C. The thermal profile of the spectral shift of α-helices bands in the region 1656-1642cm-1 confirms the same two temperature intervals for PSI monomers and only one interval for trimers. Apparently, the observed absorbance changes at the Amide I maximum during heating of PSI monomers and trimers are caused by deformation and unfolding of α-helices. The absence of absorbance changes in the interval of 20-65°C in PSI trimers is probably caused by a greater stability of protein secondary structure as compared to that in monomers. Upon heating above 80°C a large part of α-helices both in trimers and monomers converts to unordered and aggregated structures. Spectral changes of PSI trimers and monomers heated up to 100°C are irreversible due to protein denaturation and non-specific aggregation of complexes leading to new absorption bands at 1618-1620cm-1. We propose that monomers shield the denaturation sensitive sides at the monomer/monomer interface within a trimer, making the oligomeric structure more stable against thermal stress.

P700+- and 3P700-induced quenching of the fluorescence at 760 nm in trimeric Photosystem I complexes from the cyanobacterium Arthrospira platensis.

The 5 K absorption spectrum of Photosystem I (PS I) trimers from Arthrospira platensis (old name: Spirulina platensis) exhibits long-wavelength antenna (exciton) states absorbing at 707 nm (called C707) and at 740 nm (called C740). The lowest energy state (C740) fluoresces around 760 nm (F760) at low temperature. The analysis of the spectral properties (peak position and line width) of the lowest energy transition (C740) as a function of temperature within the linear electron-phonon approximation indicates a large optical reorganization energy of approximately 110 cm(-1) and a broad inhomogeneous site distribution characterized by a line width of approximately 115 cm(-1). Linear dichroism (LD) measurements indicate that the transition dipole moment of the red-most state is virtually parallel to the membrane plane. The relative fluorescence yield at 760 nm of PS I with P700 oxidized increases only slightly when the temperature is lowered to 77 K, whereas in the presence of reduced P700 the fluorescence yield increases nearly 40-fold at 77 K as compared to that at room temperature (RT). A fluorescence induction effect could not be resolved at RT. At 77 K the fluorescence yield of PS I trimers frozen in the dark in the presence of sodium ascorbate decreases during illumination by about a factor of 5 due to the irreversible formation of (P700+)F(A/B-) in about 60% of the centers and the reversible accumulation of the longer-lived state (P700+)FX-. The quenching efficiency of different functionally relevant intermediate states of the photochemistry in PS I has been studied. The redox state of the acceptors beyond A(0) does not affect F760. Direct kinetic evidence is presented that the fluorescence at 760 nm is strongly quenched not only by P700+ but also by 3P700. Similar kinetics were observed for flash-induced absorbance changes attributed to the decay of 3P700 or P700+, respectively, and flash-induced fluorescence changes at 760 nm measured under identical conditions. A nonlinear relationship between the variable fluorescence around 760 nm and the [P700red]/[P700total] ratio was derived from titration curves of the absorbance change at 826 nm and the variable fluorescence at 760 nm as a function of the redox potential imposed on the sample solution at room temperature before freezing. The result indicates that the energy exchange between the antennae of different monomers within a PS I trimer stimulates quenching of F760 by P700+.

Red chlorophyll excitation dynamics in Arthrospira platensis photosystem I trimeric complexes as studied by femtosecond transient absorption spectroscopy.

Femtosecond absorption spectroscopy was applied to study for the first time excitation dynamics in isolated photosystem I trimers from Arthrospira platensis, which display extremely long-wavelength absorption peaks. Pump-probe spectra observed at 77K in the timescale of dozens of picoseconds upon 70-fs excitation revealed two maxima near 710 and 730 nm, which correspond to red chlorophyll forms. Bleaching at 680 nm developed in ∼ 200 fs, whereas the bleaching kinetics at 710 and 730 nm exhibited two components with time constants of 1 and 5.5 ps. Comparison of the kinetics of bleaching development at 710 nm and 730 nm with that of bleaching decay at 680 nm indicated that both long-wavelength forms of trimers are populated mainly via direct energy transfer from bulk chlorophyll.

α-Cyclodextrin/aminobenzoic acid binding in salt solutions at different pH: dependence on guest structure.

Influence of Na(+) and K(+) cations on α-cyclodextrin guest-host complex formation with isomeric aminobenzoic acids was examined at different pH and temperature of 298.15 K by (1)H NMR and calorimetry methods. More pronounced influence of Na(+) on inclusion complex formation of α-CD with aminobenzoic acid anions compare to the effects of Na(+) on α-CD complex formation with zwitterionic aminobenzoic acid molecules was revealed. For the first time, the dependence of salt effects on the structure, ionization and the hydration state of the guest molecule was demonstrated and analysed on the basis of the obtained thermodynamic parameters of complex formation and calculated free energy of hydration of different ionized forms of aminobenzoic acids.

Physicochemical and structural studies of clathrate hydrates of tetrabutylammonium polyacrylates.

In this work, physicochemical and structural studies have been carried out for semiclathrate hydrates of linear (un-cross-linked) and cross-linked tetrabutylammonium polyacrylates with different degrees of cross-linking of the polymeric guest molecules (n = 0.5, 1, 2, 3%) and different degrees of substitution of proton ions of carboxylic groups in poly(acrylic acid) for TBA cations (x = 1, 0.8, 0.6). The changes in the hydrates' stability and composition depending on the outlined parameters were examined in the course of phase diagram studies of the binary systems water-tetrabutylammonium polyacrylates using differential thermal analysis method and calorimetric measurements of fusion enthalpies of the hydrates. Phase diagram studies of the binary system water-linear tetrabutylammonium polyacrylate revealed the formation of four hydrates. Based on the data of chemical analysis of hydrate crystals the compositions of all hydrates have been determined. Single-crystal X-ray diffraction studies revealed a tetragonal structure, space group 4/m, and unit cell parameters are close for different hydrates and lie in the ranges a = 23.4289-23.4713 Å and c = 12.3280-12.3651 Å (150 K). The structure can be related to tetragonal structure I typical for the clathrate hydrates of tetraalkylammonium salts with monomeric anions. Powder X-ray diffraction analyses confirmed the identity of the above crystal structure to that of the hydrates with cross-linked tetrabutylammonium polyacrylates. The behavior of TBA polyacrylate hydrates under the pressure of methane was studied and quantitative assessment of the gas content in the hydrates was made using volumetric analysis method.