Structures And Properties Of Aggregates Formed By Biological Micromolecules With Anionic Surfactants In Aqueous Solution

Self-assembly is the spontaneous organization of components into ordered patterns or structures via non-covalent interactions without human intervention. The self-assembled structures including micelles, vesicles, sponge structures, and fibers etc. were observed in mixed surfactant systems. These aggregates have attracted extensive research interests over the past decades due to their relevance to biological systems. These self-assembled structures is demonstrated to give rise to fascinating hydrogel, which possess potential applications in many fields, such as, nanotechnology, environment and biomedicine science. Herein, we designed systems based on anion surfactants mixed with biological micromolecule. The phase behaviors, microstructures, rheological properties and some of the applications of these systems were investigated. The contents of this doctoral dissertation are divided into five chapters:In Chapter â… , the concepts of self-assembly, the basic knowledge of surfactant physical chemistry, and the aggregation behavior of mixed surfactant systems were introduced. Especially, when anionic surfactants mixed with organic compounds with low molecular weights, the chain length of which is about 6 is introduced in detail. The self-assembled structures in these systems could further form gels. The classification of gels and how to design gelators, and the application hydrogels in dye and toxic waste adsorption, molecular recognition, materials synthesis, etc. were reviewed in detail. The objective and the scientific significance of this doctoral dissertation are also pointed out at the end of this part.In Chapter â…¡, Uni-lamellar and multi-lamellar vesicles were prepared by the enantiomers of a biological molecule, L-lysine or D-lysine, with a double-tail weak monoacid, di-(2-ethylhexyl) phosphoric acid (abbreviated as DEHPA), in water. With the addition of DEHPA to lysine aqueous solutions, ion-pairs can form through the acid-base reaction between the lysine cations and DEHP" anions. The self-assembled vesicles were proved to be driven by the hydrogen bonding between the side-chain amino group in lysine molecules and the polar group of DEHP" species. The combination of the polar groups of DEHPA and lysine through electrostatic interaction and hydrogen bonding reduces the cross sectional area of the hydrophilic group, improving the surface activity and inducing the microstructure transition from primitive aggregates to micelles, and to vesicles in solutions. Due to the chirality of the lysine molecules, the aggregates exhibited diverse chiral properties with the transition of the microstructures.In Chapter III, aggregation behaviors of alkaline amino acids, lysine, arginine (Arg) and histidine (His), and fatty acids (FAs) with different chain lengths in aqueous solutions were investigated. The self-assembled structures, rheological properties of samples were determined in detail. (1) In lysine/FAs/H2O system, aggregates including micelles, vesicles, sponge structures, and fibers were observed by varying the compositions and chain length of fatty acids. The sponge phase was determined in mixtures of octanoic acid and lysine by freeze fracture-transmission electron microscope (FF-TEM). Circular dichroism (CD) signals were clearly detected in the self-assembled structures because of the chirality of lysine molecules. The rheological properties of samples consisting of different aggregates of lysine-fatty acids mixtures were measured, which provide the controlling factor of chain length for the rheologies. The combination effect of non-covalent interactions including electrostatic interactions, hydrogen bonding, and hydrophobicity is supposed to be responsible for the aggregation behaviors, in which the hydrogen bonding acts as the main driving force in the self-assembled process. (2) In Arg/FAs/H2O system, different self-assembled structures were investigated in detail. Furthermore, the effect of three alkaline amino acids, lysine, arginine, and histidine, on the aggregation behavior of OA in water was compared. It was found that the phase behaviors of the lysine/OA/H2O and Arg/O A/H2O are different from His/OA/H2O system, which we ascribed to the difference in charge states of their side chains in same pH condition, when they are at the same concentration. Emulsions were successfully produced at room temperature with the tunable stability which can be modulated by the nature and proportion of fatty acid.In Chapter IV, the gelation and crystallization behavior of a biological surfactant, sodium deoxycholate (NaDC), mixed with L-taric acid (L-TA) in water are described in detail. With the variation of molar ratio of L-TA to NaDC (r=nL-TA/nNaDc) and total concentration of the mixtures, the transition from sol to gel was observed. SEM images showed that the density of nanofibers gradually increases over the sol-gel transition. The microstructures of the hydrogels are three-dimensional networks of densely packed nanofibers with lengths extending to several micrometers. Several days after preparation, regular crystallized nanospheres formed along the length of the nanofibers, which was typical among the transparent hydrogels induced by organic acids with pKa1 value< 3.4. Small angle X-ray diffraction demonstrated differences in the molecular packing between transparent and turbid gels, indicating a variable hydrogen bond mode between NaDC molecules.In Chapter V, the gelation behavior of sodium deoxycholate (NaDC), mixed with glutathione (GSH) in water is described in detail. At a fixed concentration of NaDC, the self-assembled structure transition of the hydrogels from nanofibers, to nanohelix structures, then to twist ribbons is clearly observed with the increase of GSH concentration. The CD and FT-IR spectroscopy were employed to examine the optical activity of the assembled structures in solution, and the various types of peptide secondary structure. The difference of molecular packing in these structures is demonstrated by small angle X-ray diffraction. It is found that the mechanical strength of the hydrogels is enhanced with the addition of AgNO3. Controlled growth of Ag nanoparticles at spatially arranged along the length of the nanofibers or helically on nanohelix is readily achieved by UV reduction of Ag(I) ions on the supramolecular templates.