Article pubs.acs.org/Langmuir
Catanionic Gels Based on Cholic Acid Derivatives M. Chiara di Gregorio,† N. Viorel Pavel,† Javier Miragaya,‡ Aida Jover,‡ Francisco Meijide,‡ José Vázquez Tato,‡ Victor H. Soto Tellini,§ and Luciano Galantini*,† †
Dipartimento di Chimica, Università di Roma “La Sapienza”, P. le A. Moro 5, 00185 Roma, Italy Facultad de Ciencias, Universidad de Santiago de Compostela, Avenida Alfonso X El Sabio s/n, 27002 Lugo, Spain § Centro de Electroquímica y Energía Química, Escuela de Química, Universidad de Costa Rica, San Pedro de Montes de Oca, San José 2060, Costa Rica ‡
S Supporting Information *
ABSTRACT: 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.
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INTRODUCTION Bile acids are biological amphiphiles characterized by the presence of a rigid steroid backbone and a peculiar distribution of the hydrophobic and hydrophilic regions (Figure 1). A charged head represents the main hydrophilic group, which is attached by a short chain to the condensed ring system. Other
hydrophilic spots, constituted by hydroxyl groups, are located on this system, thus conferring upon it partially polar features. The hydrophobic−hydrophilic balance of bile acids is crucial in their biological activity1−3 and it has been invoked to account for micellar cholesterol-solubilizing capacities,4 bile acidinduced liver toxicity,5 composition of aggregates and precipitates in model bile,6 and early stages of cholesterol gallstone formation.7 Because of their peculiar amphiphilic structure, bile acids form very ordered aggregates with unconventional morphologies. Moreover, it has been recently shown that modifications of the bile acid molecular structure provide new surfactants showing interesting biological features8 and self-assembly behaviors.9−13 The behaviors are different with respect to those of natural bile acids and could expand the range of applications of steroid surfactants. For example, derivatives obtained by changing the carboxylic function into an ammonium group are often reported to be hydrogelators, as well as sodium deoxycholate,14 and show sometimes interesting antimicrobial activity.15−18 Among bile acid derivatives, molecules presenting hydrophobic aromatic substituents represent a particularly interesting group since we noticed that they show uncommon features particularly attractive for Received: January 18, 2013 Revised: September 7, 2013 Published: September 9, 2013
Figure 1. Molecular structures of investigated derivatives and their precursor. © 2013 American Chemical Society
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dx.doi.org/10.1021/la402602d | Langmuir 2013, 29, 12342−12351
Langmuir
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applications. In particular, derivatives showing thermoresponsive aggregation18−20 or forming surprisingly narrow nanotube gels21 have been obtained by specific substitutions of one hydroxyl group on the rigid four-ring system. Mixtures of anionic and cationic forms of some of these aromatic substituted derivatives have been recently used to prepare bile salt-based catanionic surfactants. For this system, the formation of tubules in the whole range of anionic/cationic surfactant molar ratio was observed. The tubule charge was tuned by controlling the mixture surfactant stoichiometry.22 Mixtures of anionic and cationic surfactants in water have been a fascinating subject of extensive investigations for many years.23−26 These mixtures exhibit interesting phase behavior and properties, which mainly arise from electrostatic interactions between the oppositely charged head groups. By adjusting the composition, these interactions can be tuned to produce various microstructures with characteristic geometries. Normally, catanionic mixtures are prepared with the counterions of the two surfactants remaining in solution as a dissociated salt. In these systems, at low concentrations of the catanionic components, vesicles and micelles are generally formed, accompanied by two-phase regions and precipitates around the equimolar composition.27,28 The behavior shown by the bile salt-based mixtures has never been observed before and suggests that bile acid derivatives, because of their rigidity and their unconventional distribution of hydrophobic and hydrophilic moieties, allow the preparation of catanionic systems presenting uncommon properties that could enhance their already wide range of applications. With this background, in this work two new aromatic substituted derivatives of sodium cholate were synthesized for the preparation of catanionic mixtures. In order to start to collect some information on the effect of substitutions in different positions of the molecule, the aromatic residue was introduced at the end of the side chain. The derivatives are constituted by diamine and dicarboxylic cholic derivatives (DACD and DCCD, respectively) shown in Figure 1, which can provide cationic (DACD) and anionic (DCCD) surfactants in water. The data so far reported in literature on modified steroid surfactants are mainly related to derivatives containing a single anionic or cationic head per steroid skeleton. In some cases single charged derivatives containing more than a steroid moiety are also reported. In any case, molecules with a ionizable groups/steroid skeleton ratio ≤1 have been generally studied. In addition to an unusual position of the aromatic substituent, the molecules of this work show as a further novelty the presence of two ionizable groups and a single steroid moiety, thus increasing to a value of 2 the abovementioned ratio. The study started with the analysis of the pure surfactants by surface tension, UV absorption, and circular dichroism (CD) measurements aimed at getting information on critical aggregation concentration (cac) and supramolecular packing of the aggregates. The aggregate morphologies were studied by static and dynamic light scattering (SLS and DLS), small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM). The effects of some of the most common parameters such as pH, temperature, and concentration were considered in the analysis. Finally, catanionic mixtures were prepared by mixing the two derivatives in different proportions and characterized by CD, TEM, and rheological measurements.
Article
MATERIALS AND METHODS
Synthesis of the Dimethyl Ester of Dicarboxylic Cholic Derivative (ID). Cholic acid (0.01 mol, 4.1g) and tri-n-butylamine (0.01 mol, 2.4 mL) were dissolved in 30 mL of dioxane. The solution was cooled to 8 °C and ethyl chloroformate (0.015 mol, 1 mL) was added. The solution was stirred for 15 min and 5-amino-iso-phthalic acid dimethyl ester (0.01 mol, 2.09 g; prepared according to Vogel et al.29) dissolved in 5 mL of dioxane was added. The solution was stirred for 20 min at this temperature and for 1 h at room temperature. The solvent was removed under vacuum and the crude was purified by column chromatography with methanol/ethyl acetate (9:1) as eluent. Yield 68%. 1 H NMR (DMSO/CDCl3 mixture; 300 MHz, δ/ppm) 10.12 (s, NH−C24=O), 8.45 (s, HAr), 8.12 (s, HAr), 4.04−3.18 (m, H3, H7, and H12), 3.88 (H methyl ester), 2.5−1 (steroid skeleton and lateral chain), 0.98 (d,, H21), 0.895 (H1β), 0.80 (s, H19), 0.59 (s, H18). 13C NMR (DMSO/CDCl3; 75 MHz, δ/ppm) 173.07 (CH3−O−C=O), 166.51 (NH−C24=O), 140.07−124.28 (CAr), 71.88 (C12), 71.23 (C3), 67.08 (C7), 52.99−23.48 (CH3−O−C=O, steroid skeleton, and lateral chain), 23.26 (C19), 17.90 (C21), 13.041 (C18). Synthesis of Dicarboxylic Cholic Derivative. ID (0.01 mol, 5.998 g) was dissolved in a solution of 3 g of KOH in 10 mL of methanol. The solution was stirred at reflux for 1 h. This solution was dissolved in 250 mL of water and neutralized with HCl. A solid (DCCD) is formed and separated by filtration. The product was dried at 60 °C for 24 h. Yield 91%. 1 H NMR (DMSO/CDCl3 mixture; 300 MHz, δ/ppm) 9.69 (s, NH−C24=O), 8.38 (s, HAr), 8.22 (s, HAr), 4−3.2 (m, H3, H7, and H12), 2.5−1 (steroid skeleton and lateral chain), 0.95 (d, H21), 0.78 (s, H19), 0.58 (s, H18). 13C NMR (DMSO, CDCl3; 75 MHz, δ/ppm) 173.04 (HO−C=O), 167.55 (NH−C24=O), 140.03−124.76 (CAr), 72.66 (C12), 71.62 (C3), 67.89 (C7), 46.97−23.41 (steroid skeleton and lateral chain), 22.86 (C19), 17.69 (C21), 12.83 (C18). MALDI-TOF (m/z) [M − 3OH], 519.13; [M − 2OH], 536.77; [M + Na]+, 594.47; [M + K]+, 610.43. Theoretical: [M − 3OH], 519.30; [M − 2OH], 537.31; [M + Na]+, 594.30; [M + K]+, 610.28. Synthesis of Diamine Cholic Derivatives. ID (8.35 × 10−4 mol, 0.5 g) was dissolved in 5 mL of ethylenediamine and stirred at 65 °C for 8 h. The solvent was removed under vacuum and the crude product was washed with methanol and dichloromethane several times to remove the unreacted ethylenediamine. The crude product was dissolved in the minimum amount of methanol and precipitated by adding small amounts of ethyl acetate. The formed crystals contain
