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Hypothesis Sodium deoxycholate is a natural bile salt produced by animals and fulfilling important physiological processes. It is also used as dispersive surfactant and building block for self-assembled architectures in biology and material science. Although long debated, the study of its self-assembly in water is hereto incomplete and the models of the known aggregates are still controversial. This background suggests a complex scenario likely missing of additional mesophases. Experiments Electron and optical microscopy techniques were crossed with SAXS data for the research. Findings Novel rod, sponge, vesicle, lamellae, nanotube phases and reversible transitions among them arise at conditions (concentration, pH, temperature, ionic strength, ionic composition) fitting the physiological working environment of sodium deoxycholate. These findings enlarge the perspective towards different directions. The integration of the previous literature with this work removes any interpretative contradiction since all the structures cover the entire spectrum of phases expected for surfactants, thus being explained according to the Israelachvili’s scheme. It is not trivial that a single molecule can show such a high structural variability. This fact highlights a very versatile system. Probably it is not a coincidence that it occurs in a multitasking biomolecule. These results furnish fundamental knowledge to clarify the bile salts’ role in vivo.

Chlorophyllin and meso-tetra(4-carboxifenyl)porphyrin have been used as crosslinking agents for curing the epoxy resin of bisphenol A diglycidyl ether. Monitoring reaction by DSC revealed two peaks on calorimetric scans show. FTIR corroborates that the first signal corresponds to an esterification reaction while the second one is due to an etherification one. Thermodegradation temperatures show that the signals obtained by calorimetry do not correspond to decomposition processes.

In this work we introduce the zinc 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (ZnTPyP) macrocycle as a crosslinking agent of the epoxy resin BADGE n = 0. NMR, TEM and FTIR confirm that both homopolymerization and heteropolymerization (resulting in the formation of a pyridone) are taken place. DSC evidences two exothermic signals at 170 and 290°C, with enthalpy values of 262.3 and–20.7 J/g, respectively. Crystal powder diffraction supports that the second one corresponds to a physical process. The introduction of the ZnTPyP into the epoxy network improves the thermal stability of the material, which, although very weakly, shows ferromagnetic properties.

Ester 3-(octanoyloxy)-4-nitrobenzoic acid is a standard for the assay of the activity of phospholipase enzymes (the main toxins involved in tissue-damaging after snake bites). Because of its amphiphilic nature, it probably behaves as a surfactant but the instability of the ester bond prevents its characterization. Its ether analogue, 3-(octyloxy)-4-nitrobenzoic acid, is also an interesting compound as it is an inhibitor of the phospholipase activity and can be accepted as a model of the ester derivative. Its sodium salt, stable at basic pH, was studied by surface tension measurements, isothermal titration calorimetry, dynamic light scattering (DLS), and transmission electron microscopy (TEM). The aggregation number, fraction of bound counterions, critical micelle concentration and thermodynamic parameters involved in demicellization were obtained. The positive value for the change in the heat capacity for demicellization indicates that a larger hydrophobic surface area of each monomer is exposed to a hydrophilic environment after dissociation. Semiempirical calculations are in agreement with DLS and TEM measurements. For several carboxylate surfactants, the plot of enthalpy vs. entropy is linear. Although the slope has been named the compensation temperature, Tc, it merely is an experimental temperature without any extra-thermodynamic meaning.

Two similar macrocycles protoporphyrin IX and zinc protoporphyrin IX (ZPP) have been used as cross-linking agents for curing the epoxy resin of bisphenol A diglycidyl ether (BADGE, n = 0). The enthalpies and the activation energies of the esterification reaction of the 2 systems are very close to each other. However, the temperature of the minimum in the differential scanning calorimetry thermogram is 38°C lower for BADGE (n = 0)/ZPP, thus requiring a less energy expenditure for curing the system. By the contrary, the enthalpy and activation energies for the etherification reaction are lower and higher, respectively, for BADGE (n = 0)/ZPP suggesting that the zinc ion affects it, although the involved mechanism is unknown.

The crystal structure of the iodide salt of a quaternary ammonium derivative of cholic acid having a naphthalene group attached to the 3rd position of the steroid nucleus through an amide bond ([3,5,7,12]-3-(2-naphthyloylamino)-7,12-dihydroxycholan-24-triethylammonium iodide) has been resolved. The compound crystallizes in the P212121 orthorhombic space group (a/Å = 10.9458(3), b/Å = 12.1625(3), c/Å = 28.4706(7)). The lateral chain adopts a fully extended tttt conformation because the quaternary ammonium group cannot participate in the formation of hydrogen bonds. The iodide ion is involved in the formation of hydrogen bonds as well as the amide group and the two steroid hydroxy groups. Hirshfeld surface analysis confirms that these contacts, as well as the electrostatic interactions, stabilize the structure. The helixes around the 21 screw axis are right-handed ones.

The crystal structure of a Li+ salt of a glucosyl derivative of lithocholic acid (lithium 3α-(α- d -glucopyranosyl)-5β-cholan-24-oate) has been solved. The crystal belongs to the orthorhombic system, P212121 spatial group, and includes acetone and water in the structure with a 1:1:2 stoichiometry. Monolayers, having a hydrophobic interior and hydrophilic edges, are recognized in the crystal structure. Li+ is coordinated to three hydroxyl groups of three different glucose residues, with two of them belonging to the same monolayer. A fourth molecule, located in this monolayer, is involved in the coordination of the cation through the carboxylate ion by an electrostatic interaction, thus completing a distorted tetrahedron. All Li+-oxygen distances values are very close to the sum of the ionic radius of Li+ and van der Waals radius of oxygen. Each steroid molecule is linked to other five steroid molecules through hydrogen bonds. Water and acetone are also involved in the hydrogen bond network. A hierarchical organization can be recognized in the crystal, the helical assembly along 21 screw axes being left-handed.

A complete characterization of the aggregation process of the sodium salts of chenodeoxycholic (NaCDC) and obeticholic (NaObC) acids has been carried out by using different experimental techniques. Since it is well known that the presence of hydrophobic groups strongly reduce the critical aggregation concentrations and modifies the type of aggregation structures which can be formed, it could be expected that the introduction of an ethyl group into the structure of the chenodeoxycholic acid, to form the obeticholic derivative, could give rise to important modifications in its aggregation behavior. However, it has been observed that the aggregation numbers (determined in the range of temperatures 10–40 °C from ITC measurements) are identical to each other, a fact which was supported by the determination of the hydrodynamic radius from dynamic light scattering. This similarity was also observed for the fraction of bound counterions determined from the well-known Corrin–Harkins plot, being equal to 0.33 for both compounds. Finally, the polarity of the interior of the aggregate as measured by the ratio of the fluorescence intensities of the first and third vibronic peaks, I1/I3, of monomeric pyrene solubilized within the aggregates is also identical within the experimental error. The main differences correspond to: (1) the temperatures at which the enthalpy of demicellization is zero (31.4 (NaObC) and 26.8 °C (NaCDC)); and (2) the cmc value for NaObC is roughly half the one for NaCDC as observed from surface tension, fluorescence and ITC measurements. This implies that the free energy for the demicellization process is around 2 kJ mol−1 larger for NaObC than for NaCDC. (3) Finally, the difference in the change in the heat capacity of the aggregation (being 60 J mol−1 K−1 higher for ObCA) is due to the different number of water molecules which are lost during the monomer transfer from the bulk solution to the micellar aggregate.

The crystal structures of two ester compounds (a monomer in its methyl ester form,with anamino isophthalic group, and a dimer in which the two steroid units are linked by a urea bridgerecrystallized from ethyl acetate/methanol) derived from cholic acid are described. Average bondlengths and bond angles from the crystal structures of 26 monomers and four dimers (some of themin several solvents) of bile acids and esters (and derivatives) are used for proposing a standardsteroid nucleus. The hydrogen bond network and conformation of the lateral chain are also discussed.This standardstructure was used to compare with the structures of both progesterone and cholesterol The authors thank the Ministerio de Ciencia y Tecnología (Project MAT2013-45657-P) forfinancial support SI

The actual significance of the so-called compensation temperature Tc for micellization of surfactants is reviewed. It is demonstrated that it is possible to obtain as many Tc values as the number of temperature intervals in which the dependencies of enthalpy and entropy changes with temperature are analyzed. The value of each Tc will be the central value To of each temperature interval. These two facts suggest that Tc is simply such experimental To. Thus any physical interpretation derived from Tc is unfounded.

This work reports on the multi stimuli sensitivity of a cholic acid derivative in a buffer. The molecules self-organize in single walled tubules at room temperature and pH 8.0–9.5. By increasing the pH to 10.0–12.0, these tubules open up and form scrolls, which transform by aging into different tubular structures. A further evolution from scrolls or tubules into ribbons is induced by increasing the temperature. Moreover, a transition into rolled lamellae can be triggered by adding NaCl. All transitions are reversible and accompanied by drastic molecular rearrangements, manifested by spectroscopic and imaging techniques. Such rich multi stimuli responsiveness is usually accomplished by block copolymers or peptides and seldom by pure surfactants. The reported surfactant provides in addition an uncommon variety of structures. The sharpness of the spectroscopic response suggests that the system could be employed in physico-chemical sensors with a rather narrow dynamic range.

The compounds presented here constitute a clear example of molecular biomimetics as their design is inspired on the structure and properties of natural phospholipids. Thus novel double-armed surfactants have been obtained in which nitrilotriacetic acid plays the role of glycerol in phospholipids. The hydrophobic arms are linked to the head group through amide bonds (which is also the case of sphingomyelin): (R 1 NHCOCH 2 )(R 2 NHCOCH 2 ) NCH 2 CO 2 H (R 1 being CH 3 (CH 2 ) 11 , CH 3 (CH 2 ) 17 , CH 3 (CH 2 ) 7 CH CH(CH 2 ) 8 , and adamantyl, and R 2 = adamantyl). The dependence of the surface tension with concentration shows the typical profile of surfactants since a breaking point, which corresponds to the critical aggregation concentration ( cac ), is observed in all cases. The cac of these diarmed derivatives are about 1–3 orders of magnitude lower than those of classical monoalkyl derivatives used as reference compounds. In contrast to conventional surfactants, reversed trends in cac values and molecular areas at the solution–air interface have been observed. This anomalous behavior is tied to the structure of the surfactants and suggests that long and flexible alkyl chains should self-coil previous to the aggregation or adsorption phenomena. Above cac all compounds form large aggregates, globular in shape, which tend to associate forming giant aggregates.

Supramolecular rearrangements are crucial in determining the response of stimuli sensitive soft matter systems such as those formed by mixtures of oppositely charged amphiphiles. Here mixtures of this kind were prepared by mixing the cationic block copolymer pAMPTMA(30)-b-pNIPAAM(120) and an anionic surfactant obtained by the modification of the bile salt sodium cholate. As pure components, the two compounds presented a thermoresponsive self-assembly at around 30-35 degrees C; a micelle formation in the case of the copolymer and a transition from fibers to tubes in the case of the bile salt derivative. When both were present in the same solution they associated into mixed aggregates that showed complex thermoresponsive features. At room temperature, the core of the aggregate was comprised of a supramolecular twisted ribbon of the bile salt derivative. The block copolymers were anchored on the surface of this ribbon through electrostatic interactions between their charged blocks and the oppositely charged heads of the bile salt molecules. The whole structure was stabilized by a corona of the uncharged blocks that protruded into the surrounding solvent. By increasing the temperature to 30-34 degrees C the mixed aggregates transformed into rods with smooth edges that associated into bundles and clusters, which in turn induced clouding of the solution. Circular dichroism allowed us to follow progressive rearrangements of the supramolecular organization within the complex, occurring in the range of temperature of 20-70 degrees C.

The crystal structure of three head-to-head dimers (having two cholic acid or deoxycholic acid units) linked at carbon atoms C3 by aromatic or alkyl bridges is studied. An internal coordinates system is necessary for describing the relative orientation in the space of the two bile acid residues. Five angles (three torsion and two common ones) are necessary for defining the relative position of both steroid residues in space. Carbon atoms C3 (which always carries a α-hydroxy group in natural bile acids), and C10 and C13 (which always carry β-methyl groups) of each steroid residue are suitable for this purpose. Furthermore, the distance between each C3 carbon atoms of both steroid residues will allow one to locate the steroids in space. The three dimers selected provide a large range of values for these angles. The packing, hydrogen bond network, and location of guest in the three crystals are discussed.

Starting from the structure of ice (in which each water molecule is surrounded by other four water molecules forming a tetrahedron with a value of 4.51 A for the edge O-O distance), and the knowledge that this value also corresponds to the O7-O12 distance of the skeleton of cholic acid, it is hypothesized that two steroid cholic acid moieties, with an appropriate steroid-steroid distance and a belly-to-belly orientation, could encapsulate a single water molecule between them. To check this hypothesis two succinyl derivatives of cholic acid (a monomer and the related head-head dimer in which the succinyl group is the linking bridge) were designed. The expected "ice-like" structure is found in the crystal of the dimer. There is a hydrogen bond synergy between those participating in the "ice-like" structure, and those in which the bridge is involved with the O7-H hydroxy group and the side chain of the steroid. © 2012 Elsevier Inc. All rights reserved.

It is well-known that amines have the tendency to undergo aerial carbonation to form sec-ammonium N-carboxylates (carbamates). In fact, carbamate formation may be a quite general and functionally important phenomenon throughout biology, as carbamate moieties are key components in the biological fixation of CO2 by living organisms. Currently, there is renewed interest in the use of amino acids for removal of CO2 from gas streams that also contain oxygen. These facts justify the wide interest in research on the formation of carbamate derivatives from carbon dioxide, an aspect that this paper explores for the “amino bile acids”. The 24-amine derivatives of cholic (24-amine-CA) and chenodeoxycholic (24-amine-CDCA) acids were synthesized following literature procedures. These compounds were recrystallized in water. The empirical formula evidenced that half of the molecules retain the expected amine structure for the compound (in its protonated form) but the remaining 50% have a carbamate moiety resulting from ...

[3β,5β,7α,12α]-3[(Adamantyl-1-acetyl)-amino]-7-12-dihydroxycholan-24-oic acid (AdCH2CA) was synthesized by the reaction between 1-adamantyl acetyl chloride and the methyl ester of 3β-amino-cholic acid and hydrolysis of the ester. The acid was recrystallized from racemic 2-butanol (0.1% water). Crystals are orthorhombic (P212121) and form inclusion complexes with water and 2-butanol with a 1:1:1 stoichiometry. Only the S-enantiomer is included into the structure of the crystal, exhibiting a chameleonic mimicry with the steroid bilayers. The isolation of crystals allows the enantioresolution of the racemate with a high purity (≈99%) of S-2-butanol. The steroid molecules are disposed in an antiparallel orientation in the hydrophobic layer and a parallel orientation in the hydrophilic one.

Two βCD dimers (linked by succinic acid, 2, or ethylenediaminetetraacetic acid, EDTA, 3, bridges) and a negatively charged monomer derivative of βCD, 1, have been synthesized and their ability to solubilize cholesterol in aqueous solution was studied. The three compounds exhibit a great capacity in solubilizing cholesterol as, for instance, concentrations up to 6 mM of cholesterol were measured in the presence of 25 mM of 3. The phase-solubility diagrams of the two dimers exhibit A L type profiles while the monomer 1 follows an A P isotherm. The cholesterol/dimer complexes have 1:1 stoicheiometries while monomer 1 forms two complexes with molar ratios of 1:1 and 1:2 (cholesterol/1). The equilibrium constants are K 1:1 = (5.9 ± 0.3) × 104 M−1 and K 1:1 = (8.8 ± 0.2) × 104 M−1 for 2 and 3, respectively, and K 1:1 = 73 ± 19 M−1 and K 1:2 = 204 ± 65 M−1 for 1. The comparison of K 1:1(3) with the product K 1:1 × K 1:2 (1) reveals that a chelate effect in binding the cholesterol by 3 exists. The structure of the cholesterol/3 complex was studied by ROESY experiments and by molecular dynamics simulations.

A structural characterization of host-guest supramolecular copolymers, formed by an adamantane dimer and two beta-cyclodextrin trimers in aqueous solution, has been carried out by combining small angle X-ray scattering and light scattering experiments. A shape-reconstruction method was applied to the SAXS data to obtain relatively high-resolution conformation information, and a correlation with the experimental dynamic light scattering results was performed, by estimating the hydrodynamic radii of the reconstructed shape through a shell model method. When applied on the solutions of the trimers, the analysis provides a globular reconstructed shape with a hydrodynamic radius in agreement with the experimental one. For the polymers, elongated structures were inferred which grow both in length and in cross section by increasing the concentration. Depending on the beta-cyclodextrin trimer employed in the polymer preparation, polymerization degrees ranging between roughly 7 and 14 or 9 and 22 were obtained in the concentration range 4.00-10.0 or 3.10-6.60 mM of the trimer (6.00-15.0 or 4.65-9.90 mM of the dimer). Aggregation schemes were proposed accounting for the formation of hyperbranched, linear, and network like polymers. The experimental results are not far from those expected on the basis of the aggregation in hyperbranched structure, for which the growth of elongated aggregates can be predicted in the early stages of the polymerization. However, the coexistence of the other structures, in particular of the linear one, cannot be ruled out.

A new gemini steroid surfactant derived from 3 alpha,12 alpha-dihydroxy-5 beta-cholan-24-amine (steroid residue) and ethylenediamintetraacetic acid (spacer) was synthesized and characterized in aqueous solution by surface tension, fluorescence intensity of pyrene, and light scattering (static and dynamic) measurements. These techniques evidence the existence of a threshold concentration (cac), below which a three layers film is formed at the air-water interface. Above the cac, two types of aggregates(-)micelles and vesicle-like aggregates(-)coexist in a metastable state. Filtration of a solution with a starting concentration of 2.6 mM (buffer 150 mM, pH 10) allows isolation of the micelles, which have an average aggregation number of 12, their density being 0.28 g cm(-3). Under conditions where only the vesicle-like aggregates are detected by dynamic light scattering, a value of 5.5 x 10(4) was obtained for their aggregation number at 30 mu M, their density being 6.8 x 10(-4) g cm(-3). At high concentrations, the intensity ratio of the vibronic peaks of pyrene, I-1/I-3, (= 0.68) is very close to published values for deoxycholate micelles, indicating that the probe is located in a region with a very low polarity and far from water. A hypothesis to explain the observed aggregation behavior (small aggregates are favored with increasing gemini concentration) is outlined.

The formation of supramolecular structures initiated by a p-tert-butyl-phenyl-amide derivative of cholic acid is investigated. The initial spherical vesicles, with a rather low effective bending constant, collapse into necklaces that self-transform into tubules of small diameter. Finally, molecular tubes are generated (see figure). During the process, the geometrical constraints of fixed surface area and fixed enclosed volume are obeyed.

The crystal structures of chenodeoxycholic acid (CDCA), ursodeoxycholic acid (7beta isomer of CDCA) and their other two epimers (3beta,7alpha- and 3beta,7beta-isomers) have been resolved. The four isomers were recrystallized from p-xylene. CDCA crystal is hexagonal P6(5) while the crystals of the other three isomers are orthorhombic (P2(1)2(1)2(1) space group). Only the 3beta,7beta isomer forms an inclusion complex with the solvent with a 1:1 stoichiometry. In all cases, the three hydrogen bond sites (the two hydroxy groups, O3-H and O7-H, and the carboxylic acid group of the side chain, O24bO24a-H) simultaneously act as hydrogen bond donors and acceptors. By considering that O24a is always donor and O24b is always acceptor, the hydrogen bond sequences can be understood on the basis of the interaction between the two hydroxy groups. However the comparison between the four compounds is complicated by the existence of two molecules in the asymmetric unit in the UDCA crystal resulting in that the same hydrogen bond site (for instance O3) can be donor towards two different acceptors (either O7 or O24b). As in the case of the four isomers of deoxycholic acid (Steroids 2004, 69, 379), the other three isomers present a donor-->acceptor sequence, which is O7-->O3 when O3-H is beta and O3-->O7 when O3-H is alpha. The spatial orientation of the carboxylic acid of the side chain is referred to two almost perpendicular planes (defined by (1) the carbon atoms C1/C6-C17/C20 and by (2) the methyl groups C18-C19 and the two carbon atoms to which they are linked, C10 and C13, respectively). Only the side chain of CDCA evidences a positive deviation towards the hydrophobic beta side of the molecule.

Steroidal surfactants like bile salts have a rigid amphiphilic structure significantly different from the typical head–tail one. As a result, these molecules show peculiar features in their self-assembly behavior and solubilization and permeation abilities. Bile salts are widely used as starting materials in the preparation of synthetic derivatives by changing their amphiphilic structure and by introducing specific functionalities. Due to the steroid rigidity and the peculiar distribution of hydrophobic and hydrophilic domains, these molecules self-organize in ordered supramolecular assemblies and are particularly attractive for the bottom up construction of complex nanostructures. They often self-assemble in 1D structures such as tubes or fibers and show low molecular weight gelator features. Their tubular nanoscale structures have cross section diameters spanning a wide range of values (inner diameter 3–450 nm) and are sometimes formed through appealing pH or temperature responsive aggregations. Moreover, mixtures of these surfactants allow in some cases the preparation of mixed tubes with tunable composition and related features such as charge and sizes. The unconventional amphiphilic molecular structure of BSs dictates also remarkable abilities as carriers across tissues and membranes of many compounds (e.g. drugs, carbohydrates and ions). Therefore they are often employed as encapsulators, dispersants and transporters in complex systems. Chemical modifications can also be used to provide derivatives with improved performances. UCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela de Química UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigación en Electroquímica y Energía Química (CELEQ)

The synthesis of a novel cholic acid derivative bearing in the C-3 position a residue of tryptophan linked through an amide bond is herein described. Acidic or basic conditions are needed for the solubilization of the derivative in water. In alkaline solutions the molecule shows a self-association similar to the one of its natural precursor leading to the formation of ellipsoidal micelles which does not involve significant Trp–Trp interactions. On the contrary, in acidic conditions strong interactions between the tryptophan moieties occur, leading to the formation of a gel at low temperature. These interactions are broken upon heating and small micelles similar to those observed at high pH are formed. In both cases, fluorescence spectra suggest a polar environment for the amino acid fluorophore not remarkably affected by the self-assembly.

The complexation of three trihydroxy sodium bile salts--cholate, glycocholate, and taurocholate--and their three related sodium dihydroxy bile salts--deoxycholate, glycodeoxycholate, and taurodeoxy...

The complexation of two steroid antibiotics of the fusidane family, sodium fusidate and potassium helvolate, by β-CD and γ-CD has been studied by using 1D and 2D-NMR techniques. Both guests form 1:1 complexes with γ-CD and 1:2 (guest:cyclodextrin) complexes with β-CD. Thus, both antibiotics behave as monotopic and ditopic guests when they are complexed by γ-CD and β-CD, respectively. Both steroids enter into the cavity of the γ-CD by the side chain, reaching the central region of the steroid (rings C and D), whereas the A and B (partially) rings remain outside. For β-CD complexes, ROESY spectra show a remarkable absence of interactions of the protons of the C and D rings, whereas clear interactions corresponding to the side chain, and A and B rings are observed. The obtained equilibrium constants (see previous paper) are discussed in terms of the structures proposed for the complexes. NMR spectra of sodium fusidate are revised, and a full assignment of the 1 H and 13 C NMR spectra is presented for potassium helvolate.

Static and dynamic light scattering techniques have been used to study the aggregation behaviour of polygalacturonic acid (PG) in aqueous solution at different pH values. The dependence of apparent molecular weight, radius of gyration and hydrodynamic radius with time or concentration have been determined. Other structural parameters as contour length, Kuhn segment numbers, mass per unit length, persistence length, and average number of chains per cross section have been deduced from Casassa –Holtzer plots. The weight average molecular weight increases linearly with time suggesting that the aggregation kinetics follows a simple Smoluchowski equation in which all the kernels are identical, the aggregation rate being approximately two chains per day at pH 3.83. As a consequence of this growth the polydispersity of the aggregates increases with time. The growth of the hydrodynamic radius with time suggests that no water molecules remain trapped in the aggregate, explaining why the PG does not gel. q 2003 Elsevier Science Ltd. All rights reserved.

Forced-oscillation measurements of aqueous dispersions of an amide pectin were carried out at pH 3.83 in the presence of 50% sucrose. The results show a predominant viscous behaviour corresponding to the so-called terminal region of the general mechanical spectrum of biopolymer solutions. The polymer chains have been described by the bead-spring model with dominant hydrodynamic interactions among beads. At pH 3.31, samples are more elastic than viscous and the mechanical spectra reveal an entanglement network (weak gel), although not dense enough to store the stress during a long period of time after having been deformed. At low pectin concentration, the plot of loss moduli versus frequency shows a minimum that suggest monodisperse junction zones and the existence of two regions with different relaxation times. The short relaxation process corresponds to movements of galacturonic units inside the junction zones (local configuration adjustment), and the longer one to movements of the junction zone as a whole (rearrangements of couplings through entanglement and cross-linking). The comparison of the results obtained here at different sucrose concentrations and temperatures with those for methoxy pectins, suggests that the hydrogen bonds formed by amide groups have an important role in the formation of the gel. The role of the hydrophobic interactions in the stability of the junction zones has also been discussed.

The formation of supramolecular structures by the complexation of two bile salts--sodium cholate (NaC) and sodium deoxycholate (NaDC)--with four new head-to-head g -cyclodextrin dimers was studied by NMR techniques. All dimers form 1:2 (dimer:bile salt) stoichiometry complexes with NaC. With NaDC, linear supramolecular conglomerates of an n : n stoichiometry were obtained for all dimers. ROESY spectra confirmed the presence of electrostatic interactions when a protonated amino group is present in the linking group. The dependence of the pseudo-equilibrium constants with the electrostatic interactions and steric hindrance is discussed.

A dynamic rheological study of sodium deoxycholate gels was carried out within the linear viscoelastic region. The effects of the oscillatory frequency f, sodium deoxycholate concentration, pH, temperature, and ionic strength (NaCl) were studied. The G‘ ∝ f n relationship, the creep-recovery experiments, and the frequency−temperature superposition principle strongly suggest that the interactions in the gel are physical in nature and that its mechanical behavior can be explained by a simple relaxation mechanism. It is concluded that the hydrogen bonds are the molecular forces involved in the formation, relaxation, and strength of the gel.

The 18th International Electronic Conference on Synthetic Organic Chemistry session General Organic Synthesis A head-to-head dimer of cholic acid linked at C3 carbon atoms by a urea bridge was synthesized and the crystal structure was studied. The crystal belongs to the monoclinic crystal system, space group I2. The packing shows a bilayer structure with alternating hydrophobic and hydrophilic layers. The horizontal and vertical planes of the two cholic residues are almost parallel to each other, the vertical ones being perpendicular to the bilayers. The hydrogen bond network is two-dimensional as it does not propagate in the direction perpendicular to the bilayers. It is noteworthy that the nitrogen atoms of the urea bridge do not participate in that network. The carbonyl urea group and only one of the ester of the two side chains form hydrogen bonds with two other dimers through the steroid hydroxy O7-H and O12-H groups. This means that each dimer forms eight hydrogen bonds but only four are different

The aggregation behavior of a cationic derivative of cholic acid {[3β,5β,7α,12α]-3-(2-naphthoylamino)-7,12-dihydroxycholan-24-triethylamonium iodide} has been studied by surface tension measurements, fluorescence spectroscopy, transmission electronic microscopy (TEM), and circular dichroism. The critical aggregation concentration, the fraction of bound counterions and thermodynamic parameters for the formation of aggregates have been determined, as well as the morphology of the aggregates. TEM images support a consecutive transformation mechanism from fibers to tubules, these having a well-defined geometry and being the only structure observed at the end of the process. Intermediate observed structures are helical ribbons.

Iso-deoxycholic acid (3β,12α-dihydroxy-5β-cholan-24-oic acid, iso-DCA), the 3β hydroxy epimer of deoxycholic acid (DCA), was synthesized from deoxycholic acid. The crystals, obtained by recrystallization from p -xylene, are orthorhombic, space group P 2 1 2 1 2 1 , with a =7.3232(6), b =10.5938(16) and c=28.2957(18) A . The crystals contained no inclusion molecules, and thus differed from those of DCA when recrystallized from o -, m - and p -xylene. This absence of solvate molecules could explain the reduction of ≈3 A in the b -axis length in iso-DCA when compared with that of DCA with xylene guests.

The complexation behavior of two bile saltssodium cholate (NaC) and sodium deoxycholate (NaDC)with β-cyclodextrin (β-CD), 6-deoxy-6-amino-β-cyclodextrin (β-CDNH2), and dimer I (N,N‘-bis(6-deoxy-β-cyclodextrin)pyromellic acid diamide) was studied by NMR techniques. Complexes formed between β-CD and β-CDNH2 with NaC and NaDC have 1:1 and 2:1 (host:guest) stoichiometries, respectively. Complexes with β-CDNH2 show higher equilibrium constants than those with β-CD because of the electrostatic effect of the protonated amine group. Dimer I showed 1:2 and n:n stoichiometries with NaC and NaDC, respectively. ROESY spectra stated that bile salts enter first with their 5-C ring forward the inner cavity by the side of the secondary hydroxyl groups of cyclodextrins. In the complexes formed with β-CDNH2, the steroid body of the bile salt enters deeper in the cavity, while the carboxylated side chain is extended toward the protonated amine group at C-6, allowing an electrostatic interaction between both groups. In the c...

In order to study its guest binding and the inclusion phenomena, 6-deoxy-6-(aminoethyl)amino-β-cyclodextrin (βCDN) was synthesised and its binding properties examined. The complexation phenomena of sodium cholate (NaC) and sodium deoxycholate (NaDC) with βCDN has been monitored by the NMR method using 13C chemical shift data. The method of continuous variation ’“Job's method” has been used to determine the stoichiometry of these supramolecular complexes. The Job's plot confirms the 1 : 1 supramolecular complex for NaC: βCDN and the 1 : 2 supramolecular complex for NaDC: βCDN. The interaction of NaC and NaDC with βCDN has been obtained through two-dimensional Rotational Nuclear Overhauser Effect Spectroscopy (ROESY) NMR. Equilibrium constants were also obtained from 13C chemical shift data (C-1, C-3 & C-4) at different pH values (7, 9, & 11).

Steady-state fluorescence and time-resolved fluorescence have been used to study sodium deoxycholate aggregates at basic pH. Pyrene was used as a probe, and dimethylaniline, as a quencher. It is concluded that dynamic quenching with partially micellized quenchers and not static quenching with the quencher totally micellized, as has been often quoted in the literature, must be considered to interpret the experimental results. Fluorescence lifetimes for pyrene in the absence of quencher and the kinetic constants involved in quenching by dimethylaniline are given and compared to those for similar systems. The derived aggregation number, equal to 8 ± 2, is close to the ones determined by other authors from freezing point depression and static light scattering. The possible distortion of these small aggregates by probes is also discussed.

The G-1 dendrimer 4,4',4'',4'''-methanetetrayltetrakis(N-(3,5-dicarboxyphenyl)benzamide) has been obtained from 4,4',4'',4'''-methanetetrayltetrabenzoic acid and isophtalic acid. The compound was recrystallized from methanol and its structure resolved. The crystal belongs to the tetragonal crystal system space group I-4 2d, the cell lengths being a=b=18.9585(16) A, and c=23.703(2) A. The crystal structure evidences the formation of cavities. Only one type of hydrogen bond is observed implying the nitrogen atom (acting as donor) and the oxygen atom of one carboxy group of the aminoisophtalic residue (as acceptor), while the other carboxy group does not participate in the network. The donor-acceptor distance is 3.024(2) A.

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 A, 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.

Freezing point depression, delta T/k, and pNa are measured and analyzed for aqueous solutions of trihydroxy (NaTC) and dihydroxy (NaDC and NaTDC) bile salts. The results show the existence of break points in the plot of delta T/k vs molality at 0.018, 0.013, and 0.007 m, respectively, in good agreement with previous published critical micelle concentration values. Above the break point bile salts form aggregates with average aggregation numbers of 2.59 +/- 0.12 (NaTC), 5.82 +/- 0.04 (NaDC), and 5.42 +/- 0.47 (NaTDC). Fractions of bound counterions are also deduced, being close to 0.3 for the three bile salts studied. This indicates that only one counterion is bound for every three monomers in the aggregate. The different structural models published for the bile salt aggregates are discussed.

The formation of tubules by p-tert-butylphenylamide derivatives of chenodeoxycholic and ursodeoxycholic acids in aqueous solution is investigated. The critical aggregation concentrations of the new surfactants are much lower than those of ursodeoxycholate and chenodeoxycholate, indicating the enhanced surfactant properties resulting by the presence of the hydrophobic p-tert-butylphenyl group. The molecular areas at the air-water interface suggest the formation of monolayer films with molecules upright oriented. The shape of the aggregates was investigated by TEM. The main structure present in solution corresponds to tubules. The estimated value for the wall thickness of tubules suggests that a bilayer structure is formed. Host of positively charged latex beads by tubules suggests that their inner and outer surfaces are negatively charged. The acid form of the chenodeoxycholate derivative was recrystallized from toluene and its crystal structure analyzed. © 2012 Elsevier Inc. All rights reserved.

[3β,5β,7α,12α]-3-(4-t-Butylbenzoilamine)-7,12-dihydroxycholan-24-oic acid was synthesized and recrystallized from chlorobenzene and acetone. Orthorhombic P212121 and monoclinic P21 crystals were obtained, respectively, and both crystals include solvent and water molecules with a 1:1:1 stoichiometry. In the second case, there are two nonequivalent molecules of the steroid in the unit cell. In both crystals, a crossing structure results for the molecular packing, stabilized by hydrogen bonds between the steroid molecules. In the second crystal, water links steroid molecules of the same type, which are not directly connected through hydrogen bonds. The steroid molecules define helicoidal assemblies along 21 screw axes. The handedness in the crystal in chlorobenzene is unambiguously defined by following the criterion proposed by Miyata et al., as the steroids are in a belly-to-belly disposition around the axis. This motif does not appear in the crystal in acetone, and other criterion is required. On the basis of the fact that a staircase and its banister have the same handedness and using the crystal in chlorobenzene as reference, the additional criterion has been established. According to it, in the absence of a belly-to-belly orientation, the handedness must be defined by keeping the bile acid with its right side oriented toward the axis. Pitch angles of the helices and tilt angles of the molecules are also determined. Chirality, 2011. © 2011 Wiley-Liss, Inc.