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Indian Journal of Pure & Applied Physics Vol. 49, July 2011, pp. 451-459 Thermodynamic and transport studies on some basic amino acids in aqueous sodium acetate solution at different temperatures S Thirumaran* & P Inbam *Department of Physics [DDE], Annamalai University, Annamalainagar 608 002, India E-mail: [email protected] Received 3 September 2010; revised 24 February 2011; accepted 24 March 2011 Ultrasonic velocity (U), density (ρ) and viscosity (η) of three amino acids namely L-arginine, L-lysine and L-histidine in aqueous sodium acetate solution (0.4 mol. kg-1) as a function of composition at 298.15, 308.15 and 318.15 K, have been measured. Using these experimental values, the acoustical parameters such as adiabatic compressibility (β), molal hydration number (nH), apparent molal compressibility (ϕK), apparent molal volume (ϕV), limiting apparent molal compressibility (ϕK0), limiting apparent molal volume (ϕV0), the constants (SK, SV) and viscosity B-coefficient of Jones-Dole equations were calculated for all the three systems. These parameters have been thoroughly analysed and eventually emphasizing the possible molecular interactions in terms of structure-making and structure-breaking effects of the above amino acids in the solvent mixture. Keywords: Molal hydration number, Limiting apparent molal volume, Adiabatic compressibility, Apparent molal compressibility 1 Introduction For the past two decades, the hydration of proteins through volumetric and ultrasonic measurements has been investigated, since these properties are sensitive to the degree and nature of hydration1. Due to the complex molecular structure of proteins, direct study is difficult. Therefore, the useful approach is to study simpler model compounds, such as amino acids which are building blocks of proteins. Most of the studies on amino acids2,3 have been carried out in pure and mixed aqueous solution. Amino acids and peptides are the fundamental structural units of protein. The investigation of volumetric and thermodynamic properties of amino acids and peptides in aqueous and mixed aqueous solvents has been the area of interest of a number of researchers4,5. Proteins are formed by polymerizing monomers that are known as amino acids, because they contain an amino (−NH2) and a carboxylic acid (−COOH) functional group. The chemistry of amino acids is complicated by the fact that the –NH2 group is a base and the −COOH group is an acid. In aqueous solution, an H+ ion is therefore, transferred from one end of the molecule to the other end to form a Zwitterion. Zwitterions are simultaneously electrically charged and electrically neutral. They contain positive and negative charges, but the net charge on the molecule is zero. Most of these amino acids differ only in the nature of the R-groups. Amino acids with non-polar substituents are said to be hydrophobic (water hating). Amino acids with polar R-groups that form hydrogen bonds with water are classified as hydrophilic (water loving). The remaining amino acids have substituents that carry either negative or positive charges in aqueous solutions are neutral pH and are therefore strongly hydrophilic. In the present paper, the amino acids at neutral pH which are taken up for study are L-arginine, L-lysine and L-histidine which are all polar R-groups. Knowledge of the interactions responsible for stabilizing the native state of globular protein in aqueous solution is essential to understand its structure and function. Due to complex structure of protein, the study of conformational stability and unfolding behaviour of globular protein have proved quite challenging and still remains a subject of extensive investigations. Therefore, protein model compounds such as amino acids and peptides, which are basic components of proteins have been investigated in detail with respect to their thermodynamic properties in aqueous and mixed aqueous solutions. The effects of salts on the stability of protein structures and some electrolytes have a tendency to disrupt some of the structural features of proteins, 452 INDIAN J PURE & APPL PHYS, VOL 49, JULY 2011 whereas other electrolytes show property to study such structures. The study of the thermodynamic ability of the native structure of proteins has proved quite challenging6. Salt solutions have large effects on the structure and properties of proteins including their solubility, denaturation, dissociation into sub-units. Amino acids are the fundamental structural unit of proteins. But L-amino acids are used in many biological processes in human body like transamination, decarboxylation and metabolism. On the other hand, L-amino acids are also involved in intracellular metabolism and operate specific transport systems of the plasma membrane. Hence, the study of these model compounds (amino acids) in aqueous salt solutions is of more significance in understanding the effects of salts on biomolecules. Various researchers have studied the interaction between some amino acids and simple slats7,8 which act as stabilizer/destabilizer, but a few studies are available about the behaviour of amino acids in the presence of organic salts9,10. Most of the works on amino acids has been carried out in dilute electrolytic solutions. Although various studies of amino acids are available in the presence of electrolytes having divalent cations, but no report has been found in the presence of organic salts having univalant cation. Sodium acetate is widely used in molecular biology applications. It is used in the purification and precipitation of nucleic acids, protein crystallization, staining of gels in protein gel electrophoresis. Large scale applications of sodium acetate include its use as retardant in plastic manufacturing as a mordant in dyeing and in the tanning of leather. Therefore, in order to understand the behaviour of proteins in aqueous salt solutions, the authors have studied the thermodynamic and transport studies of some amino acids in aqueous sodium acetate solution at different temperatures. In the present study, it has been reported that the values of density, viscosity and ultrasonic velocity have been measured for the amino acids, L-arginine, L-lysine and L-histidine in aqueous sodium acetate solution at different temperatures. Various parameters like adiabatic compressibility (β), molal hydration number (nH), apparent molal compressibility (φk), apparent molal volume (φv), limiting apparent molal compressibility (φK0) and its related constant (SK), limiting apparent molal volume (φV0), its related constant (SV) and viscosity-B coefficients of JonesDole equations have been evaluated and discussed in terms of ion-solvent, ion-ion interactions occurring between the amino acids and aqueous sodium acetate solution. 2 Experimental Details Analytical reagent grade (AR) and spectroscopic reagent grade (SR) with minimum assay of 99.9% of L-Arginine, L-lysine and L-histidine were obtained from E-Merk, Germany Chemicals. Fresh conductivity water has been used for preparing aqueous sodium butyrate solution. Required amount of water and sodium acetate were taken to prepare at 0.4 m (molality) of solution in a clean dry conical flask with ground stopper. The required amount of amino acids for a given molality was dissolved and similar procedure has been adopted for different molalities of other amino acids. All above solutions were used on the day they were prepared. An electronic digital balance [Model: SHIMADZU AX-200] with an overall accuracy of ± 1×10−4 g has been used for this purpose. The density was determined using a specific gravity bottle by relative measurement method with an accuracy of ±0.01 kgm−3. An Ostwald’s viscometer of 10 ml capacity was used for the viscosity measurement. An Ultrasonic Interferometer with working frequency at 3 MHz [Model: F-81, Mittal Enterprises, New Delhi] with overall accuracy of ± 3 ms−1 has been used for velocity measurement. An electronically digitally operated constant temperature bath (RAGAA Industries, Chennai) has been used to circulate water through the double-walled measuring cell made up of steel containing the experimental liquid at the desired temperature. The accuracy in the temperature is ± 0.1 K. 3 Theory and Calculations Adiabatic compressibility (β) is given by: β= 1 U 2ρ ... (1) Molal hydration number (nH) has been computed using the relation: nH = n1 β 1 − n2 β0 …(2) where β and β0 are the adiabatic compressibilities of solution and solvent, respectively, n1 and n2 are the number of moles of solvent and solute. THIRUMARAN & INBAM: THERMODYNAMIC AND TRANSPORT STUDIES ON AMINO ACIDS Apparent molal compressibility (φk) is given by: φK = β M 1000 ( ρ 0β-ρβ 0 ) + 0 mρ 0 ρ0 ... (3) where β, ρ and β0, ρ0 are the adiabatic compressibility and density of solution and solvent respectively, m the molal concentration of the solute and M the molecular mass of the solute. ϕK is the function of m as obtained by Gucker11 from Debye Huckel theory12 and is given by: φ K = φ 0K + S K m 1/ 2 ... (4) where ϕK0 is the limiting apparent molal compressibility at infinite dilution and SK is a constant. ϕK0 and SK were obtained by least square method. Apparent molal volume (ϕV) is obtained by: φV = M 1000 (ρ0 − ρ ) + mρ 0 ρ0 ... (5) The apparent molal volume has been found to differ with concentration according to Masson’s empirical relation13 as: φV = φ0V + SVm½ ... (6) where ϕV0 the limiting apparent molal volume at infinite solution and SV is a constant and these values were determined by least square method. The importance of viscometric study of electrolyte solution in mixed solvent is well established14,15. The entire viscosity data have been analysed in the light of Jones-Dole semi-empirical equation16, η = 1 + Am½ + Bm η0 …(7) Eq. (7) may be expressed as: η −1 η 0 = A + Bm½ m½ …(8) where η and η0 are the viscosities of the solution and solvent, respectively and m is the molal concentration of the solute-solvent system. A and B are constants which are definite for a solute-solvent system. A is known as the Falkenhagen17 coefficient which 453 characterises the ionic interaction and B is the JoneDole or viscosity B-coefficient which depends on the size of the solute and nature of solute-solvent interactions. 4 Results and Discussion The experimental values of density, viscosity and ultrasonic velocity for different molal composition of each of the amino acids viz, L-arginine, L-lysine and L-histidine in aqueous sodium acetate solution at different temperatures, which are presented in Table 1. The values of adiabatic compressibility, molal hydration number, apparent molal compressibility, apparent molal volume, limiting apparent molal compressibility, limiting apparent molal volume and the constants SK and SV and viscosity B-coefficient are given in Tables 2-4. Further, Figs 1-5 show the variations of adiabatic compressibility, molal hydration number, apparent molal compressibility, apparent molal volume and limiting apparent molal compressibility with molal concentration of L-arginine, L-lysine and L-histidine at different temperatures 298.15, 308.15 and 318.15 K. Here the curves are drawn using least square fitting. In all the amino acid systems from Table 1, the values of density, viscosity and ultrasonic velocity increase with increase of molal concentration of amino acids. And the same, except ultrasonic velocity decreases with rise in temperature. The ultrasonic velocity (U) from Table 1, increases with increase in the concentration of the solute as well as rise in temperature. Such an increase in ultrasonic velocity (U) clearly suggesting the molecular association is being taking place in these liquid mixtures. The factors apparently responsible for such behaviour may be the presence of interactions caused by the proton transfer reactions of amino acids18 and hydrophilic nature of aqueous sodium acetate19. Density (ρ) is a measure of solvent-solvent and ionsolvent interactions. Increase of density with concentration indicates the increase in solvent-solvent and solute-solvent interactions, whereas the decrease in density indicates the lesser magnitude of solutesolvent and solvent-solvent interactions. Increase in density with concentration is due to the shrinkage in the volume which in turn is due to the presence of solute molecules. In other words, the increase in density may be interpreted to the structure-maker of the solvent due to the added solute. Similarly, the decrease in density with concentration indicates structure-breaker of the solvent. It may also be true that solvent-solvent interactions bring about a INDIAN J PURE & APPL PHYS, VOL 49, JULY 2011 454 Table 1 — Values of density (ρ), viscosity (η) and velocity (U) in aqueous sodium acetate solution Density, ρ/(kg/m3) Molality m (mol.Kg−1) 298.15 308.15 318.15 Viscosity, η/(×10−3 Nsm−2) Temperature (K) 298.15 308.15 318.15 Velocity, U/(m/s) 298.15 308.15 318.15 0.6856 0.6916 0.6976 0.7036 0.7096 0.7156 0.7216 1546.50 1546.98 1547.46 1547.94 1548.42 1548.90 1549.38 1551.00 1551.48 1551.96 1552.44 1552.92 1553.40 1553.88 1556.50 1556.98 1557.46 1557.94 1558.42 1558.90 1559.38 0.6856 0.6936 0.7016 0.7096 0.7176 0.7256 0.7336 1546.50 1546.92 1547.34 1547.76 1548.18 1548.60 1549.02 1551.00 1552.30 1553.10 1554.20 1555.30 1556.10 1557.10 1556.50 1556.95 1557.40 1557.85 1558.30 1558.75 1559.20 0.6856 0.6896 0.6936 0.6976 0.7016 0.7056 0.7096 1546.50 1546.88 1547.26 1547.64 1548.02 1548.40 1548.78 1551.00 1551.38 1551.76 1552.14 1552.52 1552.90 1553.28 1556.50 1556.88 1557.26 1557.64 1558.02 1558.40 1558.78 System – I: L-arginine 0.00 0.02 0.04 0.06 0.08 0.10 0.12 1018.50 1019.10 1019.70 1020.30 1020.90 1021.50 1022.10 1012.40 1013.00 1013.60 1014.20 1014.80 1015.40 1016.00 1009.90 1010.50 1011.10 1011.70 1012.30 1012.90 1013.50 1.0600 1.0660 1.0720 1.0780 1.0840 1.0900 1.0960 0.8480 0.8520 0.8580 0.8640 0.8700 0.8760 0.8820 System – II: L-lysine 0.00 0.02 0.04 0.06 0.08 0.10 0.12 1018.50 1019.40 1020.30 1021.20 1022.10 1023.00 1023.90 1012.40 1013.30 1014.20 1015.10 1016.00 1016.90 1017.80 1005.20 1006.10 1007.00 1007.90 1008.80 1009.70 1010.60 1.0600 1.0680 1.0760 1.0840 1.0900 1.0960 1.1040 0.8480 0.8540 0.8600 0.8660 0.8720 0.8780 0.8840 System – III: L-histidine 0.00 0.02 0.04 0.06 0.08 0.10 0.12 1018.50 1019.50 1020.51 1021.55 1022.59 1023.70 1024.80 1012.40 1013.40 1014.45 1015.55 1016.65 1017.80 1018.80 1005.20 1006.20 1007.25 1008.30 1009.40 1010.60 1011.70 1.0600 1.0640 1.0680 1.0720 1.0760 1.0800 1.0840 bonding, probably hydrogen bonding between them. Usually the values of density and viscosity of any system vary with increase in concentration of solutions. The change in structure of solvent or solutions as a result of hydrogen bond formation or dissociation or hydrophobic (structure-breaking) or hydrophilic (structure-forming) character of solute. That is hydrogen bond forming or dissociating properties can, thus, be correlated with change in density and viscosity20. The increase in ultrasonic velocity (U) in these solutions may be attributed to the cohesion brought about by the ionic hydration. When the amino acids are dissolved in aqueous sodium acetate, the water molecules are attracted to the ions strongly by the electrostatic forces, which introduce a greater cohesion in the solution. Thus, cohesion increases with increase of amino acid concentration in the solutions. The increased associations obtained in these solutions may also be due to water enhancement brought by the increase in electrostriction in the presence of sodium acetate. The electrostriction effect, which brings about the shrinkage in the volume of solvent caused by the zwitterionic portion of the 0.8480 0.8520 0.8560 0.8600 0.8640 0.8680 0.8720 amino acid. Such a similar effect was reported by earlier researchers21. Table 2 presents the variation of adiabatic compressibility (β) with molal concentration of amino acids. The values of β in all the amino acids systems show a decreasing trend. The adiabatic compressibility’s values are larger in L-arginine system than those of other amino acid systems. This shows that the molecular association is greater in L-arginine. Amino acid molecules in the neutral solution exist in dipolar form and then have stronger interactions with the surrounding water molecules. The increasing electrostrictive compression of water around the molecules results in a larger decrease in the compressibility of the solutions. The interaction between the solute and the water molecules in the solvent is referred to as hydration. The positive values of hydration number increase as appreciable solvation of solutes. This is an added support for the structure promoting nature of solutes as well as the presence of dipolar interaction between the solute and water molecules. This also suggests that compressibility of the solution will be less than that of the solvent. As a result, the solutes will gain THIRUMARAN & INBAM: THERMODYNAMIC AND TRANSPORT STUDIES ON AMINO ACIDS mobility and have more probability of contacting the solvent molecules. This may enhance the interaction between solute and solvent molecules. The perusal of Table 2 shows that the values of hydration number (nH) are positive in all the systems studied and such positive values of nH indicate the appreciable salvation of solute. In the present study, it is observed that the values of hydration number Table 2 — Values of adiabatic compressibility (β) and molal hydration number (nH) in aqueous sodium acetate solution Molality m (mol.Kg−1) Adiabatic Molal hydration compressibility number β(×10−10m2N−1) (nH ×10−1) Temperature (K) 298.15 308.15 318.15 298.15 308.15 318.15 decreases in L-arginine, L-lysine systems and increases in L-histidine system with increasing molalities of the solute. However, the nH values increase with rise in temperature in all the three systems. The decreasing values of nH which indicate the increase in solute-solvent interaction and viceversa. Such a decrease in nH values with increase of molality of the solute concentration leading to the reduction in the electrostriction. This indicates the sodium acetate has a dehydration effect on the amino acids. Table 3 — Values of apparent molal compressibility (ϕk) and apparent molal volume (ϕv) in aqueous sodium acetate solution Molality m (mol.Kg-1) System – I: L-arginine 0.00 0.02 0.04 0.06 0.08 0.10 0.12 4.1052 4.1003 4.0953 4.0904 4.0854 4.0805 4.0756 4.1061 4.1011 4.0961 4.0912 4.0862 4.0813 4.0763 4.1063 4.1013 4.0963 4.0914 4.0864 4.0815 4.0765 … 3.5569 3.5514 3.5490 3.5487 3.5452 3.5418 … 3.5620 3.5565 3.5550 3.5538 3.5502 3.5469 … 3.5681 3.5625 3.5600 3.5598 3.5562 3.5529 4.1052 4.0994 4.0936 4.0877 4.0819 4.0761 4.0703 4.1061 4.1002 4.0944 4.0888 4.0831 4.0774 4.0711 4.1063 4.1002 4.0942 4.0882 4.0822 4.0762 4.0702 … 0.4000 0.3993 0.3991 0.3988 0.3982 0.3979 … 0.4010 0.3985 0.3929 0.3926 0.3920 0.3914 … 0.4131 0.4123 0.4121 0.4118 0.4112 0.4108 … 4.8631 4.9486 5.0273 5.0635 5.1110 5.1119 … 4.8806 4.9667 4.9916 5.0418 5.1302 5.1363 0.02 0.04 0.06 0.08 0.10 0.12 4.1052 4.0992 4.0931 4.0870 4.0808 4.0744 4.0680 4.1061 4.1000 4.0937 4.087 4.0809 4.0743 4.0683 4.1063 4.1002 4.0939 4.0877 4.0812 4.0744 4.0680 … 4.8483 4.8688 4.9043 4.9192 4.9668 4.9944 3.6835 3.6815 3.6794 3.6774 3.6753 3.6733 3.6951 3.6930 3.6909 3.6889 3.6868 3.6848 3.7082 3.7061 3.7040 3.7020 3.6999 3.6978 29.2841 29.2842 29.2843 29.2844 29.2845 29.2846 29.4606 29.4607 29.4608 29.4609 29.4610 29.4611 29.6716 29.6717 29.6718 29.6719 29.6720 29.6721 44.2686 44.2687 44.2689 44.2691 44.2692 44.2694 44.5857 44.5858 44.5860 44.5862 44.5863 44.5865 49.2343 50.4690 51.7037 52.3211 53.1853 52.5268 49.5870 50.8305 51.2450 52.0741 53.5663 53.7321 System – II: L-lysine System – III: L-histidine 0.00 0.02 0.04 0.06 0.08 0.10 0.12 Apparent molal Apparent molal compressibility volume –ϕk(×10-8 m2 N−1) −ϕv(×m3 mol−1) Temperature (K) 298.15 308.15 318.15 298.15 308.15 318.15 System – I: L-arginine System – II: L-lysine 0.00 0.02 0.04 0.06 0.08 0.10 0.12 455 0.02 0.04 0.06 0.08 0.10 0.12 4.7321 4.7291 4.7261 4.7230 4.7200 4.7170 0.02 0.04 0.06 0.08 0.10 0.12 5.0298 5.0466 5.0901 5.1101 5.1768 5.2133 4.7516 4.7354 4.6928 4.6899 4.6869 4.7277 4.8530 4.8498 4.8466 4.8434 4.8402 4.8370 44.0036 44.0038 44.0039 44.0041 44.0042 44.0044 System – III: L-histidine 5.0521 5.1499 5.2474 5.2943 5.3611 5.3037 5.0778 5.1763 5.2067 5.2709 5.3890 5.3988 48.9395 49.1849 49.7577 50.0440 50.9031 51.3941 Table 4 — Values of limiting apparent molal compressibility (ϕV0), Limiting apparent molal volume (ϕk0), and their constants SK, SV and A and B parameters of Jones-Dole equation Amino Acids L-arginine L-lysine L-histidine L-arginine L-lysine L-histidine Limiting apparent molal compressibility ϕk0 /(×10−8 m2 N−1) 298.15 308.15 318.15 –3.69 –4.74 –4.88 –3.70 –4.77 –4.87 –3.71 –4.86 –4.84 Constant SK / (×10−8 N–1 m–1 mol–1) 298.15 308.15 318.15 Limiting apparent molal volume ϕk0/(× m3⋅mol–1) 298.15 308.15 318.15 6.36 7.83 –9.00 –29.29 –44.02 –46.95 5.92 2.24 –1.40 5.62 8.50 –1.60 –29.47 –44.28 –46.89 –29.68 –44.60 –46.53 Constant, SV / (N–1 m–1 mol–1) 298.15 308.15 318.15 A (× dm−3/2 m-1/2) 298.15 308.15 318.15 B (×dm3 mol−1) 298.15 308.15 318.15 0.0475 0.0726 –12.08 -0.2123 0.0115 -0.5748 1.0572 0.3131 0.2852 0.0476 0.0711 –18.32 0.0461 0.0725 –20.77 -0.2744 -0.0002 0.0004 -0.3465 0.0003 0.0002 1.3208 0.3544 0.2373 1.7017 0.5823 0.2908 456 INDIAN J PURE & APPL PHYS, VOL 49, JULY 2011 Fig. 2 — Variation of molar hydration number with molality Fig. 1 — Variation of adiabatic compressibility with molality The values of apparent molal compressibility (φK) and apparent molal volume (φV) are presented in Table 3. The following observations have been made from apparent molal compressibility (φK) and apparent molal volume (φV) of L-arginine, L-lysine and L-histidine in aqueous sodium acetate solution at different temperatures are: (i) The values of the (φK) and (φv) are all negative over the entire range molality of amino acids. (ii) The (φk) values are increasing with increasing molality of the solute in L-arginine, L-lysine systems, whereas a reverse trend is observed in L-histidine. (iii)The (φv) values decrease with increasing molality of solute in all three systems. (iv) However, both (φK) and (φV) decrease with rise in temperature in all the three systems studied. (v) The maximum values of apparent molal compressibility (φK) as well as apparent molal volume (φV) are obtained for L-arginine system, which suggests electrostriction and hyperphilic interactions occurring in these systems, thereby, indicating the presence of solute-solvent interactions. (vi) From the magnitudes of (φK) and (φV), the molecular association between the three systems of amino acids are of the order: L-arginine > L-lysine > L-histidine. All the above observations clearly suggest that the negative values of (φK) indicate ionic, dipolar and hydrophilic interactions occurring in these systems. Since more number of water molecules are available at lower concentration of sodium acetate, the chances for the penetration of solute molecules into the solvent molecules are highly favoured. The increasing values of (φK) in the concerned systems reveal that less strengthening in solute-solvent interactions existing in these mixtures. THIRUMARAN & INBAM: THERMODYNAMIC AND TRANSPORT STUDIES ON AMINO ACIDS Fig. 3 — Variation of apparent molal compressibility with molality Further, the negative values of (φV) in all the systems indicate the presence of solute-solvent interactions. The decreasing value of (φV) is due to strong ion-solvent interaction and vice-versa. The negative value of (φV) indicates electrostrictive salvation of ions22. From the magnitude of (φV), it can be concluded that the strong molecular association is found in L-arginine mixtures than other two systems and hence, L-arginine is a more effective structuremaker than other two amino acids. The Limiting apparent molal compressibility (ϕk0) values provide information regarding the solutesolvent interaction and its related constant (SK) of the solute-solute interaction in the solution, which are presented in Table 4. The ϕK0 values are negative in all the systems and decrease with rise in temperature. Such a negative values of ϕK0 for all the systems reinforce the earlier view that the existence of solutesolvent interactions. 457 Fig.4-Variation of apparent molal volume with molality Fig. 5 — Variation of limiting apparent molal compressibility with molality The values of SK exhibit both positive and negative values and vary non-linearly with rise of temperature. This behaviour shows that the existence of ionion/solute-solute interactions in all the three systems. It is well known that solutes which are causing electrostriction lead to decrease in the compressibility 458 INDIAN J PURE & APPL PHYS, VOL 49, JULY 2011 of the solution. This is reflected in the negative values of φK of amino acids in aqueous sodium acetate solution. Hydrophilic solutes indicate negative compressibility as well as ordering of solutes which is introduced by them in water structure23. Table 4 presents the values of Limiting apparent molal volume (ϕV0) and its related constant SV, which exhibit negative values in all the three systems studied. Possession of positive values suggests the presence of strong solute-solvent interactions and vice-versa. Further, the ϕV0 values decrease with rise of temperature. The decreasing trend is due to disruption of side group hydration by that of the charged end. The decrease in ϕV0 may also be attributed to the increased hydrophilicity/polar character of the side chain of the amino acids. Table 4 presents the positive values of SV in L-arginine, L-lysine mixtures which indicate the presence of strong solute-solute interaction, whereas the negative value of SV in L-histidine predicts the weak solute-solute interaction. Further, these values indicate the induced effect of sodium acetate on the solute-solute interaction, which have the possibility of both the increasing polar part of the amino acids and dependence of the behaviour of sodium acetate on the concentration in aqueous medium. In the present amino acid systems, it may be presumed that the interactions may be taking place as: (1) Ion-dipolar/hydrophilic group interactions between the ions of sodium acetate (Na+,CH3 COO−) and (NH3+,COO), (−OH) group of amino acids19. (2) Ion-hydrophilic group interaction between the ions of sodium acetate and polar parts of amino acids. (3) Hydrophilic-ionic interaction between the Na+, COO− group of sodium acetate and Zwitterionic centres of the amino acids. (4) Hydrophilichydrophobic interaction between the Na+, COO− group of sodium acetate molecules and OH of the amino acids. Viscosity is an important parameter in understanding the structure as well as molecular interactions occurring in the solutions. From Table 1, it is observed that the values of viscosity increase with increase in solute concentration in all the systems. This increasing trend indicates the existence of molecular interaction occurring in these systems. The role of viscosity B-coefficient has also been obtained. From Table 4, it is observed that the values of A are positive as well as negative and B-coefficients are positive in all systems. Since A is a measure of ionic interaction24 it is evident that there is a weak as well as strong ion-ion interactions present in the liquid mixtures. The B-coefficient which is known for measure of order or disorder introduced by the solute in the solvent. It is also a measure of solutesolvent interaction. The behaviour of B-coefficient in all the amino acids suggests the existence of strong solute-solvent interactions. The magnitude of B-values is higher in L-arginine which clearly confirms the amino acid L-arginine is acting as effective structure-maker in aqueous sodium acetate solution. Similar trends of interaction studies studied for other amino acids in aqueous sodium acetate solution have been reported earlier19, which supports the present investigation. From the magnitude of B-coefficient, it can be concluded that the molecular interactions between the amino acids are of the order L-arginine > L-lysine > L-histidine. The above conclusion is an excellent earlier agreement with that drawn from φk and φv data. 5 Conclusions It may be concluded that the existence of ionsolvent or solute-solvent interactions resulting in attractive forces which promote the structure-making tendency, while ion-ion or solute-solute interactions resulting in dipole-dipole, dipole-induced dipole and electrostrictive forces which enhance the structurebreaking properties of amino acids. And eventually, by analysing all the evaluated parameters which clearly suggest that L-arginine is a strong structure maker in aqueous sodium acetate solution over the other two amino acids. Hence, in the present study the molecular interaction follows the order: L-arginine > L-lysine > L-histidine. It is also noticed that the strength of the molecular interaction weakens with rise of temperature which may be due to weak intermolecular forces and thermal dispersion forces. Ultrasonic velocity, density and viscosity have been measured for three amino acids viz, L-arginine, L-lysine and L-histidine in aqueous sodium acetate solutions at 298.15, 308.15 and 318.15 K, which have biological and biochemical relevance. The dipolar (zwitterions) characteristics of these organic liquid molecules shed light on solute-solvent interactions in aqueous sodium acetate mixtures which proved to be the most interesting due to its univalent character. There is much scope for further studies in these systems by varying pH of the solution and temperature which may reveal more about hydrogen bonding interaction as well as other interaction existing between solute-solvent molecules. Hence it is THIRUMARAN & INBAM: THERMODYNAMIC AND TRANSPORT STUDIES ON AMINO ACIDS evident that the ultrasonic velocity measurement in the given medium serves as a powerful probe in characterising the physico-chemical properties of that medium. 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