Study On Thermodynamic Property Of Amino Acid In Sugar Alcohol Aqueous Solution

Protein is an important part of all living organisms,as well as the material basis of life activity and morphological structure.Amino acids,which have been used extensively as the most significant model compound of protein,are the basic building blocks of protein.Sugar alcohol is a kind of polyol,they can not only stabilize the natural conformation of spherical protein,but also remarkably affect the denaturalization and solubility of protein.By the researches on the volumetric and viscometric properties of amino acids in the sugar alcohols aqueous solutions,we could obtain some necessary information about interactions of the amino acids and sugar alcohols molecules,and then further understand the mechanism of the protein stability through sugar alcohols.In this paper,the densities and viscosities of glycine,L-alanine,L-valine,L-threonine and L-arginine in mannitol,sorbitol,xylitol aqueous solutions at T =(293.15 to 323.15)K were measured by vibrating tube and iVisc capillary viscometer,and experimental data were compared with the literature values.Moreover,the extended Guimar?es equation and Jones-Dole equation were utilized to correlate the densities and viscosities of solutions,respectively.The apparent molar volume,limiting partial molar volume,transfer partial molar volume,viscosity B-coefficients and the free energies of activation per mole of solvent and solute were calculated from the experimental densities and viscosities and discussed based on the cosphere overlap model and transition state theory,respectively.The results show that both the values of the apparent molar volume and limiting partial molar volume for five amino acids increase in the order: glycine < L-alanine < L-threonine < L-valine < L-arginine.For glycine,L-alanine and L-valine,ion-hydrophilic interaction predominates in ternary solutions.For L-threonine and L-arginine,the predominant interactions are ion-hydrophilic and hydrophilic-hydrophilic interactions.According to the transition state theory,glycine tends to act as structure-breaker in sugar alcohol aqueous solutions,whereas L-alanine,L-valine,L-threonine and L-arginine are likely to be structure-makers.The group contribution method was applied to analyze the limiting partial molar volume and viscosity B-coefficient.The contributions of the group(NH3+,COO-),CH2,OH,CNHNHNH2 in sugar alcohol aqueous solutions were obtained,and all values are positive.The results suggest that the group contribution to the limit partial molar volume increase in this order: OH < CH2 <(NH3+,COO-)< CNHNHNH2.The variation rule of group contribution to viscosity B-coefficients with temperature shows that CH2 prefers to be structure-maker in solutions,and other groups are structure-breakers.