Article pubs.acs.org/JPCB
Manipulation of the Gel Behavior of Biological Surfactant Sodium Deoxycholate by Amino Acids Xiaofeng Sun,†,‡,§ Xia Xin,*,†,‡ Na Tang,† Liwen Guo,‡ Lin Wang,‡ and Guiying Xu*,†,‡ †
National Engineering Technology Research Center for Colloidal Materials, Shandong University, Shanda nanlu No. 27, Jinan 250100, P. R. China ‡ Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, Shanda nanlu No. 27, Jinan 250100, P. R. China § China Research Institute of Daily Chemical Industry, Wenyuan Road No. 34, Taiyuan, Shanxi 030001, P. R. China S Supporting Information *
ABSTRACT: Supramolecular hydrogels were prepared in the mixtures of biological surfactant sodium deoxycholate (NaDC) and halide salts (NaCl and NaBr) in sodium phosphate buffer. It is very interesting that with the addition of two kinds of amino acids (L-lysine and L-arginine) to NaDC/NaX hydrogels, the gel becomes solution at room temperature. We characterized this performance through phase behavior observation, transmission electron microscopy, scanning electron microscopy, X-ray powder diffraction, Fourier transform infrared spectra, and rheological measurements. The results demonstrate that the gels are formed by intertwined fibrils, which are induced by enormous cycles of NaDC molecules driven by comprehensive noncovalent interactions, especially the hydrogen bonds. Our conclusion is that the presence of halide salts (NaCl and NaBr) enhances the formation of the gels, while the addition of amino acids (L-lysine and L-arginine) could make the breakage of the hydrogen bonds and weaken the formation of the gels. Moreover, its fast disassembly in the presence of amino acids allows for the release of substances (i.e., the dye methylene blue) entrapped within the gel network. The tunable gel morphology, microstructure, mechanical strength, and anisotropy verify the role of halide salts and amino acids in altering the properties of the gels, which can probably be exploited for a variety of applications in future.
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INTRODUCTION Supramolecular self-assembly, which is a branch of smart chemistry, focuses on chemical systems made up of a discrete number of assembled molecular subunits or components.1 Among them, the development of self-assembling small molecular hydrogels has received considerable attention in soft-material research due to their potential applications in cosmetics, biomaterials, sensors, drug delivery, stimuli-responsive materials, and pharmaceutical formulations.2−4 Of particular importance are those hydrogels made from biocompatible fragments because they can be safely employed in biomedical applications,5,6 such as amino acid derivatives,7 cholic acid derivatives,8 carbohydrate systems,9 and peptides.10,11 In particular, bile acids or their salts, which belong to cholic acid derivatives, possess an amphiphilic structure with a steroidal backbone that is more complex than alkane surfactants. Thus, they were reported to generate special aggregation properties12−14 and have a number of biological functions such as solubilization of cholesterol, absorption of dietary fats and fat soluble vitamins, and removal of pancreatic hydrolysis products such as fatty acids.15 Besides the increasing number of studies on their physiological importance, bile salts including sodium cholate (NaC), sodium deoxycholate (NaDC), and sodium lithocholate (SLC) can spontaneously © 2014 American Chemical Society
self-assemble into gels in water driven by van der Waals, Hbonding, as well as hydrophobic effect.16−18 This is quite different from the polymeric gels, which are primarily formed due to chemical cross-linking.19 In the past few years, there has been immense interest in creating smart-materials, that is, materials that can change their properties in response to external stimuli such as light, temperature, pH, or biological targets. Great attention has been paid to the design of supramolecular hydrogels sensitive to chemical stimuli.20 The on−off switching of the selfassembly process can be used conveniently for the detection of a change in the environment (pH, concentration) as well as for the release of an entrapped component (drug, enzyme, single crystal).21,22 In this sense, drug-delivery systems based on hydrogels have been described that respond to the presence of a given amino acid or enzyme that triggers the disassembly process and release of a drug entrapped.23−25 Thus, one of the advantages of supramolecular hydrogels is that the assembled structure and its function can be efficiently controlled by appropriate design of the small component molecules.26 We Received: September 26, 2013 Revised: January 2, 2014 Published: January 6, 2014 824
dx.doi.org/10.1021/jp409626s | J. Phys. Chem. B 2014, 118, 824−832
The Journal of Physical Chemistry B
Article
Figure 1. Structures of NaDC, L-Lys, and L-Arg. The isoelectric point (pI) of the amino acids is also included.
sample solutions with desired concentrations of each component was then prepared by mixing different amounts of each stock solution to a final volume of 4 mL. The solutions were mildly stirred at room temperature until all components were evenly mixed. Then, the samples were equilibrated at 20.0 ± 0.1 °C for at least 4 weeks before the phase behavior was inspected. Release Study of the Dye Methylene Blue. One mL of the dye-methylene-blue (0.2 mM)-loaded NaDC (200 mM) hydrogels was prepared, and 3 mL of L-Arg solution (200 mM) was added on top of the gel phase. After 6 h, the system turned into a solution (100% release), but if the L-Arg solution was replaced with pure water, the hydrogels were not affected macroscopically even after 2 days. This result suggested that in the presence of water there was no release of the dye (
