Studieson Thesynthesis Of Quaternary Ammoniumcation Icsurfactants And Their Interaction With Themodelproteins

Surfactants and protein are very common in nature, and both of them are amphipathic molecules, thus they are easy to interact with each other. The complex formed by surfactants and proteins have been widely used in food, medicine, industry and cosmetic etc, and it is very significant to study the interactions between surfactants and proteins.Six kinds of quaternary ammonium surfactants with different alky chain lengths were synthesized. They include N-dodecy1-N-(2-hydroxyethyl)-N,N-dimethyl ammonium bromide(DHDAB), N-dodecyl-N,N-di(2-hydroxyethyl)-N-methyl ammonium bromide (DDHAB), N-tetradecyl-N-(2-hydroxyethyl)-N,N-dimethyl ammonium bromide(THDAB), N-tetradecyl-N,N-di(2-hydroxyethyl)-N-methyl ammonium bromide(TDHAB), N-cetyl-N-(2-hydroxyethyl)-N,N-dimethyl ammonium bromide(CHDAB) and N-cetyl-N,N-di(2-hydroxyethyl)-N-methylammonium bromide(CDHAB). The reaction temperature was controlled at 62℃. Relative to alkyl bromide, alcohol amine should be excessive. The solvent used here was acetone, and whole reaction time was about 6 hours. The products were characterized by IR, NMR and EA, and the thermostability was analyzed by TGA. It turns out that the stable surfactants have longer alkyl chains and more hydroxyethyl groups.Electrical conductivity measurement was applied to determined the critical micelle concentration (CMC) of six kinds of surfactants at different temperature (298、303、308、313、318 and 323 K), and surface tension was also used to determine the CMC of six kinds of surfactants at 298 K. Moreover, the CMC of six kinds of surfactants in Tris-HCl buffer solution (pH = 7.0, contain 0.10 mol/L NaCl) was measured by isothermal titration calorimetry (ITC) at 298 K. The results showed that electrical conductivity, surface tension and ITC were the effective methods for CMC measurement. The CMC of surfactants in buffer solution was smaller than that in aqueous solution, which illustrates that buffer solution could decrease the CMC of surfactants remarkably. The CMC of the six surfactants follow the order: CHDAB> CDHAB> THDAB> TDHAB> DHDAB> DDHAB. It states that the surfactant with longer alkyl chain and more hydroxyethyl groups have smaller CMC value. Furthermore, by comparing the CMC of surfactants at different temperatures and salt concentrations, it could get the conclusions that the CMC of surfactants increases with increase of temperature, and the CMC of surfactant decreases with increase of salt concentration.The interactions between six kinds of surfactants and bovine serum albumin (BSA) were investigated by UV-Vis, fluorescence spectrometry, ITC and determination of particle size and Zeta potential. The conclusions were as follow:(1) By using UV-Vis, it was found that surfactants interact with BSA to form the complexes. (2) In fluorescence spectrometry, Stern-Volmer equation was utilized to discuss the interactions between surfactants and BSA at low concentration of surfactant solutions, and results showed that surfactants with longer alkyl chain and more hydroxyethyl groups could facilitate the interactions. The fluorescence quenching of BSA was caused by static quenching; by using synchronous fluorescence spectrometry, it can be found that the microenvironment hydrophobicity nearby BSA tryptophan residues was strengthened with the increase of the concentration of surfactant solution. The conformation of BSA has been slightly changed after the interaction. Furthermore, the fluorescence intensity of tryptophan residues declined faster than the increase of Tyrosine fluorescence intensity, which means that the six surfactants primarilyinteracted with the tryptophan residues of BSA. (3) The ITC experiments turned out that the electrostatic interaction and hydrophobic interaction were the main interactions at low concentration of surfactant solution. The hydrophobic layer on the surface and secondary structure of BSA were damaged and changed when the temperature was increased, and it led to absorb more heat. Finally, when the enthalpy trends to be stable, it means that the interactions between BSA and surfactant were completely finished. (4) The particle size and Zeta potential results showed that the high concentration of surfactant solutions resulted in the destruction the structure of BSA, then it had very little impact on itsζ-Potential.The interactions between the six surfactants and lysozyme (LYS) were analyzed by UV-Vis, DLS, ITC and fluorescence spectrometry. The results are listed below:(1) In UV-Vis, six kinds of surfactants could interact with LYS to generate the complexes. (2) In fluorescence spectrometry, the results from Stern-Volmer equation showed that DHDAB and DDHAB did not interact with LYS at low concentration, while THDAB, TDHAB, CHDAB and CDHAB could interact with LYS strongly at low concentration. The fluorescence enhancement was ascribed to the static enhancement by forming the complexes. Surfactants with longer alkyl chain and more hydroxyethyl groups had stronger interactions with LYS. The results of synchronous fluorescence showed that BSA endogenous fluorescence mainly came from the tryptophan residues. The enhancement of emission spectrum was caused by the interactions between the six surfactants and the tryptophan residues in LYS molecules. (3) In ITC experiments, the interactions between surfactants and LYS were very strong when the concentration of surfactant solution was lower than CMC. When the concentration of surfactant solution was higher than CMC, the interactions were weak. (4) By particle size and Zeta potential, LYS was absorbed and packed by surfactants, and led to the increase of particle size; surfactant stopped influencingζ-Potential of LYS once the CMC was reached.