18-crown-6-sodium cholate complex: thermochemistry, structure and stability

18-crown-6, one of the most relevant crown ethers, and sodium cholate, steroidal surfactant classified as natural bile salt, are components of novel, synthesized coordination complex ; 18-crown-6-sodium cholate (18C6•NaCh). Like crown ethers, bile salts act as building blocks in supramolecular chemistry in order to design new functionalized materials with a desired structure and properties. In order to obtain thermal behavior of this 1:1 coordination complex, thermogravimetry and differential thermal analysis were used, as well as microscopic observations and differential scanning calorimetry. Temperature dependent infrared spectroscopy (IR) gave a detailed view into phase transitions. The structures during thermal treatment were observed with powder X-ray diffraction, and molecular models of the phases are made. Hard, glassy, colorless compound 18C6•NaCh goes through crystalline – crystalline polymorphic phase transitions at higher temperatures. The room temperature phase is indexed to a triclinic lattice, while in the high temperature phases molecules take randomly one of the two different configurations in the unit cell, resulting in the 2-fold symmetry. The formation of cholesteric liquid crystalline phase occurs simultaneously with partial decomposition, followed by the isotropisation with simultaneous and complete decomposition at much higher temperature, as obtained by IR. The results provide valuable information about the relationship between molecular structure, thermal properties, and stability of the complex, indicating the importance of an appropriate choice of cation, amphiphilic, and crown ether unit in order to synthesize compounds with desired behavior

Synthesis and complexation of polytopic adamantane-based probes

The complexation of adamantane derivatives with cyclodextrins has been the subject of numerous studies during the last twenty years.1-18 This is due to several reasons. First, adamantane derivatives form strong complexes with β-cyclodextrin and the equlibrium constants are the strongest which can be found for the formation of this kind of inclusion complexes. This is because of the adamantyl residue perfectly fits inside the β-cyclodextrin cavity since the radios of this residue is slightly larger thant the radius inside the cavity available for guest in this cyclodextrin. Second, unimers simultaneously having guest (adamantyl residue) and host (β-cyclodextrin residue) moieties in their structure have been obtained and by self-association formed linear supramolecular structure. Third, adamantyl dimers have been synthesized and complexed with β-cyclodextrin dimers to form the so called “chelate complexes”. Fourth, adamantyl dimers have used to obtain linear and dentritic-like supramolecular polymers when they are complexed with polytopic hosts derived from β-cyclodextrin. Finally, polytopic hosts and polytopic guests can be used to form other macromolecular assemblies. In such cases, high viscosity enhancements have been observed, which are maximum for a composition in which the stoichiometry is one host (βCD residue):one guest (adamantyl residue). Thus the synthesis of both polytopic hosts and guests have a growing importance in designing new supramolecular entities mainly supramolecular polymers. The aim of this paper is the synthesis of adamantyl oligomers (dimers and trimers) which can later be used as polytopic guests

Synthesis, characterization and self aggregation of a new neo-pentylamide cholic derivative (Na-n-penC)

The self-aggregation in aqueous solution of a new neo-pentyl amide of the 3-β-amino derivative of cholic acid (Na-n-penC) has been investigated in aqueous solution by surface tension and steady state-fluorescence spectroscopy of pyrene (used as a probe). The nature of the agregates was determined by transmission electron microscopy (TEM) revealing that vesicles are formed. The structure of the compound in the solid state was resolved by X-ray spectroscopy. The synthesis of the compound is also given

Study on the structure of host-guest supramolecular polymers

A detailed characterization of a host−guest supramolecular copolymer, formed by adamantane and β-cyclodextrin dimers (Ad2 and βCD2, respectively) in aqueous solution, has been carried out by combining small-angle X-ray scattering and light scattering experiments with molecular dynamics (MD) and Monte Carlo (MC) simulations. First, the solutions of the monomers were studied by a straightforward analysis of the scattering data. Afterward, the complex given by Ad2 and two β-cyclodextrin molecules was investigated by correlating scattering results and MD simulations, to characterize the host−guest linkage. Finally, a detailed interpretation of the polymer scattering data was achieved by MC simulations. These simulations were performed on a single supramolecular aggregate and, in view of its peculiar shape, the complete polymer structure was considered without resorting to more general but simplified chain models. The comparison between simulations with and without excluded volume interaction points out that, at our ionic strength (sodium azide 150 mM), the polymer is close to the theta condition. Fits by reverse MC methods show that the polymer presents a shrunk conformation in solution, but it does not close in stable cyclic structures, as generally hypothesized for this kind of small oligomer. However, a polymerization degree of about 8 (4 Ad2 and 4 βCD2) was estimated which does not show any concentration dependence

BRANCHED HOST-GUEST SUPRAMOLECULAR POLYMERS

The obtainment of sophisticated structure by assembling molecules through non-covalent interactions represents a fundamental topic in supramolecular chemistry. In this field, a lot of work has been addressed towards the preparation of supramolecular polymers, which are generated by a directional and reversible connection of monomers. In particular, during the past few years, data on several host-guest linear supramolecular polymers, exploiting the hosting properties of cyclodextrins, have been published. Polymers have been obtained either by mixing unimers carrying complementary units, i.e. host and guest sites, or complementary monomers having two interacting host or guest moieties. Recently, moreover, branched structures have been prepared by mixing tritopic host and ditopic guest derivatives. Although very few papers have been published on this subject, these structures seem to be very interesting either because they are particularly new or because of the applicative importance of molecules with similar structure (dendrimers). In the light of these results, in this work, we reported preparation and characterization of polymers by connection of tritopic host and ditopic guest derivatives, exploiting the highly favorable interaction between the adamantyl group and the beta-cyclodextrin cavity. The structure of the polymers were studied by combining Small Angle X-Ray Scattering and Static and Dynamic Light Scattering measurements. Besides the conventional analysis of the scattering data, the 3D reconstruction of the electronic density distribution was performed from the SAXS spectra. Moreover, the hydrodynamic radii were estimated, from the shape of the electronic density distribution, and compared with the experimental ones

Supramolecular Tubules of Bile Salt Derivatives

In the past few years it has been greater and greater the interest towards the fabrication of micro or nano structures, for several applicative purposes. These preparations are often based on the formation of supramolecular structures, obtained by self assembly of organic molecules in solution. In particular the self assembly properties of amphiphilic molecules have been exploited because of their ability to generate aggregates with different morphologies, depending on the molecular shape and solution conditions. Among the various surfactants, the bile salts (BS) and some of their derivatives (DBS) seem to be particularly interesting for two main reasons: i) their ability of generating a large variety of supramolecular structures; ii) the fact that, being the bile salts biological surfactant and their derivatives obtained by slight modification, they are expected to be biocompatible and, therefore, potentially useful in biomedical applications. Among the various supramolecular morphologies, tubular structures are especially important since nanotubes can be involved in the preparation of several outstanding nanostructured systems such as membranes, sensors, optoelectronic devices and interconnected liposomes networks. Among the tubule forming surfactants the BS and the DBS have focused the attention of many researchers because of their ability of generating tubules in a wide range of diameter values. In this contribution we report on some of tubule forming DBS obtained by increasing the hydrophobic moiety of sodium cholate. In particular, we will focus on a derivative of sodium cholate. By starting from a viscous aqueous solution of this surfactant, in bicarbonate buffer (pH~10) and at room temperature, it was observed that the tubule formation is induced if the temperature is raised to a value around 40°C. The final tubules have diameters of about 450 nm and a length as large as 7 microns We reported a deep characterization of the tubule formation kinetics by using static light scattering, circular dichroism, small angle X-ray scattering along with transmission electron and optical microscopies. In such a way we tried to provide a well characterized example of self assembling kinetic in the formation of surfactant tubules

Stimuli responsive Bile Salt derivatives

Variations of the hydrophobic–hydrophilic balance of bile salts provide derivatives with unconventional amphiphilic structure and new self assembly properties. In particular interesting stimuli responsive associations are observed in derivatives obtained by substituting the hydroxyl group in position 3 of sodium cholate with a hydrophobic aromatic group. It has been already shown that one of these compounds forms thermoresponsive gels providing a reversible transition from fibrils to tubules at 35-37 °C. Here a pH responding self assembly is reported for a new derivative, presenting a naphtilamide residue as a substituting group. It is shown that in aqueous solutions it forms short supramolecular tubules at low pH which transform into more elongated structure at high pH values. Circular dichroism and Light Scattering measurements show that a drastic rearrangement of the molecular packing take place during the transition, occurring around pH 9.0. Molecules showing a stimuli responsive self-assembly provide smart nanoscopic structures which find application in nanoscience. Due to the biological origin of their precursors, stimuli responsive bile salt derivatives could be potentially useful for bio medical nanotechnologies

pH Sensitive Tubules of a Bile Acid Derivative: a Tubule Opening by Release of Wall Leaves

Tubules formed by self-assembly of organic molecules have a vast potential in nanotechnology applications and the introduction of sensitivity to stimuli into self-assembly tubules represents a particularly attractive feature. Here we report on the preparation and characterization of a molecule obtained by chemical modification of a natural bile acid, a biological surfactant, that self-assembles in pH sensitive tubules in aqueous solutions. The tubules, that are rigid, single-walled and with a diameter of 60 nm, form at pH 8-9 and open up when the pH is increased. The transition is reversible, it occurs in the pH range of 9-10 with an opening mechanism that is remarkably different from those so far proposed in the literature. It involves a release of wall layers similar to leaves, and is determined by a drastic pH-triggered change in the molecular arrangement, which in turn induces a radical modification of the wall curvature. The description of the morphological transformation is performed by means of cryogenic transmission electron microscopy and represent, to our knowledge, the first detailed visualization of pH stimulated tubule opening. UV and circular dichroism spectroscopies are used to investigate the evolution at the molecular level

Catanionic gels based on cholic acid derivatives

In this paper, the preparation and characterization of an anionic and a cationic surfactant obtained by chemical modifications of a natural bile acid (cholic acid) are reported. The bile acid was modified by introducing a diamine or a dicarboxylic aromatic residue on the lateral chain. The pure cationic surfactant self-assembles in a network of fibers with a cross-section gyration radius of about 15.1 Å, providing hydrogels with a pH-dependent compactness. On the other hand, the anionic molecule gives rise to prolate ellipsoid micelles. Homogeneous catanionic mixtures have also been obtained, with molar fraction of each surfactant ranging from 0.125 to 0.875. At total surfactant concentration of 0.05% (w/v), the mixtures form gels of fibrils partially arranged in secondary twisted superstructures. Comparison of this concentration with the minimum gelation concentration of the pure cationic derivative (0.16% w/v) suggests that, in the mixtures, the presence of the electrostatic component in self-assembly of the molecules allows the formation of gels starting from more dilute samples. In view of these achievements, this work suggests that catanionic mixtures can be exploited to enhance the efficiency of gelators. © 2013 American Chemical Society.In this paper, the preparation and characterization of an anionic and a cationic surfactant obtained by chemical modifications of a natural bile acid (cholic acid) are reported. The bile acid was modified by introducing a diamine or a dicarboxylic aromatic residue on the lateral chain. The pure cationic surfactant self-assembles in a network of fibers with a cross-section gyration radius of about 15.1 Å, providing hydrogels with a pH-dependent compactness. On the other hand, the anionic molecule gives rise to prolate ellipsoid micelles. Homogeneous catanionic mixtures have also been obtained, with molar fraction of each surfactant ranging from 0.125 to 0.875. At total surfactant concentration of 0.05% (w/v), the mixtures form gels of fibrils partially arranged in secondary twisted superstructures. Comparison of this concentration with the minimum gelation concentration of the pure cationic derivative (0.16% w/v) suggests that, in the mixtures, the presence of the electrostatic compone