Thermodynamic Studies On Amino Acids In Aqueous Solutions Of Poly-hydroxyl Compounds

Amino acids, which have been used widely as the most important biological model compounds, are not only the basic unit of proteins, but also the important active substances in living organisms. Sugars and polyols, which help in stabilizing the natural conformation of globular proteins, play a significant role in the process of proteins' solubility, denaturalization, folding and unfolding. The chief reasons of studying these systems are to obtain both the information about solvated solute-solute interactions in aqueous solutions and better understanding of mechanism of proteins' stabilization by polyols.Enthalpies of solution of L-serine and L-threonine in aqueous solutions of isopropanol, 1, 2-propanediol, propanetriol and glucose were measured at 298.15K with microcalorimeter RD-496Ⅲ. We also calculated enthalpies of transfer of L-serine and L-threonine from water to aqueous solutions of isopropanol, 1, 2-propanediol, propanetriol, glucose and discussed all these results in detail comparing with previous reports about glycine.It turns out that the enthalpies of transfer of amino acids from water to aqueous solutions of small acyclic alcohol are positive values which prove the dominance of endothermic dehydration. And the enthalpies of transfer of L-serine and L-theronine from water to aqueous glucose solution are generally negative proving the exothermic straight interaction between amino acid and cosolvent molecule is more obvious than the endothermic interactions. Although the structure of glucose molecule, which is cyclic with several hydroxyls, is considerably different with those of small acyclic alcohol molecules, it shows some similar property with that of glycerol.Moreover, the densities of glycine, L-alanine, L-serine and L-threonine in water and aqueous D-mannitol solution were measured at 298.15 K and apparent molar volumes and limited partial molar volumes of these amino acids calculated. The data were used to deduce transfer partial molar volumes from water to aqueous D-mannitol solution and hydration numbers for further discussion.The result shows that the transfer partial molar volumes of amino acids from water to aqueous D-mannitol solutions generally rise along with the promotion of D-mannitol's concentration, but they aren't linear with it for the complexity of its structure. The values of transfer partial molar volumes display in the following order: L-threonine＜L-alanine＜L-serine＜glycine illuminating the difference among the hydrophilicity of their side chains. And the theoretical hydration numbers of amino acids decline with the increase of D-mannitol's concentration indicating the amino acid molecules interact with mannitol more than water molecule.