Interactions Between Quaternary Cationic Surfactants And Bovine Serum Albumin

In this thesis, the interactions between N-dodecyl-N-(2-hydroxyethyl)-N, N-dimethyl ammonium bromide (DHDAB), N-dodecyl-N,N-di(2-hydroxyehtyl)-N-methyl ammonium bromide (DDHAB), N-tetradecyl-N-(2-hydroxyethyl)-N, N-dimethyl ammonium bromide (THDAB), N-tetradecyl-N,N-di(2-hydroxyehtyl)-N-methyl ammonium bromide (TDHAB),N-cetyl-N-(2-hydroxyethyl)-N,N-dimethyl ammonium bromide (CHDAB) and N-cetyl-N,N-di(2-hydroxyehtyl)-N-methyl ammonium bromide (CDHAB)and bovine serum albumin (BSA) are studied by the measurements of isothermal titration calorimetry and fluorescence spectroscopy.The critical micelle concentration (CMC) of these surfactants in Tris-HCl buffer solution and the interaction enthalpies of BSA with the surfactants have been determined by isothermal titration calorimetry (ITC). The influences of temperature, NaCl and hydrophilic headgroups on the interaction of DDHAB with BSA have been investigated in detail. The ITC results show that with increasing concentration of surfactant the interaction enthalpy curve of BSA exhibits two peaks before and after the critical micelle concentration. With the growth of temperature the first endothermic peak position remain unchanged and the second peak changed from an exothermic one to an endothermic one. With the addition of NaCl, the CMC value as well as the first endothermic value increases, while the second endothermic peak position moves toward the lower concentration of the surfactant. With the increase of the number of hydroxyethyl of the surfactants, the first endothermic peak value decreases, while the second one increases and its position moves toward the lower concentration of the surfactant. These changes indicate that the hydrophobic interaction changes with varying the temperature, while the electrostatic interaction varys with the addition of NaCl in two aspects, the electrostatic repulsive force would decrease or the electrostatic attraction force would enhance under a proper concentration range of NaCl. The curves of interaction enthalpy-concentration of a surfactant between BSA and THDAB/TDHAB/CTAB/CHDAB/CDHAB exhibit two or three peaks, which are quite different from that of BSA with DDHAB. It is the difference of CMC values that leads to the different extent of electrostatic interaction, dehydration, hydrophobic interaction and surfactant micellization. As a result, different peak images are obtained.Fluorescence spectroscopy has been used to investigate the effects of carbon chain length and hydrophilic group on relative fluorescence intensity value (Io/I). The fluorescence quenching mechanism at low surfactant concentrations has been discussed with the Stern-Volmer equation. The changes of system microenvironment are studied by the synchronous spectroscopy. The fluorescence results show that the fluorescence intensity (I) of BSA decreases with the increase of the surfactant concentration, coupled with the blue shift of the maximum wavelength. The value of Io/I increases significantly with the growth of carbon chain length, but increases slightly as the number of hydrophilic group increasing. The value of dynamic quenching constant (Ksv) increases with increasing of carbon chain length, temperature and hydrophilic groups, which indicates that the interaction between surfactant and BSA becomes stronger. The main quenching procedure is ascribed to dynamic quenching. The positive values of BSA-surfactant interaction enthalpy change (AH) and entropy change (AS) represent that the hydrophobic interaction plays a key role in the interaction of BSA with the surfactants. It can also be seen from the synchronous spectra that the surfactant mainly interacts with the tryptophan residues compared to tyrosine residues and the conformation of BSA induced by surfactants.