Effect Of Additives On Aggregation Behaviors Of Sodium Deoxycholate

Bile salts are important biosurfactants, which exist in the living bodies of vertebrates, and often act as solubilizing or emulsifying agents for absorption of dietary lipids, or as gallstone solubilizing agents in clinical medicine. All of them possess a rigid steroid backbone having polar hydroxyl groups on the concave a-face and methylene groups on the convex?-face. This arrangement creates unique physiochemical properties for such a class of molecules, being different from those of conventional surfactants with a linear hydrocarbon chain. It is of great importance that the aggregation behavior of bile salts should be investigated.To our knowledge, the effect of additives on the aggregation behavior of the traditional surfactant, such as CTAB, SDS and Triton X-100, has been paid much more attention. However, details on the interactions between bile salts and additives have not been investigated deeply up to now. In this thesis, sodium deoxycholate (NaDC) is employed to study the effect of additives on its aggregation behavior. Surface tension, dilational viscoelasticity, molecular dynamic simulation and steady-state fluorescence spectra of pyrene of NaDC aqueous solution are investigated in the absence and presence of three kinds of alkaline amino acids, namely L-Lysine (L-Lys), L-Arginine (L-Arg) and L-Histidine (L-His), in order to study the aggregation behaviors of NaDC at the air/water surface and in aqueous solution. The effect of amino acids on the solubility of fenofibrate in NaDC solution was studied using UV-vis spectrum method. The rheological properties of NaDC hydrogel as a function of concentration of NaDC and salts were investigeted by rheological measurements. This research may open an avenue to gain a better knowledge about the behavior of such systems in the biological environment. This thesis is divided into five parts.In the first section, the properties of bile salt, the study progress on the aggregation behaviors of bile salts and the effect of additives on the aggregation bahavior of NaDC are summarized. In the second section, the aggregation behaviors of NaDC at air/water surface were investigated via surface tension and oscillating bubble measurements in the absence and presence of three alkaline amino acids. The results of surface tension show that NaDC has lower ability in reducing surface tension of water, because NaDC molecules orient at the surface with an oblique direction and tend to aggregate together, which is approved by the molecular dynamic simulation. L-Lys is the most efficient one among three amino acids in reducing critical aggregation concentration (cac) of NaDC in aqueous solution. The influence of amino acids on the dilational rheological properties of NaDC was studied using the drop shape analysis method in the frequency range from 0.02 to 0.5 Hz. The results reveal that the absolute modulus passes through a maximum value with increasing NaDC concentration. The addition of amino acids increases the absolute modulus of NaDC solution and the maximum value is observed at much lower concentration. From the perspective of structures of amino acids, the performance of L-Arg is similar to that of L-His, and both of them bring out smaller effect on the absolute modulus than that of L-Lys. From the above results, it may be presumed that electrostatic and hydrophobic effects are important impetus during the interaction between amino acids and NaDC at air/water surface. Hydrogen bonding is so ubiquitous in the system that the difference of hydrogen bonding between NaDC and amino acid is ignored.In the third section, the influence of three alkaline amino acids on the aggregation behaviors of NaDC in phosphate buffer (pH=7.0) was studied at 25?. The fluorescence probe technique of pyrene was employed to determine accurately the cac, polarity of microenvironment and aggregation numbers for NaDC aggregate. The added amino acids can effectively reduce the cac values and micropolarity of NaDC, indicating that it is easier for NaDC to aggregate together and lie compactly in the presence of amino acids. Meanwhile, the aggregation numbers of NaDC are increased evidently, meaning that more NaDC molecules connect together to form stable aggregates. It is worth mentioning that the performance of L-Arg is similar to that of L-His, and both of them bring out smaller effect on the above parameters than L-Lys. In view of this, it may be inferred that both electrostatic and hydrophobic interaction are responsible for the interaction between NaDC and amino acids in aqueous solution.In the fourth section, the ability of NaDC to solubilize fenofibrate in the absence and presence of amino acids was studied using UV-vis spectrum method. The solubility of fenofibrate is increased with increasing NaDC's concentration and temperature. At fixed NaDC concentration, the enthalpy of solution,?SolHm can be calculated from the slopes of the Van't Hoff plots. The characteristics of transfer enthalpy and entropy for fenofibrate from buffer to NaDC solution reveal that,?trH and T?trS decrease in the negative region with increasing concentration of NaDC. In the negative region, drug dissolution is enthalpy favorable and entropy unfavorable, and enthalpy effect is overcomes the entropy effect. The solubility of fenofibrate is increased with increasing concentration of amino acids. From the perspective of structures of amino acids, the performance of L-Arg is similar to that of L-His, and both of them bring out smaller effect on the solubility of fenofibrate than that of L-Lys. From the above results, it may be presumed that electrostatic, hydrophobic effects and hydrogen bonding are important impetus during the solubilization of fenofobrate in NaDC/amino acid solution. Moreover, the micropolarity, cac values and aggregation numbers of NaDC in the absence and presence of amino acids can be employed to interpret the solubilization process.In the fifth section, the rheological behavior of NaDC hydrogel was investigated in the presence of sodium halide, including NaCl, NaBr and NaI. The added sodium salts is favorable to the growth of NaDC aggregates in a direction and thus the entangled network is formed due to the interaction between NaDC and sodium halide. However, other inorganic salts, such as KCl, MaCl2, CaCl2 and AlCl3 are impossible to induce the formation of NaDC hydrogel. With increasing concentration of NaDC, three parameters, including the viscosity, elastic modulus and retardation time, increase remarkably. It can be inferred that the increasing NaDC molecules are favorable to the formation of network.These parameters first incerase and then decrease slowly with increasing concentration of sodium salts. At a low salt content, the added salt can effectively screen the elactrostatic interaction among hydrophilic groups of NaDC and thus factor the formation of network. Addition of more salt makes excessive aggregation and eventually the gelator molecules start to escape from the gel network by precipitation. Among three kinds of sodium halide, NaCl is most effective in improving the strength of NaDC hydrogel, while Nal bring out smaller effect than those of NaCl and NaBr. This sequence is in agreement with the lyotropic series, indicating that the ion with samller radius has stronger hydration effect and thus the interaction between it and water is much stronger. Undoubtedly, the NaDC molecules are easy to aggregate in the presence of smaller ions.