Interaction of L-Threonine in aqueous THFA and in mixed aqueous solution at 298.15K

: Densities  and viscosities  of L-Threonine in water and in mixed aqueous solution of tetrahydrofurfurylalcohol at 298.15K. On the basis of these data ,the apparent molal volumes v  ,partial molal volumes at infinite dilution v  ° , the slop v S ,transfer volume ) ( t r v   , Gibbs free energy of activation for viscous flow of solution ΔG* 1,2 and the B-coefficient have been calculated using Jones-Dole equation . The obtained thermodynamic properties have been discussed in terms of intermolecular interactions.


Introduction
In order to understand various fundamental processes like denaturation of proteins, folding / lunfolding processes, stability of proteins etc. in aqueous solution , knowledge of various solute-solvent and solutesolute interactions is very important. Due to complex structure of proteins , direct investigation of the solute /solvent effect on these biological macromolecules is very challenging, therefore, a useful approach is to study simple model compounds such as amino acid which are the monomer units of proteins molecules. The interpretation of behavior of amino-acids is quite helpful in understanding the water-protein interactions in solutions.
Moreover volumetric, viscometric studies of amino acids in water and solutions of organic solvents can provide valuable information for understanding protein unfolding [1][2][3] and the hydrophobic interactions of non-polar side chains [4]. The amino acids are high water solubilities suggests that they exist in an ionic form (zwitter ion). In physiological media such as blood , membrans , cellular fluids , etc. , where happens to be involved in an important manner , the zwitter ionic (dipolar) character of these compounds has an important bearing on their biological functions [5]. The interactions occurring between amino acids and other components are 698 reflected by their thermodynamic properties [6][7][8][9][10]. L-threonine (abbreviated as Thr.), classified as polar amino acid, is an essential α-amino acid . It was discovered as last of 20 common proteinogenic amino acids with two chiral centers. Threonine is used for single enantionmer (25,3R). The water and THF has proved to be most interesting solution due to hydrogen bond interactions of water with THFA [11][12][13]. The mixed aqueous solvent (with different percent w/w) can influence the solubility behaviour of amino acids . Conaequently thermodynamic properties , enthalipes , heat capacities , apparent molal volumes and viscosities of amino acids and peptide in mixed aqueous solvents is useful to obtain information about various types of interactions in these solutions [14][15][16][17][18][19].

Experimental
Amino acid L-Theonine obtained from fluka company is Analar and used without any further treatment. THFA was purchased from Merck with purity higher than 99% and used without further purifications.
The viscosity  were determined using as uspended -level ubbelohde viscometer described by findly [20], in a bath controlled to Foolk for all measurements.

Result and Discussion:
The apparent molal volume ( v  ) is calculated using the following equation solution , m is calculated using the following relation where c is the molar concentration . Since the limiting apparent molal volume ( v  ) were obtained from the relation [22].
 is the partial molal volume at infinite dilution which gives information about solute hydrophobicity , hydration properties and is also a measure of solute-solvent interaction. v S is slop indicating solute-solute interactions which is also known as volumetric pair wise interaction coefficient [23]. From transition state theory the Gibbs free energy of activation for viscous flow of solution ΔG*(J.mol -1 ) at a given temperature and composition is given by the equation [24][25]. The values of ΔG* is calculated via equation 3, the values of ΔG* increasing with increasing THFA% in solution .
The viscosity of Threonine in water increases with increase in concentration of solute. The viscosity of water plus THFA in the absence of Threonine (stock solution ) is slightly more than the viscosities when Threonine is add to it. However, the viscosity of threonine in the presence of water and THFA(solvent) at different molarities increase.
It evidences the existence of molecular interactions occurring in the system. In terms of Jones-Dole coefficients gives the value of transfer Bcoefficient. The coefficient B represents measure of order or disorder introduced by co-solute in to water. The value of B-coefficients is positive which shows strong alignment of THFA with Threonine and thus higher Bcoefficients at high concentrations of these organic solutes, thus the positive values of B-coefficients suggesting the solute-solvent interactions.
The standard partial molar volumes of transfer, ) (tr v   , at infinite dilution from water to aqueous THFA solution have been evaluated from