INTRODUCTION

Aim of the present study is to study the molecular interaction on the physico-chemical and thermodynamical studies of amino acids in mixed aqueous solutions and the effect of these compounds on water structure. Such a study helps in better understanding of the interactions occurring between amino acid molecules and entities present in mixed aqueous medium in the living cells. Although a lot of attention has been given to the behaviour of amino acids in different salt-water mixed solvents (Bhat and Ahluwalia, 1985; Wadi and Goyal, 1992; Ogawa and Suji, 1987) very few studies have been carried out on amino acids in (carbohydrate and water) mixtures (Parfenyuk et al., 2004) probably due to the complex nature of their interactions. Information has been obtained about the interactions between carbohydrates and proteins from x-ray crystallography (Newcomer et al., 1981), NMR spectra, computer calculation (Bock et al., 1985), chromatography data (Ogawa and Suji, 1987) and kinetic studies (Miller et al., 1983, 1980). However, thermodynamic studies of these compounds in solutions are rare.

Therefore, the study of carbohydrate-protein interactions is very important for immunology, biosynthesis, pharmacology and medicine (Parfenyuk et al., 2004). In addition to their importance to the food and pharmaceutical industries, the simple saccharides have received considerable attention for their ability to protect biological molecules and structures against the stresses induced by freezing and drying process and during subsequent storage. Polyhydroxy compounds are well-known stabilizing agents for the native state of proteins/enzymes. Stabilizing potency of polyols and saccharides are related to the number or configuration of the hydroxy groups. These considerations led us to undertake the study of amino acids L-serine, L-glutamine and L-asparagine in concentration range from (0.02 to 0.1 m) in mass percentage of aqueous D-glucose solutions ranges from 0 to 40% by 10% increments at 298.15 K.

In this study, we report the values of Density, viscosity and Ultrasonic velocity of 0.02 to 0.1 molality of L-serine, L-glutamine and L-asparagine in aqueous D-glucose solutions at 298.15 K. Various physical and thermodynamical parameters like adiabatic compressibility (β), molal hydration number (n_H), apparent molar compressibility (φ_k), apparent molar volume (φ_v), limiting apparent molar compressibility , limiting apparent molar volume and their constants (S_K, S_V), transfer volume and viscosity A and B coefficient of Jones-Dole equation (Jones and Dole, 1929), respectively were calculated from the density, viscosity and ultrasonic velocity data. All these parameters are discussed in terms of solute-solvent and solute-co-solute interactions occurring in the amino acids in aqueous D-glucose solutions.

MATERIALS AND METHODS

Analytical (AR) reagent grade with minimum assay of 99.9% of L-serine, L-glutamine, L-asparagine and D-glucose were obtained from Emerck Germany and SD. Fine Chemicals India was used as such without further purification. Water used in the experiments was deionised and distilled and was degassed prior to making solutions. Solutions of D-glucose were prepared by mass and used on the day they were prepared. The mass percentage of D-glucose in these solutions ranged from 0 to 40% by 10% increments. The density was determined using a specific gravity bottle by relative measurement method with an accuracy of ±0.01 kg m^-3. The weight of the sample was measured using electronic digital balance with an accuracy of ±0.1 mg (Model: SHIMADZU AX 200). An Ostwald’s Viscometer (10 mL capacity) was used for the viscosity measurement and Efflux time was determined using a digital Chronometer to within ±0.0 1 s. An ultrasonic interferometer having the frequency 3 MHz (MITTAL ENTERPRISES, NEW DELHI, MODEL F-81) with an overall accuracy of ±0.1% has been used for velocity measurement. An electronically digital operated constant temperature bath (RAAGA Industries) has been used to circulate water through the double walled measuring cell made up of steel containing the experimental solution at the desired temperature. The accuracy in the temperature measurement is ±0.1 K.

Theory and Calculations
Using the measured data, the following acoustical and thermodynamical parameters have been calculated using the standard relations.

(1)

Molal hydration number has been computed using the relation

(2)

Where:

The apparent molar compressibility has been calculated from relation.

(3)

Where, β, ρ and β₀, ρ₀ are the adiabatic compressibility and density of solution and solvent, respectively, m is the molar concentration of the solute and M the molecular mass of the solute. φ_k results can be fitted by the equation:

(4)

Where:

	=	The limiting apparent molar compressibility at infinite dilution
Sk	=	A constant

and S_k of Eq. 4 have been evaluated by the least-square method.

The apparent molar volume φ_v has been calculated using the relation:

(5)

The apparent molar volume φ_v has been found to differ with concentration according to Masson (1929) empirical relation as:

(6)

Where:

	=	The limiting apparent molar volume at infinite dilution
Sv	=	A constant and these values were determined by least-square method

Transfer volumes of each amino acid, from water to aqueous D-glucose solutions were calculated using the equation

(7)

The importance of viscometric study of electrolyte solution in mixed solvent systems is well established (Chauhan et al., 2002; Patial et al., 2002). The entire viscosity data have been analysed in the light of Jones-Dole semi empirical equation (Jones and Dole, 1929).

(8)

Where:

A and B are constants which are definite for a solute-solvent system. A is known as the Falkenhagen coefficient which characterises the ionic interaction and B is the Jones-Dole or Viscosity B-coefficient which depends on the size of the solute and the nature of solute-solvent interactions.

RESULTS AND DISCUSSION

From the experimental values of density, viscosity and ultrasonic velocity some acoustical parameters such as adiabatic compressibility (β), hydration number (n_H), the apparent molar compressibility (φ_k), apparent molar volume (φ_v), limiting apparent molar compressibility , limiting apparent molar volume and their constants (S_K, S_V), transfer volume and viscosity B-coefficient of Jones-Dole equation were calculated and the results were given in Table 1-4.

In all the three systems the values of adiabatic compressibility Table 2 decrease with increase in concentration of solute (amino acids) as well as increase in concentration of D-glucose in water. The decrease in adiabatic compressibility is attributed to the influence of the electrostatic field of ions (NH₃⁺ and COO^–) on the surrounding solvent molecules so called electrostriction. The magnitude of β values are larger in L-serine than other two amino acids. The larger β values which show molecular association/interaction is greater in L-serine than that of other two amino acids. Amino acid molecules of neutral solution exist in the dipolar form and thus have stronger interaction with the surrounding water molecules. The increasing electrostrictive compression of water around the molecules results in a large decrease in the compressibility of solutions.

The interaction between the solute and the water molecules present in the solvent is said to be hydration. From the Table 2 it is observed that the values of n_H are positive in all systems studied and such positive values of n_H indicate an appreciable solvation of solutes. The values of n_H are found to increase with increasing the content of L-serine but it decreases in L-glutamine and L-asparagine. Further, these values decrease with rising of D-glucose content in all the three systems studied. The decreasing values of n_H which indicate the increase in solute-co-solute interaction with the increase in D-glucose concentration leading to the reduction in the electrostriction. This shows that D-glucose has a dehydration effect on the amino acids.

Table 1:	Values of density (ρ) viscosity (η) and ultrasonic velocity (U) of amino acids in aqueous D-glucose solutions at 298.15 K

Table 2:	Values of adiabatic compressibility (β) and hydration number (nH) of amino acids in aqueous D-glucose solutions at 298.15 K

Table 3:	Values of apparent molal compressibility (φK) and apparent molal volume (φV) of amino acids in aqueous D-glucose solutions at 298.15 K

The following observations have been made on apparent molar compressibility (φ_k) and apparent molar volume (φ_v) (Table 3) of the amino acids in aqueous D-glucose solution.

Table 4:	Values of limiting apparent molar compressibility (φK0), limiting apparent molar volume (φV0) and their constants SK and SV, transfer volumes(ΔVφ0) and A and B coefficients of Jones-Dole equation of amino acids in aqueous D-glucose solutions at 298.15 K

The above observations clearly suggest that the negative values of φ_k and φ_v in all systems indicate the presence of solute-solvent interactions. The negative values of φ_k indicate hydrophilic and ionic interactions occurring in these systems. Since, more number of water molecules are available at lower concentration of D-glucose, the chances for the penetration of solute molecules into the solvent molecules is highly favoured. The observed behaviour of φ_k reveals that strengthening of the solute-solvent interaction in all systems studied. The decrease in φ_v is due to strong ion-ion interaction and vice-versa. The negative values of φ_v indicate electrostrictive solvation of ions. From the magnitude of φ_v, it can be concluded that stronger molecular association is found in L-serine than other two amino acids and hence L-serine is a more effective structure maker.

The values of provides information regarding solute-solvent interaction and S_K that of solute-solute interaction in the solution. One can notice from the Table 4. values are negative in all systems studied except 0 and 40% of L-serine and 30% of L-glutamine. Appreciable negative values of for all systems reinforce our earlier view that the existence of solute-solvent interaction. The values of S_K exhibit both negative and positive and vary non-linearly in L-glutamine and L-aspargine, but it is found to negative in L-serine mixtures. This behaviour shows that the existence of ion-solute/solute-solute interaction in all the systems studied. S_K parameter, which represents the solute-solute interaction is positive and decreases with increasing the mass percentage of D-glucose. It is well known that solutes causing electrostriction lead to decrease in the compressibility of the solution. This is reflected by the negative values of the φ_k of amino acids in aqueous D-glucose solutions. Hydrophilic solutes often show negative compressibility as well, due to the ordering that is introduced by them in water structure (Prakash et al., 1964).

The volume behaviour of a solute at infinite dilution is satisfactorily represented by , which is independent of the solute-solute interactions and provides information concerning solute-solvent interactions. Table 4 reveals that the values of are both positive and negative in L-serine but it is found to be negative for other two amino acids. Large positive values suggesting the presence of strong solute-solvent interactions and vice-versa in these media.

Fig. 1:	Transfer volumes ΔVφ0 vs mass percentage of D-glucose at 298.15K

Further values decrease in L-serine and L-glutamine and the same increase for L-asparagine with increasing the mass percentage of D-glucose. increase due to the reduction in the electrostriction at the terminals where as it decreases due to disruption of side group hydration by that of the charged end. The decrease in may be attributed to the increased hydrophilicity/polar character of the side chain of the amino acids. The S_V values (Table 4) for L-serine and L-glutamine are found to be negative and positive for L-aspargine. The negative values of S_V indicate the presence of stronger solute-solute interaction and less complex ion formation taking place in the systems. Generally speaking the types of interaction occurring between the amino acid molecules and D-glucose molecules can be classified as follows:

The values (Table 4) can also be explained on the basis of co-sphere overlap model in terms of solute-co-solute interactions. According to this model, hydrophilic-ionic group interactions contribute positively, whereas hydrophilic-hydrophobic group interactions contribute negatively to the values. Therefore, from Fig. 1, the negative values observed for L-serine and L-glutamine suggest that the latter type of interactions are dominating over the former.

Table 4 shows that the values of the A coefficient either negative or positive for all amino acids over the entire composition range of aqueous D-glucose at 298.15 K indicating the presence of solute-solute interactions. The values of the B-coefficient for all the amino acids in aqueous D-glucose solutions are positive indicating that the ion-solvent interactions are stronger. So they behave as structure makers. Further, the values of B-coefficient increase with increasing of D-glucose content in L-serine which suggests that solute-solvent interactions are stronger than other two amino acids.

CONCLUSION

In summary, volume and compressibility data have been determined for L-serine, L-glutamine and L-asparagine in aqueous D-glucose solution at 298.15 K and the results have been used to study the molecular interaction in the solutions. From the magnitude of φ_v and the values of B-coefficient it can be concluded that L-serine posses greater molecular association than the other two amino acids in aqueous D-glucose solutions. The transfer volume calculated from suggest increase in hydrophilic and hydrophobic group interaction with increase of the side-chain length of the amino acids. Similar types of interactions were also suggested by Ali et al. (2006) in their study which supports our present ultrasonic investigation.