Francisco Meijide

Complexation of Bile Salts by Natural Cyclodextrins

The complexation of three trihydroxy sodium bile salts--cholate, glycocholate, and taurocholate--and their three related sodium dihydroxy bile salts--deoxycholate, glycodeoxycholate, and taurodeoxycholate--by f -, g - and n -cyclodextrins ( f -CD, g -CD and n -CD), has been studied by using 1D and 2D-NMR techniques. Trihydroxy bile salts form 1:1 complexes with g -CD and n -CD, while dihydroxy bile salts form 1:2 (bile salt:cyclodextrin) complexes with g -CD and 1:1 complexes with n -CD. ROESY experiments stated that the side chain, ring D, and part of ring C of the steroid body of the bile salts, are included into the cavities of g -CD and n -CD, in 1:1 complexes. The A ring of the steroid body is included into the cavity of the second g -CD in the 1:2 complexes. The only structural difference between related bile salts is the existence or not of a hydroxyl group at C7. The bigger cavity of n -CD allows this region of bile salts to be located inside the cyclodextrin cavity and therefore n -CD does not discriminate between both types of bile salts. However, in trihydroxy bile salts, this region clearly remains outside the g -CD cavity. The absence of the C7 hydroxyl group enlarges the hydrophobic region of the steroid body, allowing a favourable interaction with the hydrophobic cavity of a second g -CD. It is concluded that g -CD molecularly recognises bile salts, distinguishing di- from trihydroxy ones, while n -CD does not. The steroid body of bile salts is too big to enter into the f -CD cavity and only an interaction between their side chain and f -CD is observed.

Catanionic Tubules with Tunable Charge

Thethree-dimensionalstructureswithnanoscopicdimensionsthat are yielded by the self-assembly of lipids and surfactantsare of particular interest for their applications in nano-technology.Intheseapplications,thepossibilityofcontrollingthe charge of the particles allows the regulation of funda-mental aspects, such as the ability of the particles to loadmolecules (drugs, DNA, proteins, etc.), to aggregate, and topenetrate membranes. Within the possible surfactant supra-molecular architectures, tubular structures have recentlydrawn much research interest.

Synthesis and Characterization of a New Gemini Surfactant Derived from 3α,12α-Dihydroxy-5β-cholan-24-amine (Steroid Residue) and Ethylenediamintetraacetic Acid (Spacer)

A new gemini steroid surfactant derived from 3alpha,12alpha-dihydroxy-5beta-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 microM, their density being 6.8 x 10(-4) g cm(-3). At high concentrations, the intensity ratio of the vibronic peaks of pyrene, I1/I3, (=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.

Rheological behaviour of an amide pectin

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.

Kinetics of formation of supramolecular tubules of a sodium cholate derivative

We report a kinetic study of the supramolecular tubule formation of the bile salt derivative [3β,5β,7α,12α]-3-(4-t-butylbenzoilamine)-7,12-dihydroxycholan-24-oic acid sodium salt (Na-tbutPhC). At high bicarbonate buffer concentration (pH∼10) this salt shows gelator properties. Starting from gels or viscous solutions, the tubule formation is triggered by increasing the temperature beyond the critical value of 34–36 °C. For gels, when the process takes place, the transition to sols occurs. The process is easily triggered and can be followed by several techniques. We used static light scattering (SLS), circular dichroism (CD), small angle X-ray scattering (SAXS) along with transmission electron (TEM) and optical microscopies. The CD results show that fibrils with a clockwise arrangement of the bile salt derivative are present in the samples at room temperature. When the tubule formation starts, evolutions of the CD and SLS profiles are observed indicating that the formation process begins with the aggregation of the fibrils accompanied by a simultaneous peculiar reciprocal reorientation of the surfactant molecules. After that, as pointed out by the long time evolution of the curves, a slow transformation towards the final well defined tubules occurs, involving an adjustment of the molecular packing. In the meanwhile, the slow ordering of the tubule walls in well spaced layers takes place, as inferred by SAXS. The TEM images show that short disordered tubules are formed, because of the aggregation of fibrils, in the beginning. Moreover they highlight a final elongation of the tubules taking place without a further aggregation of fibrils. Optical microscopy frames, collected during the process, point out that the tubules grow singly even at quite a high concentration, thus supporting the data interpretation.

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.

Formation of tubules by p-tert-butylphenylamide derivatives of chenodeoxycholic and ursodeoxycholic acids in aqueous solution.

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.

Spectra and structure of complexes formed by sodium fusidate and potassium helvolate with β- and γ-cyclodextrin

Abstract 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.

Complexation of 6-Deoxy-6-(aminoethyl)amino-β-cyclodextrin with Sodium Cholate and Sodium Deoxycholate

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 ’“Jobs method” has been used to determine the stoichiometry of these supramolecular complexes. The Jobs 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).

Characterization of carbon nanotube dispersions in solutions of bile salts and derivatives containing aromatic substituents.

Bile salts (BS) are known to solubilize high weight fractions of carbon nanotubes (CNTs) in aqueous solutions. Here, the efficiency of derivatives of bile salts (BSDs) containing aromatic substituents in dispersing single-wall CNTs (SWCNTs) has been investigated in order to check whether the presence of aromatic residues, because of their affinity toward carbon nanotube surfaces, determines improvements of the BS dispersion efficiency (DE). Electric arc and CoMoCAT SWCNTs were analyzed. The results, reported for the two surfactant concentrations of 0.06 and 1.0 wt %, show that the DE of BSDs depends on the position, orientation, and structure of the introduced aromatic residues. In the case of the CoMoCAT SWCNTs, at low surfactant concentration a DE improvement is observed in BSDs where the aromatic residue is linked either to carbon 3, located on the rigid four-ring system, or to the side chain. For the latter, this improvement is also enhanced in double-charge derivatives and kept at high surfactant concentration. It was also observed that at low concentrations of surfactant, the DE values of BSs and BSDs are usually larger than those of the more conventional detergent sodium dodecylsulfate.