Volumetric Studies of Some Amino Acids in Aqueous 1,4- Dioxane Solution at 308.15K

The Density measurement was carried out in aqueous solution of glycine, L-alanine, Lvaline, L-Leucine and L-phenyl alanine in 10% 1,4dioxane solution at 308.15 K. The values of apparent molar volume, limiting apparent molar volume have been evaluated from density data. These values are used for calculatating the number of water molecule hydrated (nH) to the amino acids. Transfer volumes at infinite dilution from water to aqueous 1,4dioxane solution have been also calculated. Group contribution to partial molar volumes has been determined for the amino acids. Transfer parameter have been interpreted in terms of solute-co solute interaction on the basis of co-sphere overlap model. All these parameters are related to type and extent of intermolecular interactions in binary liquid mixtures. All the results were interpreted in the light of ion -ion and ion – solvent interactions and of structural effect of solutes in solutions.


INTRODUCTION
One can notice that there is lack of volumetric data up to high concentration range of amino acids and electrolytes. Very less data are available for amino acids in 1,4-dioxane systems 8 . There is need of examining the effect of amino acids on the properties of electrolyte. So it is preferred to study properties of model compounds like amino acids instead of complex bio-molecules. In order to understand the effects of ionic species on amino acids in general, various properties of amino acids in aqueous 1,4-dioxane solutions are studied.

MATERIAL AND METHODS
Five amino acids namely glycine, Lalanine, L-valine, L-Leucine and L-phenyl alanine of highest purity were obtained from Sigma chemicals Co. Amino acids were dried in vacuum oven for 24 hrs on kept over P 2 O 5 in vacuum desiccators. 1,4-dioxane was refluxed and then distilled over sodium metal using a fractionating glass column. The middle fraction distilling at 373 K was collected for use. All the solutions were prepared on the molarity basis. The samples were weighted on a mettler balance having accuracy of 0.01mg. Water used to prepare solutions was obtained by distilling deionised water over alkaline KMnO 4 and it was thoroughly degassed prior to its use. The specific conductance of the water used was less than 0.055 × 10 -6 s cm -1 The densities of the solutions were measured using a single capillary pycnometer made up of borosil glass with a bulb of total volume of 8 cm 3 and capillary with internal diameter of 0.1cm was chosen [9][10][11][12][13][14][15] for the present work. The details pertaining to calibration experimental set up and operational procedure have been previously described [9][10][11][12][13][14][15] . An average of triplicate measurement was taken in to account. The reproducibility of density measurement was +-3 X 10 -5 g cm -3 . The temperature was constantly maintain by controlled temperature water bath (Gemini scientific instruments, Madras) having accuracy of ± 0.01 °C.
In the present study, densities of these solutions are increasing with increase in concentration of amino acids. The plot of density with the concentration amino acids in 10% aqueous 1,4-dioxane are found to be linear in all the cases.
The apparent molar volumes (V  ) were calculated from the density data using well-known expression 16,17,18 ... (1) Where  0 and  s are densities of solutions and solvent respectively. C is molarity and M is the molar mass of solute. The resulting values of apparent molar volumes (V  ) with the molar concentration (M) of the amino acids in 10% aqueous 1,4-dioxane at 308.15 K are reported in table. 1.1. Comparison with earlier results 19 shows that Values of V  increase with increase in temperature in aqueous 1,4-dioxane. It is also found that V  increases linearly with increase in size of alkyl V  side chain of the amino acids in aqueous 1,4-dioxane. It indicates that the solute-solvent interactions increases with increase in size of the alkyl side chain of amino acids and with the concentration of amino acids 205 . It is accordance with earlier results 19 The variation of apparent molar volumes with the square root of molar concentration can be represented by Mason's equation 20 .
... (2) Where 0 V  is the limiting value of the apparent molar volume (equal to the partial molar volume at infinite dilutions) and S v is the experimental slope. The Values of 0 V  and S v obtained by least square fitting of the V  values to equation 2 for various amino acids in aqueous 1, 4-dioxane, which are reported in table 1.
Limiting apparent molar Volume ( 0 V  ) and the experimental slope have been employed to indicate the type of interaction 207-208 . The effect of temperature on 0 V  has been interpreted in terms of the solute-solvent interactions while that on experimental slope in terms of solute-solute and solute-solvent interaction 21,22 . The positive value of S v suggesting that the added electrolyte behave as a structure maker in the solvent while negative value of S v suggest the structure breaker capacity in the solvent.
All the amino acids studied have positive 0 V  value in binary aqueous solution of aqueous 1,4-dioxane. It is also found that 0 V  increase linearly with the size of alkyl side chain of amino acids and increases with increase in temperature. It indicates that the co-solute-solvent interaction increase both on increasing temperature and the size of alkyl side chain of amino acids 23 .
Since S v is related to solute-solute interaction. It is evident from table 1 that the values of the slope S v for all amino acids in aqueous 1,4dioxane are negative suggesting weak solute-solute interaction in the system. The experimental slope S v increases with increase in temperature, suggest that more and more solute is accommodated in void space left in packing the large associated solvent molecule and such enhance the structure of the solvent. However S v is found to decrease with increase in temperature of L-valine in aqueous 1,4dioxane, suggesting the decrease in solute-solute interaction with the rise in temperature indicating structure-breaking effect of L-valine in aqueous 1,4dioxane. It is found from table 4.6 that the values of values in case of L-valine L-leucine and L-phenyl alanine indicates the effect of hydrophobic parts. That is the interactions between the 1,4-dioxane and zwitter ionic center of amino acids increase with increase in temperature. For L-valine the interaction between non-polar group of L-valine and 1,4-dioxane are predominant. The overall effect is that the charged end group of glycine and L-alanine influence electrostatically the surrounding water molecule the so-called electrostriction 23 . In other word, the hydration co-sphere of NH 3 + 100 -. Which are more hydrated than that of aqueous 1,4-dioxane will be affected to greater extent than the later. The positive V  0 values results that the dehydration of solute and co-solute occurs more in case of glycine and L-alanine 23 . S. Li. et al., 24 also reported positive V  0 values for different amino acids from water to aqueous glucose solutions. Now to explain partial molar volume data, different models have been used. Franks et. al., 25 have shown that partial molar volume at infinite dilution of a non electrolyte is a combination of two factors by the following equation.
... (4) Where is V int the intrinsic molar volume of the non-hydrated solute V s is the contribution due to the interaction of the solute with water. Some workers 26,27 have suggested that the V int is made of the following type of contribution.
... (5) Where 1 V V is the Van der walls volume 28,29 and V void is the volume associated with void or empty space. For electrolyte zwitter ionic solutes, this equation was modified by Shahidi et. al . 26 to find contribution of one molecule to partial molar volume of a hydrophobic solutes as ... (6) Where V shrinkage is the volume due to shrinkage this is due to interaction of hydrogen bonding sites with water molecules. Assuming that V v,w and V void have the same magnitude in water and aqueous 1,4dioxane positive 0 V   values of glycine and L-alanine might arise from the decrease in V shrinkage in aqueous 1,4-dioxane. The interaction 1,4-dioxane with the zwitter ionic center of amino acids (glycine and Lalanine) reduces the effect electrostriction of water, thereby causing a decrease in V shrinkage . In other words some water molecule may be released as bulk water in presence of 1,4-dioxane. It brings about the increase in volume of the solvent 30 thereby the reducing the strong interactions between amino acids and water. This results in positive volume of transfer from water to aqueous 1,4-dioxane solution observed in case of glycine and L-alanine. Thus a positive 0 V   for glycine and L-alanine results from  Values can further be rationalized by co-sphere overlap model developed by Gurney 31 and Frank and Evans 32 . According this property of water molecules in the hydration co sphere depend on the nature of solute molecule. When two solute particles come close enough such that their co-sphere over lap. Some of the co sphere material is displaced and this is accompanied by changes in the thermodynamic parameters.
The interaction between aqueous 1,4dioxane and amino acids can be classified as follows 1) hydrophilic-ionic interaction occurring between zwitter ionic centers of amino acids and dipolar parts of 1,4-dioxane 2) Hydrophilichydrophobic interaction occurring between non polar parts of amino acids and hydrophobic parts of 1,4dioxane.
According to the co sphere model in terms of solute-co solute interactions, hydrophilic-ionic group interaction contributes positively, whereas hydrophilic-hydrophobic group interaction contributes negatively to the 0 V   values. In case of glycine and L-alinine, the former type of interactions is predominant over the latter and for L-valine, Lleucine, L-phenyl alanine hydrophilic-hydrophobic group interaction are dominating over the hydrophilic-ionic group interaction. It may noted from A. Pal and S. Kumar's 33 findings that values of L alanine are less than those of glycine and L-valine in solutions. This is in line with the earlier conclusion drawn on the basis of volume of shrinkage that varies solute-co-solute interactions occur in these system which contribute to different extents, depending on the particular amino acids solution. The overall effect is that the solute -co solute interactions are predominant over the solute-solvent interaction as obtained in glycine and L-alanine .
The hydration of solute molecule in water is explained on the basis of Frank and Wen 34 model of solute-solvent interaction, which pictures three different solvent interactions, which pictures three different solvents structure regions in the neighborhood of the solute. Just out side the molecule, there is layer of immobilized and compressed water as a result of electrostrictive and other attractive forces exerted the solute. The solute this is surrounded by slightly less compressed or "structure broken "region of water molecule distantly affected by these forces. The outermost layer is bulk water, which possesses the typical tetra coordinated hydrogen-bonded structure not affected by any of the above forces. Compressibility measurements indicate the changes in the first two layers of solvent around the solute molecule. In case of carbohydrate molecule, the water structure is slightly disturbed by the hydrogen -bonded network around the solute; this holds the water around the solute firmly, making the hydration layer even less compressible.
The number of water molecule nH hydrated to the amino acids were calculated using the method given by 35,36,37 ... (7) Where From the computed values of n H, It is found that in all the concentration, each in aqueous 1,4dioxane molecule is closely bound and forms a complex in cluster organization with a fixed number of water molecules.

Conclsion
In summary, we have shown ion-amino acid interaction parameters from volumetric properties of glycine, L-alanine, L-valine, L-Leucine and L-phenyl alanine in 10% aqueous 1,4-dioxane at 308.15 K. The partial molar volumes of transfer ( 0 V   ) from water to aqueous have been calculated from measured quantities The more positive value of 0 V   for glycine and L-alanine indicates the dominance of the charged group  3 NH and COOwhile negative 0 V   values in case of L-valine L-leucine and L-phenyl alanine indicates the effect of hydrophobic parts. That is the interactions between the 1,4-dioxane and zwitter ionic center of amino acids increase with increase in temperature. For L-valine the interaction between non-polar group of L-valine and 1,4-dioxane are predominant. The overall effect is that the charged end group of glycine and L-alanine influence electrostatically the surrounding water molecule the so-called electrostriction 151 .Also from the computed values of n H, It is found that in all the concentration, each in aqueous 1,4-dioxane molecule is closely bound and forms a complex in cluster organization with a fixed number of water molecules.