Studies On The Thermodynamics Of Dipeptides In Aqueous Electrolytes And Urea Solutions

Proteins are the material base depended by the structures and life activities in biological systems. Because of the structure complexities of proteins, it is extremely difficult to investigate thermodynamics properties directly. It has been common practice to use amino acids and oligopeptides as model compounds since they make up of the protein structures. The features that distinguishes dipeptide model from others reported in the literature are that the compounds chosen as models are themselves portions of proteins, i.e. peptide molecules and that they could solute easily in aqueous solutions. Investigations on the thermodynamic behavior of model compounds in aqueous solution would provide information about mechanism of action of proteins in aqueous solution, however, most proteins exist in the complicated environment containing many organic substances, which leads to significant interests in investigation on the interaction in aqueous solutions between model compounds and organic molecules present in living organisms or possessing functional group identical.The water structure hypothesis predicts that structure-breaking solutes stabilize proteins and that structure-making solutes destabilize proteins. A lot of experiments have proved that electrolytes and urea are the structure-breaker. Electrolytes could stabilize the protein conformation, and urea is the notable denaturant. There were papers reported that the changes of radium of anion and cation had different effect on the thermodynamic properties of peptides. To study systems consist of dipeptides, sodium halide (potassium chloride or urea) and water, we can obtain (i) the information that contributes to the growing body of knowledge about solute solvation and solute interactions in aqueous media, and (ii) a better understanding of their role played in the conformational stability and unfolding behavior of proteins, and then we want to explain the questions above.The configuration of glycylglycine is the simplest in dipeptides. The densities and solution enthalpies of glycylglycine in aqueous sodium halide and potassiumchloride solutions were determined. The transfer Enthalpy and transfer partial molar volume of glycylglycine from water to above mixed solvent were obtained and discussed in terms of solute-solute interaction between dipeptide and ions, which also bring a new insight into the structure of the mixed solvent mentioned above. Using the consphere overlap model explained these tendencies and gained conclusions: Interactions between peptide bond or charged centers of glycylglycine and ions are dominant. The solvation of glycylgycine in electrolyte solution is the enthalpy driven. The transfer volume depend less on temperature. Electrostatic effect is dominating in the transfer properties of glycylglycine arising from the changes of anion radium. Because of breaking the hydrogen bond in the solvation layer ofpeptide bond, the rule of cation is contrary with anion.Hydrophobic interaction plays an important role on stabilizing the configuration of proteins. N-glycyl-DL-alanine and N-glycyl-L-valine were choused as the model compounds because of hydrophobic groups. Their densities and enthalpies of solution in aqueous sodium chloride and urea solutions were determined at 298.15 K. The transfer enthalpy and transfer partial molar volume from water to above mixed solvent were obtained and discussed. Analyzing the results, we gained the conclusions: The transfer of hydrophobic group from water to NaCl solution is the process that is entropy favorable and enthalpy unfavorable. Hydrophobic-hydrophile interaction dominates in enthalpy contribution. Urea could displace water in the solvation shell of nonpolar group accompanying an increase in entropy, and urea also could reduce the energy by forming the hydrogen bond with around water. Urea also could displace water in the solvation shell of peptide bond and form the multiple hydrogen bonds with it, which could reduce the energy more. So the transfer of hydrophobic groups and peptide bond from water to aqueous urea solution is the process that is favorable for entropy and enthalpy.In conclusion, the effect of electrolyte on protein stability might be the model in which nonpolar groups get together avoiding to contact with water, which could reduce the energy. The denaturalization of urea on protein might be the model in which urea denatures protein by diminishing the hydrophobic effect by displacing water molecules from the solvent shell around nonpolar groups and binding directly to amide unite via hydrogen bonds. These models are in good agreement with recentresults of MD simulation.