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The epoxy resin diglycidyl ether of Bisphenol A (BADGE n = 0) has been cured with a new synthesized hardener (2-adamantylethanamine) and the crosslinking reaction was characterized by DSC. Values of 413.3 J/g and 95°C have been obtained for the enthalpy of the reaction and the glass transition temperature, respectively. The experimental results obey Kamal's model over all conversion range of temperatures (70°C-100°C). The activation energies of the mechanisms involved in the curing reaction have been determined for both the autocatalytic and the n-order mechanism, the values being 63.3 and 29.8 kJ/mol, respectively. The value for Tg is 23°C higher than the one for (BADGE n = 0)/amantadine, while the activation energy for the n-order mechanism is around 13 kJ/mol lower. This is consistent with a higher steric effect of the adamantyl group in the second hardener since it will hinder the opening the oxirane ring by the nitrogen atom of the amino group. As the polymerization reaction progress, this effect will disappear as the distance adamantyl-oxirane increase when new oxirane groups react with the hydroxyl groups (autocatalyzed reaction). Consequently, by selecting the appropriate cross-linking agent, it is possible to simultaneously increase Tg while reducing theactivation energy, two effects which may be desirable for some industrial applications of the material.

The effect of doping the bisphenol A diglycidyl ether (DGEBA)/m-xylylenediamine (mXDA) system with gold nanoparticles (AuNP) has been studied with differential scanning calorimetry (DSC), thermogravimetric analysis, dynamic mechanical analysis (DMA), and dielectric analysis (DEA). The evolved heat (ΔHt), the glass transition temperature (Tg), and the associated activation energies of this relaxation process have been determined. Below a certain concentration of AuNPs (=8.5%, in mg AuNP/g epoxy matrix), Tg decreases linearly with the concentration of AuNPs, but above it, Tg is not affected. The degree of conversion α of this epoxy system was analyzed by the semiempirical Kamal’s model, evidencing that diffusion correction is required at high values of α. Activation energy values suggest that AuNPs can cause some impediments at the beginning of the crosslinking process (n-order mechanism). The slight difference between the initial decomposition temperature, as well as the temperature for which the degradation rate is at a maximum, for both systems can be accepted to be within experimental error. Mechanical properties (tension, compression, and bending tests) are not affected by the presence of AuNPs. Dielectric measurements show the existence of a second Tg at high temperatures, which was analyzed using the Tsagarapoulos and Eisenberg model of the mobility restrictions of network chains bound to the filler.

Cholic acid is a trihydroxy bile acid with a nice peculiarity: the average distance between the oxygen atoms (O7 and O12) of the hydroxy groups located at C7 and C12 carbon atoms is 4.5 Å, a value which perfectly matches with the O/O tetrahedral edge distance in Ih ice. In the solid phase, they are involved in the formation of hydrogen bonds with other cholic acid units and solvents. This fact was satisfactorily used for designing a cholic dimer which encapsulates one single water molecule between two cholic residues, its oxygen atom (Ow) being exactly located at the centroid of a distorted tetrahedron formed by the four steroid hydroxy groups. The water molecule participates in four hydrogen bonds, with the water simultaneously being an acceptor from the 2 O12 (hydrogen lengths are 2.177 Å and 2.114 Å) and a donor towards the 2 O7 (hydrogen bond lengths are 1.866 Å and 1.920 Å). These facts suggest that this system can be a nice model for the theoretical study of the formation of ice-like structures. These are frequently proposed to describe the water structure found in a plethora of systems (water interfaces, metal complexes, solubilized hydrophobic species, proteins, and confined carbon nanotubes). The above tetrahedral structure is proposed as a reference model for those systems, and the results obtained from the application of the atoms in molecules theory are presented here. Furthermore, the structure of the whole system allows a division into two interesting subsystems in which water is the acceptor of one hydrogen bond and the donor of another. The analysis of the calculated electron density is performed through its gradient vector and the Laplacian. The calculation of the complexation energy used correction of the basis set superposition error (BSSE) with the counterpoise method. As expected, four critical points located in the H…O bond paths were identified. All calculated parameters obey the proposed criteria for hydrogen bonds. The total energy for the interaction in the tetrahedral structure is 54.29 kJ/mol, while the summation obtained of the two independent subsystems and the one between the alkyl rings without water is only 2.5 kJ/mol higher. This concordance, together with the calculated values for the electron density, the Laplacian of the electron density, and the lengths of the oxygen atom and the hydrogen atom (involved in the formation of each hydrogen bond) to the hydrogen bond critical point, suggests that each pair of hydrogen bonds can be considered independent of each other.

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.

Different crown ethers (4-aminobenzo-15-crown-5 (4-aminobenzo-15-C5), 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (diaza-18-crown-6), tetraazacyclododecane-1,4,7,10-tetraacetic acid (H4DOTA) and tetraazacyclododecane-1,4,7,10-tetraacetamide (H2ODDA)) were used as curing agent for bisphenol A diglycidyl ether (BADGE, n = 0). The maximum enthalpy change for all systems except that formed by the epoxy resin with H4DOTA corresponds to a stoichiometric ratio, since from this value the reaction enthalpies decrease when the proportion of epoxy increases. Heteropolymerization reaction occurs in all the crown ethers. Etherification reactions occur at temperatures much lower (30 °C less) than for the porphyrin systems studied in which a second signal appears at 300 °C. The etherification is evidenced by a slight shoulder in the thermograms for H4DOTA and H2ODDA. The systems BADGE (n = 0)/4-aminobenzo-15-C5 and BADGE (n = 0)/diaza-18-crown-6 improve the thermal stability of the epoxy resin by 30 °C approximately while the improvement for BADGE (n = 0)/H4DOTA and BADGE (n = 0)/H2ODDA is about 60 °C. © 2017 Society of Chemical Industry.

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.

Two ether and one ester derivatives of the 4-nitro-3-hydroxybenzoic acid were synthesized and characterized. The in vitro antimicrobial and cytotoxic activities of the three novel compounds were also evaluated. The aromatic derivatives showed antibacterial activity against one of the four microorganisms tested and two compounds (C8 and NOBA) had a lower IC50 in HeLa cells. Ministerio de Ciencia y Tecnología/[MAT2013-45657-P]/MICITT/Costa Rica UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias de la Salud::Instituto de Investigaciones en Salud (INISA) UCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela de Química UCR::Vicerrectoría de Docencia::Salud::Facultad de Medicina::Escuela de Medicina

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.

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 illustrates that macrocycles can be used as crosslinking agents for curing epoxy resins, provided that they have appropriate organic functionalities. As macrocycles can complex metal ions in their structure, this curing reaction allows for the introduction of that metal ion into the resin network. As a result, some characteristic physical properties of the metallomacrocycle could be transferred to the new material. The bisphenol A diglycidyl ether (BADGE, n = 0) and hemin (a protoporphyrin IX containing the Fe(III) ion, and an additional chloride ligand) have been chosen. The new material has been characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier Transform Infrared (FT-IR), Nuclear Magnetic Resonance (NMR), Transmission Electron Microscopy (TEM), and magnetic susceptibility measurements). Fe(III) remains in the high-spin state during the curing process and, consequently, the final material exhibits the magnetic characteristics of hemin. The loss of the chlorine atom ligand during the cure of the resin allows that Fe(III) can act as Lewis acid, catalyzing the crosslinking reactions. At high BADGE n = 0/hemin ratios, the formation of ether and ester bonds occurs simultaneously during the process. The authors thank the Ministerio de Ciencia y Tecnología, Spain, Project MAT2010-61721 for financial support. ECV also thanks Dr. Senén Paz and GAIRESA for a scholarship SI

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.

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 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.

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 formation of inclusion complexes between methyl orange (MO) and β-cyclodextrin (β-CD) was studied extensively using UV-visible absorption spectroscopy. In neutral-to-basic media, where MO is an anion, two complexes of stoichiometries 1:1 and 1:2 (guest:host) are formed, as determined by principal components analysis of the absorption data. Global analysis yields precise values of the corresponding association constants and the spectra of each species: free MO, complex MO · CD and complex MO · 2CD. In acid media, where MO is a zwitterion, only 1:1 complexes are formed, but the system becomes complicated due to tautomerization of MO. The microscopic equilibria among the two tautomers and their corresponding complexes were resolved and estimations of the microscopic association constants were obtained. Finally, the application of MO complexation to the quantification of bound β-CD in a polymer sample is presented, with a detailed description of the experimental conditions to be used and a critical discus...

Cyclodextrins are a group of cyclic oligosaccharides which have shown to improve pharmaceutical properties of drugs, such as solubility and stability, by forming inclusion complexes. Diloxanide furoate has direct amoebicide action on the intestinal lumen and little solubility in water. In this work, solubility, NMR, fluorescence techniques, and kinetic methods were used to determine the equilibrium constant for the formation of the β-cyclodextrin–diloxanide furoate complex, K1:1. It is shown that the kinetic approach is the best method for such a determination.

The complexation of 3-[(3-cholamidopropyl)dimethylammonio]-1- propanesulfonate (CHAPS) by β- and γ-cyclodextrin (β- and γ-CD) has been studied by using 1D- and 2D-NMR techniques. For the CHAPS/β-CD system both 13C and 1H NMR spectra exhibit split lines consistent with slow exchange rate between one or more complexes and free species. Job's plots suggest that complexes with 1:1 stoichiometry are formed. However, ROESY experiments suggest that β-CD is complexing the steroid nucleus at two sites, the first one being mainly the side chain of the guest, and the second one being the A/B rings. The conclusion is that two isomeric 1:1 complexes, C11Y, and C11 X are obtained. The concentrations of free and complexed species were measured from the integration of 1H NMR signals, allowing an independent determination of the two microscopic equilibrium constants, K 11Y and K11X, associated with the formation of the two isomeric complexes, and the macroscopic equilibrium constant, K11 (= K11X + K11 Y). A good agreement between the values obtained is observed. The slow exchange rate for the formation and decomposition of the C 11Y, complex is understood as a barrier penetration problem for the side chain to cross through the cyclodextrin cavity since the methyl groups linked to the charged nitrogen atom of the side chain are bulky enough to force a large distortion of the β-CD during the penetration. However, the origin of the energy barrier for the complexation of the wide end of the steroid nucleus leading to a slow exchange regime is unknown. The travelling of the CHAPS side chain through a larger cyclodextrin cavity should be faster. Therefore, complexation studies between CHAPS and γ-CD were undertaken. Now, only one type of complex was observed and the fast limited exchange was obeyed. The value for the equilibrium formation constant was obtained from the analysis of the chemical shift displacements of carbon atoms of γ-CD with increasing CHAPS concentration. ROESY experiments suggest that the side chain is unfolded outside the cyclodextrin cavity forming a rotaxane-type structure.

The hydrolysis of benzoic anhydride (Bz2O) in the presence of β-cyclodextrin (β-CD) was studied in aqueous solution as a function of pH, temperature and ionic strength, at 25°C. The experimental rate constant versus pH profiles show that, in the region of spontaneous water reaction (pH 3.0–6.5), β-CD inhibits the reaction and the isotope effect indicates that the rate determining step of the reaction corresponds to the water-catalyzed nucleophilic attack of water on the carbonyl group of Bz2O. Conversely, whereas inhibition is observed at pH 6.0, catalysis of the hydroxide ion reaction is observed at pH 8.0 and it is found that the activation entropy is responsible for the catalytic phenomena in the basic hydrolysis of benzoic anhydride. Copyright © 2004 John Wiley & Sons, Ltd.

The formation of inclusion complexes with beta-cyclodextrin was studied for several popular fungicides of different types: prochloraz, 2-phenylphenol, thiophanate methyl, 8-hydroxyquinoline, and benalaxyl. Phase solubility diagrams showed that in all cases complexation takes place, leading to an important increase of water solubility in prochloraz and benalaxyl. Equilibrium association constants could be determined from the phase solubility data and from NMR titrations in the case of 2-phenylphenol. Because of the low solubility of the complex formed between 8-hydroxyquinoline and beta-cyclodextrin, the corresponding association constant could not be determined. The solid complexes of fungicide-cyclodextrin were prepared and isolated by different methods. The isolation of real complexes and not physical mixtures was confirmed in the cases of prochloraz, 2-phenylphenol, and benalaxyl by differential scanning calorimetry.

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

International audience; Cyclodextrin-adamantane conjugates have been prepared and their ability to form supramolecular polymers was tested. It appears that they are either insoluble when the link is too rigid or they form self-included derivatives inhibiting the formation of the polymer.

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.

The importance of possible electrostatic effects in the complexation of cyclodextrins has been investigated. The complexation features of the anionic guest molecule, 2-(p-toluidinyl)naphthalene-6-sulfonate (TNS), with the positively charged cyclodextrin, 6-deoxy-6-amino-β-cyclodextrin (NH3+CD), and with the neutral β-cyclodextrin (β-CD) were compared. Although TNS is a widely used fluorescent probe for the studies of complexation with cyclodextrins, its behavior is still not completely understood. In this paper we show that the complexation of TNS with β-CD involves the simultaneous formation of two different 1:1 complexes, resulting from inclusion of the toluidin moiety or the naphthalenesulfonate moiety in a cyclodextrin unit, and a 1:2 complex, which is due to the complexation of the two aromatic moieties of TNS with two cyclodextrin molecules. In the complexation of TNS with NH3+CD, the same species and equilibria are present, although significant structural differences of their complexes and variatio...

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.

The curing reaction of a system consisting of a diglycidyl ether of bisphenol-A (n = 0) and hemin (a protoporphyrin IX containing an iron ion and an additional chloride ligand) was studied with a differential scanning calorimeter. A maximum value of -488.3 ± 8.4 J g-1 was obtained for the enthalpy of the reaction. The kinetics of the process was studied by the isothermal method, observing that it obeys to Kamal's model, with an overall reaction order equal to 3. From the dependence of the kinetic constant with temperature, the activation energy, activation enthalpy, and activation entropy were determined. The ratio of the kinetic constants associated to the autocatalytic and nth order terms of the reaction rate, together with the thermodynamic activation parameters suggest a trend to the autocatalytic path mechanism with increasing temperatures. This study demonstrates that macrocycles can be used as cross-linking agents for curing epoxy resins and that when metallomacrocycles are used, metal ions can be introduced into the network structure. Copyright © 2013 Wiley Periodicals, Inc.

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.