Studies On The Thermodynamics Of Amino Acids In Aqueous Oxyacid Salts And Amide Derivatives Solutions

Amino acids have been useful models for understanding the thermodynamic behavior of proteins. Investigations on the thermodynamic behavior of amino acids in mixed aqueous solutions would not only provide information about interaction between solute and solvent but also the mechanism of action of proteins. As a part of the project supported by National Natural Science Foundation (No. 20273061), this paper consists of the following five parts.The fast part (chapter 1) summarizes the studies on the thermodynamic theories and properties of protein model molecules.In the second part (chapter 2) the enthalpies of solution of glycine, L-alanine and L-serine in aqueous solutions of LiNO₃, NaNO₃, KNO₃, Na₂SO₄ and NaClO₄ were determined by RD496-â…¡microcalorimeter at 298. 15 K. Enthalpy of transfer of amino acids were derived and interpreted in terms of electrostatic interaction and structural interaction. The results show that: 1) the transfer enthalpies (Δ_trH) of amino acids in the five oxacid salts solutions are positive except L-alanine in aqueous Na₂SO₄ solutions. The transfer enthalpies of amino acids from water to NaNO₃, KNO₃, Na₂SO₄ and low concentration NaClO₄ and LiNO₃ solutions vary in the sequence L-serine＜glycine＜L-alanine while glycine＜L-serine＜L-alanine in NaClO₄ and LiNO₃ solutions at the concentration above 1.5 mol·kg^-1. The difference may be due to the effect of anions and cations association at high concentration. 2) Anions exert much marked influences on the transfer enthalpy than cations. In most salts solutions, structure interactions influenceΔ_trH more strongly than electrostatic interactions. 3) There is an approximate parallel between the potency of the anions in inducing protein denaturation and the exothermicity of the anion-glycine ineractions. In the third part (chapter 3), the densities of glycine, L-alanine and L-serine in aqueous solutions of LiNO₃, NaNO₃, KNO₃, Na₂SO₄ and NaClO₄ have been measured at 298.15 K with an Anton Paar Model 55 densimeter. Apparent molar volumes, limiting partial molar volumes, transfer volumes (Δ_trV) and hydration numbers have been calculated. The results show that: 1) all the transfer volumes of amino acids are positive and increase with increasing concentration of salts. At the same concentration of salts, the relative order ofΔ_trV is L-serine＞glycine＞L-alanine. 2) The dominating contribution to transfer volume comes from the electrostatic interaction between the head groups of amino acids and ions. The order of transfer volume of the same amino acid in oxyacid salts solutions is: Na₂SO₄＞NaNO₃＞KNO₃＞NaClO₄＞LiNO₃. 3) The destabilizing effect of ions on protein has approximately the same order as the decreasing transfer volumes of glycine.The forth part (chapter 4) have studied the enthalpies of transfer of glycine, L-alanine and L-serine in aqueous solutions of N-methylformamide, N, N-dimethylformamide, N-methylacetamide and N, N-dimethylacetamide with the same method as the second part. The results show that: 1) The transfer enthalpies of amino acids in the four amide solutions are positive except L-serine in low concentration NMF solutions and the relative order is L-alanine＞glycine＞L-serine in the same amide solutions. 2) The structure interaction between head groups of amino acids and amides have a large contribution toΔ_trH. The interaction type between amino acids and amides are dominated by 1:1 from the results of the enthalpic pair interaction coefficients. 3) For the same amino acid the value of transfer enthalpy is consist with the hydrophobicity of amides i. e. DMA＞NMA＞DMF＞NMF＞AD＞FA.In the last part (chapter 5), the volumetric properties of glycine, L-alanine and L-serine in aqueous solutions of NMF, DMF, NMA and DMA have been studied using the same method as the third part. The volumes, of transfer of amino acids were discussed in terms of solute-solute interaction between amino acids and amides. It has been show that: 1) the transfer volumes of amino acid are related with the relative hydrophobicity of amino acids and amides. Hydrophilic-hydrophilic interactions are predominant in the case of glycine and L-serine in the four amides solutions and L-alanine in higher concentration NMF solutions, and hydrophobic-hydrophobic interactions are predominant in the case of L-alanine in NMA, DMF and DMA solutions and low concentration NMF solutions. Positive transfer volumes are observed for glycine and L-serine in the presence of the four amides and L-alanine in higher concentration NMF solutions. 2) The relative order of the transfer volume in the same concentrations amides solutions is L-alanine＜L-serine＜glycine except in m＞2.5 mol·kg^-1 DMA solutions, while in m＞2.5 mol·kg^-1DMA solutions the order is L-alanine＜glycine＜L-serine. 3) DMA has significant effect but NMA, NMF and DMF have small influence on the transfer volumes of glycine and L-serine. The transfer volumes of glycine and L-serine in higher concentration DMA solutions are much larger than in the other three amides. For L-alanine, NMA, DMF and DMA seem have little influence on the transfer volumes, but NMF has markedly influence and the transfer volumes of L-alanine in higher concentration NMF solutions are larger than that in other amides solutions. 4) An increase in temperature increases the standard partial molar volumes but decreases the transfer volumes of amino acids in DMA solutions.