Aggregation Behavior Of Imidazolium Ionic Liquids And Interactions Between Ionic Liquids And Proteins

Ionic liquids (ILs) are a class of green solvents. Recently, ILs with various types and functions have been synthesized, and used in various fields. Protein is an indispensable material in living body, which plays an important role in life process. Protein/surfactant system has wide applications in pharmaceutics, cosmetic, biological systems, and food products. The studies of interactions between protein and surfactant may help people to understand the importance of hydrophilic-hydrophobic and other weak interactions in the biological macromolecules solution. The studies may reveal the secrets of life processes.The interaction between protein and surfactant is mainly focus on the traditional surfactant such as CTAB, SDS, nonionic surfactant, and block copolymer, less attention has been paid to protein/surface active ILs. The long-chain imidazolium IL is a new class of amphiphilic molecules, has many unique properties. Thus, it is expected to exhibit quite different interaction behavior with protein from tranditional surfactant. In this thesis, the aggregation behavior of imidazolium ILs, CnmimBr (n=12,14,16), is studied firstly. Then, the interactions of bovine serum albumin (BSA)/CnmimBr system, lysozyme/CnmimBr system, andβ-lactoglobulin (β-Lg)/CnmimBr system are investigated by surface tension, electrical conductivity, fluorescence spectra, circular dichroism, and isothermal titration microcalorimetry. This thesis is divided into four parts.1. The long-chain imidazolium ILs, C12mimBr, C14mimBr, and C16mimBr can form micelles in water. The hydrocarbon chain affects the micelle formation, similar to conventional ionic surfactants. With the increase of the hydrocarbon chain, the hydrophobicity strengthens, which is beneficial to the micelle formation, and thus the values of CMC become small. Compared with conventional cationic surfactants, alkyltrimethylammonium bromide and alkylpyridinium bromides, the surface activity of CnmimBr is higher, e.g., smaller CMC. Self-association of CnmimBr is studied by surface tension measurements over a temperature range. Thermodynamic parameters of micellization and the enthalpy-entropy compensation temperature are determined. The process of micellization is entropy-driven. The enthalpy-entropy compensation study confirms the effect of hydrocarbon chain length on micellization, i.e., the longer the hydrocarbon chain, the easier it is to form aggregates. Isothermal titration microcalorimetry measurement shows that C14mimBr and C16mimBr act as ideal surfactants in the micellization process because there are no solute-solute interactions whereas C12mimBr does not behave ideally because of solute-solute interactions in the high concentration solutions.2. The interaction between BSA and CnmimBr (n=12,14,16) is investigated by surface tension, electrical conductivity, fluorescence spectra, circular dichroism, and isothermal titration microcalorimetry. The combination occurs by electrostatic interaction at low CnmimBr concentration and by hydrophobic interaction at high CnmimBr concentration. CnmimBr stabilizes the secondary structure of BSA at low concentration but destroys the secondary structure of BSA at high concentration. BSA can combine with C14mimBr per gram of about 12 mol, and the binding capacity is slightly different with different temperature. The fluorescence quenching of tryptophan residue by CnmimBr leds to the fluorescence quenching of BSA. The tryptophan residue at the surface of BSA is not quenched by CnmimBr, so the interaction between tryptophan residue and CnmimBr is hydrophobic interaction. Compared with traditional cationic surfactants, tetradecyltrimethylammonium bromide (TTAB), C14mimBr is superior in both protection and destruction of BSA.3. The interaction between lysozyme and CnmimBr (n=12,14,16) is investigated by surface tension, electrical conductivity, fluorescence spectra, circular dichroism, and isothermal titration microcalorimetry. The combination occurs by intensively hydrophobic interaction. Increasing the hydrocarbon chain of IL is beneficial to the interaction. The addition of C12mimBr and C14mimBr dose not cause the fracture of disulfide bond in lysozyme. However, C16mimBr can cause the fracture of disulfide bonds. CnmimBr destroys the p-sheet structure of lysozyme at low concentration, but dose not destroy the a-helix structure. The high concentration of CnmimBr causes denaturation of lysozyme ultimately. CnmimBr dose not interact with tryptophan residue, which is different from CnmimBr/BSA system. CnmimBr changs the intrinsic fluorescence intensity of lysozyme by changing the solubility of tryptophan residues.4. The interaction betweenβ-Lg and CnmimBr (n=12,14,16) is investigated by surface tension, electrical conductivity, fluorescence spectra, and circular dichroism. The combination occurs by intensively interaction. It is electrostatic interaction at low CnmimBr concentration and CnmimBr causes the fracture of one disulfide bond in (3-Lg. CnmimBr can interacts by electrostatic interaction with the more amino acid residues which are exposed after the fracture of one disulfide bond. It is hydrophobic interaction at high CnmimBr concentration and the interaction affects the values CMC to a great degree. The low concentration of CnmimBr can destroy the second structure ofβ-Lg. CnmimBr chang the intrinsic fluorescence intensity ofβ-Lg by changing the solubility of tryptophan residues, just like CnmimBr/lysozyme system.