Studies On The Thermodynamic Properties Between The Solute-Solute Interactions In The Aqueous Solutions Of Protein Model Molecules

Amino acids, which have been used extensively as the most important biological model compounds, are not only the basic building blocks of proteins, but also the typical zwiterionic compounds and important active materials in living things.Sugars and polyols help in stabilizing the naitve conformation of globular proteins.There has been significant interest in the investigation on the interaction in aqueous solutions between amino acids and organic molecules present in living organisms or possessing functional group identical. The principle reasons for studying such systems are to obtain (i) the information that contributes to the growing body of knowledge aboutsolute solvation and solute-solute interactions in aqueous media, and (ii) a beter understanding of their role played in the conformational stability and unfolding behavior of proteins.This paper mainly consists of the following four parts.The first part summarizes the current status of studies on the thermodynamic properties of sytems containing portein model molecules.In the second part.enthalpies of mixing of aqueous amino acids solutions (glycine, L-alanine, L-serine, L-valine, L-proline, L-threonine)with aqueous 2,2,2-Trifluoroethanol (TEF) solutions have been determined at 298.15 K ,303.15 K and 310.15 K by the LKB-2277 flow microcalorimetric system. These results along with enthalpies of dilution of these aqueous solutions have been used to obtain the enthalpic interaction coefficients (h_xy,h_xxy, etc.) in terms of the McMillan-Mayer theory. The enthalpic pairwise interactions between amino acids and TEF have been discussed by solute-solute interactions.The results show: (1) The disprepancy of h_xy values between amino acids and TEF can were determined by the side-chain of amino acids. Amino acids with different side-group can make different contributions to h_xy. (2) Under the three temperatures there has a big difference for the h_xy values between amino acids and TEF molecules. For the same kind of amino acid, the h_xy value between amino acid and TEF molecules at 310.15 K is larger than that of 298.15 K and 303.15 K. Because of the synergistic effect of the hydrogen bonds, the intermolecular hydrophobic interactions strengthen and subsequently the partial dehydration of the dydroxyl groups enhance when the temperature increases. The higher the temperature, the synergistic effect of the hydrogen bonds more obvious. Therefore, the h_xycoefficients between amino acids and TEF at 310.15 K appear relatively larger positive values. This probably is the main reason that TEF is used as a reversible denaturant. (3) Proline is a natural amino acid that has one pyrrole ring. Its special structure makes it important to the properties of polypeptide. On one hand, the cyclic structure weakens the attracting interactions between proline molecules due to the steric effect, which makes solvation easy. The results indicate that the pyrrole ring has the bigger hydrophobic interaction. The hxy value between Proline and TEF molecules increases with the increment of temperature.In the third part, the heterotactic enthalpic interactions have been investigated between the six kinds of amino acids studied above, DMAC and DMSO and the interaction mechanism has been discussed.The results show: (1) The h_xy values between the six kinds of amino acids, DMAC and DMSO molecules are all positive. The h_xy coefficients at 298.15 K are in the following sequence: h_xy(Gly) xy(Ala) xy(Val) < h_xy(Ser)xy(Thr)< h_xy(DMAC). This can be ascribed to the the different structures of the protein model molecules. (2) Compared with the amino acids, the DMAC polar part is a conjugated system, which is surrounded by three methyl groups. This can lead to a very big steric effect and subsequently the hydrophilic-hydrophilic interaction between DMAC and DMSO molecules weaken. But the hydrophobic-hydrophobic interaction between them strengthen greatly because of the existence of the two methyl groups connected with the nitrogen atom. Therefore, the h_xy(DMAC) coefficient displays a larger positive value.The forth part, the enthalpies of mixing of the DMAC aqueous solution with the saccharides (glucose, sucrose) aqueous solutions have been determined at 298.15 K by the LKB-2277 flow microcalorimetric system. These results along with enthalpies of dilution of these aqueous solutions have been used to obtain the enthalpic interaction coefficients (h_xy,h_xxy, etc.) in terms of the McMillan-Mayer theory. The enthalpic pairwise interactions between DMAC and saccharides have been discussed from the point view of solute-solute interactions.The results show: The h_xy values between DMAC and glucose and sucrose molecules are all positive. At 298.15 K, the h_xy coefficients increase in the order: h_xy (glucose) < h_xy(sucrose). Sucrose is a dimer formed by a glucose molecule and a fructose molecule through the 1,4-glucoside bond. But owing to many complex factors, such as the intermolecular hydrogen bonding or steric hindrance between two monomers, two hydroxyl groups have been consumped during the course of the formation of glucoside bond for the monosaccharide. So the thermodynamic property of sucrose is between glucose and fructose, not the simple adduct of these two monosaccharide molecules.