Calorimetric Studies On Interactions Between Bio-Model Molecules And Surfactants

Most of proteins usually coexist in the complicated aqueous environment with many different substances. Studies on the thermodynamics of proteins in the different solutions are helpful to understand the interactions between proteins and different solvent molecules. Because of the complex structures of proteins, it is extremely difficult to directly investigate their thermodynamic properties. It is common to treat amino acids, peptides, and amides as model compounds of proteins. Investigations on the thermodynamic properties of model compounds in aqueous solution can provide information about the behavior of proteins in aqueous solution. Therefore, it is interesting to investigate the thermodynamics of model compounds in different aqueous solutions.Surfactants are extensively employed in pharmaceutical and biotechnological processes. The surfactant-protein interactions can modulate the functional properties of proteins. It is very important to investigate the origin and nature of surfactant-protein interactions both qualitatively and quantitatively.Microcalorimetry is an advanced technique in the modern thermodynamic and thermochemical researches. It has been extensively used in the thermodynamic research of life system containing multicomponents.The present work consists of the following foure parts:(1) The enthalpies of solution of five amino acids, glycine,l-alanine,l-valine, l-serine, and l-threonine, and two glycine peptides, diglycine, and triglycine, in aqueous surfactant solutions of sodium dodecyl sulphate(SDS), dodecyltrimethylammonium bromide(DTAB), N-dodecyl-hydroxyethyl-dimethyl ammonium bromide, N-tetradecyl-hydroxyethyl-dimethyl ammonium bromide, and N-cetyl-hydroxyethyl-dimethyl ammonium, were determined at 298.15 K by microcalorimetry. The transfer enthalpies of the amino acids and glycine peptides from water to aqueous surfactant solutions were obtained. The different changing phenomena of the transfer enthalpy, which is responding to the different concentrations of surfactant solutions, have been discussed. The effects of different side chains of amino acid molecules and the peptide bond of peptide molecules on the interactions between amino acid/peptide and surfactant molecules have been compared.It is found that the change tendencies of the transfer enthalpy are similar for the amino acids and the glycine peptides. At relatively low concentrations of the surfactant solutions, the amino acids and glycine peptides mainly interact with the head groups of the surfactant molecules. The main interactions come from dehydration and ion/hydrophilic-ion interaction between the zwitterions of amino acid or peptide bond of peptide and the head group of surfactant. With the increase of hydrophile and the number of peptide bond, the concentration range of hydration decreases. The contributions of the hydrophobic side chains of the amino acids to the transfer enthalpy are positive. In contrast, the contributions of the peptide bond of peptide are negative. When the amino acid and peptide molecules insert into the micelles and interact with the hydrophobic tail groups of the surfactant molecules, the influences of the hydrophobic side chains and the peptide bond on the transfer enthalpy are positive.(2) The enthalpies of solution of glycine,l-alanine,l-serine,l-threonine, diglycine, and triglycine in aqueous polyol solutions of mannitol, sorbitol, inositol, and xylitol, were determined at 298.15 K by calorimetry. Transfer enthalpies of amino acids and glycine peptides from water to polyol solutions were obtained. The influences of molecular structures on the transfer enthalpy have been analyzed.It has been observed that all of the values of the transfer enthalpy are negative and decrease with the increase of the concentrations of polyol in the aqueous solutions, which indicates that the ion/hydrophilic-hydrophilic interaction is primary. The hydrophilic side chains and peptide bond enhance the negative values of the transfer enthalpy. In contrast, the hydrophobic side chain has positive contributions to the transfer enthalpy. The value of transfer enthalpy of mannitol is the minimum one because mannitol molecules need more energy to overcome the hydration bonds than other polyol molecules. On the contrary, the value of the transfer enthalpy of the hydrophobic inositol molecule is maximal. The value of the transfer enthalpy of xylitol is close to that of sorbitol because the difference between their molecules is only one hydroxyl group.(3) The dilution enthalpies of two amides, N-methylformamide, and N, N-Dimethylformamide, in aqueous surfactant (sodium dodecyl sulphate, cetyltrimethylammonium bromide, N-dodecyl-hydroxyethyl-dimethyl ammonium bromide, N-tetradecyl-hydroxyethyl-dimethyl ammonium bromide, and N-cetyl-hydroxyethyl-dimethyl ammonium) solutions were determined using the isothermal titration calorimetry. The homogeneous enthalpic interaction coefficients over the whole investiageted concentration ranges of the surfactant solutions have been calculated. The results have been used to discuss the interactions between amine and surfactant molecules. The effects of different structures of amine molecules on the homogeneous enthalpic interaction coefficients have been considered.It can be seen that the homogeneous enthalpic interaction coefficients are positive, which indicates that the primary interactions are dehydration and structure recomposition. The hydrophilic-ion interaction between the peptide bond of an amine molecule and the head group of a surfactant molecule has negative contributions to the homogeneous enthalpic interaction coefficients, and the hydrophilic-hydrophobic interaction between the peptide bond of an amine molecule and the tail group of a surfactant molecule has positive contributions to the homogeneous enthalpic interaction coefficients. Hence, the values of the homogeneous enthalpic interaction coefficients change from increase to decrease with increase of the surfactant concentration in the aqueous solutions. With increasing the number of methyl group, the homogeneous enthalpic interaction coefficients increase.(4) The interactions of Bovine Serum Albumin (BSA) with two surfactants, N-dodecyl-hydroxyethyl-dimethyl ammonium bromide, and N-tetradecyl-hydroxyethyl-dimethyl ammonium bromide, have been investigated by the isothermal titration calorimetry from 298.15 K to 318.15 K. The thermodynamic functions, binding site number, binding constant, binding enthalpy, binding entropy, and binding Gibbs free energy, were obtained by fitting the calorimetric curves. The influences of the temperature and the length of the tail chain of surfactant on those thermodynamic functions have been discussed.The results indicate that there is only one class of sites for these surfactant molecules on the bio-macromolecules at the experimented temperature range. The ion-ion and hydrophobic-hydrophobic interactions should be mainly considered between the surfactant and BSA molecules. When the length of the tail chain of surfactants increases, the values of binding site number and binding constant decrease. With increase of temperature, the values of binding site number and binding constant of N-tetradecyl-hydroxyethyl-dimethyl ammonium bromide increase. For N-dodecyl-hydroxyethyl-dimethyl ammonium bromide, however, the change trend of binding site number and binding constant is opposite.