Indian Journal of Chemistry Vol. 39A, October 2000, pp. 1011-1018 Partial molal volumes of transfer of some amino acids from water to aqueous 1,4-dioxane solutions at298.15 K T S Banipal & Gagandeep Singh Department of Pharmaceutical Sciences, GuruNanak Dev University, Amritsar 143005, India B S Lark Departmen\ of Chemistry, Guru Nanak Dev University, Amritsar 143 005, India Received 25 November 1999; revised 23 February 2000 Apparent molal volumes of glycine, DL-n-alanine, L-valine, L-Ieucine and L- phenylalanine in water and in 2.5, 5, 10, 20,25% aqueous solutions of l,4-dioxane have been obtained from densities of their solutions at 298.15 K measured by using precise vibrating tube digital ~ensitymeter. The estimated partial molal volumes (V 2 0 ) at infinite dilution have been used to obtain corresponding transfer volumes (V 2.. tr ) from water to different l,4-dioxane-water mixtures which have been found to be positive for glycine, and both positive and negative for the remaining amino acids over the l,4-dioxane concentration range studied. The data have been interpreted in terms of solute-cosolute interactions. Concentration dependent behaviour of l,4-dioxane in aqueous solutions have also been taken into account while interpreting the V 2,tr values of amino acids. Denaturation of globular proteins in aqueous solutions is a fundamental biological process which till date is not completely understood and continues to be a subject of extensive investigations 1-6. During the denaturation process, various structural changes occur in protein solutions. The knowledge of solute-solvent and solute-solute interactions in various solvents is thus, prerequisite to understanding the process of denaturation. As the proteins are large complex molecules, direct study of their interactions is a difficult task. One useful approach to understand these interactions is to study their model compounds. Consequently, in recent years, there has been considerable interest in the determination of various thermodynamic properties of amino acids, small peptides and their derivatives in pure aqueous and. d I' 4-11 IDlxe aqueous so uttons. Mixed aqueous solvents are used extensively in chemistry and other fields to control factors like stability, reactivity and solubility of systems 9 1O 1,4- Dioxane isa widely used solvent because it is a nonhydrogen bonded cyclic ether, miscible with water in all proportions with a boiling point close to that of water. Moreover, its dipole moment is close to zero (0.45 D) and the dielectric constant can be varied over a wide range (2.2-78) in its mixtures with water 12 In the present paper, the apparent molal volumes of glycine, DL-a.-alanine, L-valine, L -leucine and L - phenylalanine in water and in 2.5, 5, 10, 20 and 25% aqueous solutions of 1,4-dioxane have been determined from density measurements at 298.15 K. The data have been used to estimate partial molal volumes of transfer for amino acids from water to aqueous 1,4-dioxane solutions- and assess the relative domination of various contributing factors. Materials and Methods Five amino acids, namely, glycine (0-7126), DL - a.-alanine (A-7502), L -valine (V-0500), L -leucine (L -8000) and L -phenylalanine (P-2126) of highest purity were obtained from Sigma Chemical Co. and used as such without further purification. l,4-dioxane (CDH,AR) was refluxed and then distilled over sodium metal using aim long fractionating glass column. The middle fraction distilling at 373 K was collected for use. Solutions of amino acids were prepared in deionized, doubly distilled water and : degassed before use. All the solutions were made on weight basis and all the weighing were done on the Mettler Balance with an accuracy of ± 1 x 10'"' g. Densities of the solutions were measured with a
1012 INDIAN J CHBM. SBC. A. OCTOBBR. 2000 Table l--densities (d) and apparent molal volumes (tv) of some amino acids in water andin aqueous 1.4-dioxane mixtures at 298.15 K m, mol kg-l. d,kg.m- 3 +,xlo'.m 3 mor 1 m,molkg-l d,kg.m- 3 +,xlo'.m 3 mor 1 Glycine in waler Glycine in 25% aqueous 1.440JUllle 0.1026 1000.292 43.33 0.0808 1020.555 44.29 0.1496 1001.749 43.45 0.2331 1025.109 44.41 0_2144 1003.737 43.61 0.3117 1027.436 44.42 0.2982 1006.280 43.74 0.3863 1029.612 44.47 0.4076 1009.516 43.96 0.4711 1032.058 44.53 0.4668 1011.238 44.08 0.6217 1036.320 44.65 0.5006 1012.213 44.18 0.7863 1040.903 44.75 Glycine in 2.5% aqueous 1.4-dioJUllle DL-a-Alanine in waler 0.1038 1002_615 43.29 0.0799 9993.30 60.46 0.1905 1005.330 43.32 0.0961 9997.91 60.48 0.2470 1007.072 43.39 0.1345 1000.872 60.52 0.2894 1008.383 43.40 0.1708 1001.875 60.62 0.3586 1010.483 43.49 0.2585 1004.283 60.72 0.3705 1010.848 43.49 0.3386 1006.466 60.79 0.4912 1014.476 43~8 DL-a-Alanine in 2.5% aqueous 1.4-dioXQlle Glycine in 5% aqueous 1.4-dioXQlle 0.1069 1002.381 60.37 0.21 78 1008.161 43.53 0.1492 1003.576 60:37 0.2947 1010.505 43.62 0.1548 1003.707 60.38 0.4089 1013.938 43.73 0.2174 1005.460 60.40 0.4832 1016.171 43.75 0.2275 1005.741 60.41 0.5572 1018.354 43.80 0.2559 1006.551 60.44 0.7165 1022.954 43.96 0.2968 1007.653 60.46 Glycine in 10% aqueous l,4-dioxqlle 0.0854 1008.233 43.50 0.2248 1012.545 43.60 0.4173 1018.332 43.82 0.5115 1021.107 43.90 0.5150 1021.227 43.87 0.7045 1026.685 44.07 0.3323 1008.624 60_49 0.4184 1010.954 60.55 0.4972 1013.075 60.63 DL-a-Alanine in 5% aqueous l,4-dioxqlle 0.1141 1004.447 60.46 0.2178 1007.345 60.49 0.2773 1008.971 60.58 0.3393 1010.666 60.60 Glycine in 20% aqueous l,4-dioxqlle 0.3930 1012.111 60.64 0.1210 1017.429 43.82 0.5538 1016.369 60_75 0.1812 1019.261 43.90 DL-a-Alanine in 10% aqueous 1.4-dioXQlle 0.2469 1021.214 43.97 0.0699 1007.711 60.47 0.2945 1022.659 44.03 0.1131 1008.919 60.53 0.3808 1025.201 44.14 0.1636 1010.351 60.38 0.4227 1026.402 44.24 0.2230 1012.013 60.36 0.6278 1032.258 44.46 0.2686 1013.262 60.41 Contd
BANIPAL et al.: PARTh\L MOLAL VOLUMES OF AMINO ACIDS 1013 Table l-oensities (d) and apparent molal volumes (+.. ) of some amino acids in water and in aqueous l,4-dioxane mixtures at 298.15 K-Contd '. m, mol kg-l d,kg.m 3... xl0 6, m 3 morl m, mol kg-i d,kg.m- 3 +..xlff, m 3 morl 0.3009 1014.186 60.30 0.0556 1007.068 90.28 0.3912 1016.626 60.40 0.0741 1007.562 90.15 0.5515 1020.893 60.49 0.0832 1007.801 90.14 DL-a-Alanine in 20% aqueous l,4-dioxane 0.1205 1008.810 89.85 0.1204 1017.430 60.82 L-Valine in JO%aqueous 1.4-dioxane 0.1495 1018.230 60.79 0.0156 1006.197 89.71 0.2169 1020.070 60.77 0.0278 1006.528 89.63 0.3133 1022.650 60.83 0.0400 1006.859 89.57 0.3344 1023.193 60.80 0.0565 1007.307 89.47 0.3981 1024.874 60.91 0.0644 1007.526 89.41 0.5659 1029.203 61.01 0.0724 1007.747 89.31 DL-a-Alanine in 25% aqueous l,4-dioxane 0.0882 1008.186 89.18 0.0910 1020.489 61.99 L-Valine in 20% aqueous 1.4-dioxane 0.1399 1021.760 62.02 0.0278 1014.579 89.14 0.2492 1024.562 62.08 0.0513 1015.202 89.30 0.2978 1025.785 62.13 0.0597 1015.424 89.36 0.3443 1026.953 62.15 0.0738 1015.796 89.39 0.4936 1030.635 62.23 0.0783 1015.906 89.49 L-Valine in water 0.0869 1016.131 89.51 0.0395 9980.97 90.63 0.1118 1016.770 89.64 0.0493 9983.54 90.64 L-Valine in 25% aqueous 1.4-dioxane 0.0696 9988.90 90.64 0.0232 1018.513 90.10 0.0747 9990.12 90.81 0.0414 1018.975 90.24 0.0831 9992.40 90.70 0.0526 1019.255 90.33 0.0872 9993.46 90.71 0.0638 1019.525 90.48 0.1268 1000.372 90.76 0.0749 1019.801 90.57 0.0861 1020.074 90.64 0.0624 L-Valiru: in i5%aqueous 1.4-dioxane 0.0932 1020.246 90.69 1000.932 90.74 0.1003 1020.418 90.74 0.0721 1001.190 90.69 0.0935 1001.745 90.71 0.1135 1002.241 90.84-0.1313 1002.732 90.66 0.1372 1002.875 90.66 0:1868 1004.153 90.68 L-Leucine in water 0.0224 9975.74 107.83 0.0440 9980.78 107.91 0.0641 9985.39 108.01 0.0703 9986.78 108.05 0.0967 9992.74 108.15 L-Valine in 5% aqueous 1.4-dioxane 0.1440 1000.322 108.34 0.0119 1005.913 90.63 L-Leucine in 2.5% aqueous J.4-dioxane 0.0358 1006.541 90.47 0.0233 9998.76 107.74 0.0483 1006.873 90.37 0.0362 1000.173 107.84 Contd
1014 INDIAN J CHEM, SEC. A, ocrober 2000 Table l-densities (d) and apparent molal volumes (~) of some amino acids in water and in aqueous l,4-dioxane mixtures at 298.15 K-Contd m, mol kg 1 d, kg.m- 3 cl>v x 10 6, m 3 mor l m, mol kg-i d, kg.m- 3 ~ xl0 6, m 3 mor l 0.0476 1000.434 107.89 0.0206 9979.33 122.4 0.0604 1000.731 107.88 0.0256 9981.40 122.5 0.0703 1000.959 107.88 L-Phenylalanine in 2.5% aqueous l,4-dioxane 0.0913 1001.442 107.86 0.0098 9997.42 122.2 0.1175 1002.046 107.82 0.0122 9998.27 122.2 L-Leucine iii 5% aqueous l,4-dioxane 0.0148 9999_40 122.2 0.0164 1000.009 122.2 0.0177 1006.140 108.01 0.0181 1000.081 122.3 0.0394 1006.633 107.88 0.0240 1000.330 122.3 0.0522 1006.930 107.76 0.0606 1007.123 107.69 L-Phenylalanine in 5% aqueous l,4-dioxane 0.0725 1007.402 107.58 0.0084 1001.701 124.1 0.0754 1007.466 107.61 0.0111 1001.812 124.2 0.1155 1008.420 107.20 0.0169 1002.046 124.3 L-Leucine in 10% aqueous 1,4-dioxane 0.0239 1006.163 106.21 0.0292 1006.290 106.28 0.0221 1002.254 124.4 0.0274 1002.462 124.5 L-Phenylalanine in 10% aqueous 1,4-dioxane 0.0368 1006.474 106.32 0.0105 1005.985 120.8 0.0518 1006.830 106.42 0.0141 1006.141 120.9 0_0526 1006.841 106.42 0.0195 1006.374 121.0 0.0780 1007.440 106.67 0.0210 1006_441 121.1 L-Leucine in 20% aqueous 1,4-diomne 0.0200 1015.160 106.43 0.0400 1015.621 106.61 0.0616 1016.113 106.72 0.0762 lot6.432 106.92 0.0806 1016.847 107.03 0.0274 1006.715 121.2 0.0352 1007.040 121.4 L-Phenylalanine in 20% aqueous l,4-dioxane 0.0072 1014.007 120.8 0.0085 1014.188 120.8 0.0117 1014.325 121.0 0.0138 1014.415 121.0 L-Leucine in 25% aqueous 1,4-dioxane 0.0154 1014.483 121.0 0.0130 1018.213 106.77 0.0256 1015.108 121.3 0.0190 1018.348 106.84 0.0303 1015.307 121.3 0.0287 1018.567 106.89 0.0361 1015.549 121.5 0.0379 1018.774 106.96 0.0519 1016.149 121.8 0.0493 1019.023 107.07 L-Phenylalanine in 25% aqueous l,4-dioxane 0.0663 1019.394 107.17 0.0095 1018.294 121.3 0.0789 1019.666 107.26 0.0101 1018.316 121.6 0.1165 1020.513 107.41 0.0164 1018.583 121.4 L-Phenylalanine in water 0.0204 1018.751 121.4 0.0049 9972.62 121.7 0.0228 1018.848 121.6 0.0087 9974.24 121.9 0.0240 1018.902 121.5 0.0099 9974.79 121.9 0.0404 1019.519 121.7 0.0149 9976.92 122.0 0.0484 1020.092 121.2
BANIPAL et al.: PARTIAL MOLAL VOLUMES OF AMINO ACIDS 1015 vibrating-tube digital densitymeter (Model DMA 60/602 Anton Paar, Austria). The details of its principle and working have been described elsewhere!3. Temperature of water flowing through the densitymeter cell was controlled within ± 0.01 K using an efficient temperature bath (Heto BirkerodlDenmark). The densitymeter was calibrated with dry air and water and all the measurements were made relative to pure water. The working of densitymeter was checked by measuring the densities of aqueous sodium chloride solutions and an excellent agreement was found with literature values 13,l4. The precision and accuracy of density measurements were better than lxlo-6, 3x 10-6 g cm 3 respectively. Results and Discussion Densities of solutions of amino acids in water and in 2.5, 5, 10, 20 and 25% (v/v) aqueous 1,4-dioxane solutions at 298.15 K as a function of molality are given in Table 1. The apparent molal volume of a solute in a given solvent with density, tk, (Table 1) was calculated from the equation q,y = MId -[(d-tk)i000/mddoj... (1) where M is the molecular mass of the solute, m is molality (mol kg l of solvent) of the solution having density d. Apparent molal volume of a solute at infinite dilution (q,. ') is its corresponding partial molal volume, i.e, V 2 0, and, in the cases, where q,y values showed negligible concentr~tion dependence, it was estimated to be the average of various q,y values. For most of the cases q,y data showed concentration dependence and corresponding V 2 0 values were determined by the least squares fitting of the data to Eq.2, - 0 q,y= V 2 + Sym... (2) where Sv is the slopels. The V 2 values in water and in aqueous 1,4-dioxane solutions have been summarised in Table 2 along with the literature values of V 2 in water. Good agreement between the experimental. and literature values has been observed7, 16-20. Partial molal volume of transfer of a solute from water to aqueous 1,4-dioxane solutions at infinite dilution (V 2,tr ) has been estimated as follows, V -o 2,tr = V02 (in aqueous 1,4-dioxane solution) - V02 (in water)... (3) Values of V \tr are illustrated in Fig. 1. V 2,tr values are both positive and negative over the concentration range of 1,4-dioxane studied except for glycme. V \tr values are positive for all the amino acids in 2.5 and 5% aqueous l,4-dioxane solutions Table 2-Partial molal volume at infinite dilution ( V. 2 ) of some amino acids in water and aqueous 1,4-dioxane solutions at 298.15 K. Compo V02X 106,m 3.mor l water 2.5% 5% 10% 20% 25% Glycine 43.15 43.19 43.37 43.41 43.66 44.23 (2.03", 0.03,,) (0.823", 0.02,,) (0.81",0.02,,) (0.93, 0.02,,) (1.28", 0.02") (0.65",0.02,,) 43.20",43.19 d, 43.14", 43.22 f DL-a-Alanine 60.36 60.25 60.39 60.42 60.71 60.85 (1.33", 0.02,,) (0.738", 0.02,,) (0.66",0.04,,) (0.49",0.03,,) (0.94",0.04") 60.50<1, 60.62", 60.39' L-Valine 90.70 90.71 90.71 89.85 88.99 89.90 (-7.18", 0.03") (-7.16", 0.03,,) (5.94",0.02,,) (8.54", 0.02") 90.78 d, 90.98" 90.8aI' L-Leucine 107.74 107.84 108.91 106.02 106.27 106.72 (4.20",0.01,,) ( 8.31",0.03,,) (8.08", 0.02,,) (8.05", 0.03,,) (6.32', 0.04,,) 107.74 d, 107.96" L- 121.49 122.26 123.95 120.49 120.72 121.46 Phenylalanine (40.70",0.04,,) (21.07", 0.03,,) (27.02 8, 0.03,,) (20.60", 121.48 d 0.02,,) Slope, b standard deviation, ~ef. 7; dref. 16; "Ref. 17; fref. 18; iref. 19; ~ef. 20
1016 INDIAN J CHEM, SEC. A, OCTOBER 2000 3.0r---------------. 1 ~ Glycine 2.5... 0.'. 2 ---b--- DL-oG -Alanine 2.0 3 -'-0-.- L - Vali ne 1.5. ~-.. + -L-Leucine \. 5- -0.. L -Phenylalanine 1.0 /0 0.5.!:;.. 0::.-. ON I> -0.5-1.0-1.5-2.0-2.5 L...---5..1.----L10---'15---2'-O--2SJ.--.J -4 of 1,4'- Dioxane Fig. 1-V 2.tI versus % of l,4-dioxane with the exception of very small negative values for DL-a-alanine in 2.5% aqueous l,4-dioxane solution. The magnitude of ifo 2 tr decreases from glycine to DLa-alanine in 2.5 and 5% and then increases regularly from DL-a-alanine to L -valine to L -leucine to L - phenylalanine. In 10, 20 and 25% aqueous 1,4- dioxane solutions, if \tr values are positive for glycine and DL-a-alanine (the magnitude is higher for glycine) and negative for L -valine, L -leucine andl - phenylalanine. To rationalise the behaviour of amino acids in aqueous l,4-dioxane solution, it will be worthwhile to first discuss the behaviour of 1,4-dioxane in aqueous solution. From heat capacity and </lv data Kiyohara et a1. 2! have reported that 1,4-dioxane behaves like a hydrophobic solute in highly concentrated region (5m) whereas in lower concentration range (lm) its interaction with water is not so strong and does not involve any structural changes. Kay and Broadwater 22 have reported that dipolar nature of l,4-dioxane interferes with the process by which hydrocarbon segments of apolar molecule bring about an enhancement of long range order in aqueous mixtures. Gancl 3 considered it only as inert while Harnann 24 showed that it is not essentially inert. From it's positive compressibility of electrostriction, Lo Surdo et al.25 have concluded that 1,4 dioxane is only slightly hydrated in aqueous solution. From viscosity data Lark et al.!9 have indicated that l,4-dioxane is a very weak structure-breaking solute. Tada et al.26 from conductance measurements have suggested that l,4-dioxane is a structure breaker. Consequently it appears that, at small percentages, it might be behaving as a weak structure breaker and at higher percentages as a hydrophobic solute.. Now to explain partial molal volume data, different models have been used. Franks et al. 27 have shown that the partial molal volume of a non-electrolyte is a combination of the intrinsic volume of non-electrolyte. and volume due to it's interaction with the solvent. The intrinsic volume has been considered to be made up of two types of contributions. V int = Vvw + V void Shahidi et al.28 modified this equation to include the contribution of interactions of a non-electrolyte solute with the solvent. where O's is the shrinkage in volume produced by the interactions of hydrogen bonding groups present in the solute with water molecules and n is the potential number of hydrogen bonding sites in the molecule. For electrolytes and zwitterionic solutes the shrinkage is caused by electrostriction and finally if 2 can be evaluated as It has been assumed 29 that Vvw and V void have the same magnitude in water and in mixed solvents for the same class of compounds. Therefore the observed positive if 2.tr values can be attributed to the decrease in volume of shrinkage and negative if 2.tr values to the increase in volume of shrinkage in the presence of 1,4-dioxane. This may be attributed to various types of interactions occurring between amino acids and the 1,4-dioxane molecules having different contributions to if\tr. if 2.tr values can further be rationalised by cosphere overlap model developed by Gurney30 and. Frank and Evans 3!. According to this; properties of water molecules in the hydration cosphere depend on the nature of solute molecules. When two solute. particles come close enough such that their cospheres overlap, some of the cosphere material is displaced and this is accompained by changes in thermodynamic parameters. The following types of
BANIPAL et al.: PARTIAL MOLAL VOLUMES OF AMINO ACIDS 1017 interactions are possible: (i) ion-dipolar interactions occurring between zwitterionic centres of amino acids and diplolar parts of 1,4-dioxane (ii) hydrophobicdipolar interaction between non-polar parts of amino acids and dipolar part of 1,4-dioxane, and, (iii) hydrophobic - hydrophobic interactions occurring between non-polar parts of amino acids and hydrophobic parts of 1,4-dioxane. According to the cosphere overlap model, the first. type of interactions results in positive volumes of transfer whereas the other two result in negative volumes of transfer. Therefore, the presently obtained positive values for glycine and DL-a-alanine,(except in 2.5% aqueous 1,4-dioxane solution), L-valine, L leucine and L-phenylalanine upto == 5% aqueous 1,4- dioxane solutions show that the first type of interactions (due to ion-dipolar) dominate over the other two types. The negative values for L-valine, L leucine and L-phenylalanine (due to increasing hydrophobicity) at higher, (10, 20 and 25%) concentration range suggest that the second and third types of interactions overweigh the eff~t of the first. This is in line with the earlier conclusion drawn on the basis of volume of shrinkage that various solutecosolute interactions occur in these systems which contribute to different extents, depending upon the particular amino acid and 1,4-dioxaneconcentration. Keeping in view the concentration dependent behaviour of 1,4-dioxane in water the volumes of transfer of amino acids from water to aqueous 1,4- dioxane solutions can further be rationalised. The Y \tr values observed from increasing positive glycine to L-phenylalanine (except very small negative value in the case of DL-a-alanine at 2.5% ) in the lower percentage region, i.e., up to == 5% of aqueous 1,4-dioxane may be attributed to contributions from ion-dipolar interaction whereas the increasing hydrophobic hydration may be attributed to the increase of non-polar side chain in the amino acids. The negative y0 2,tr volume of the order of - O.l1x 10-6 m 3 mor l, in the case of DL-a.-alanine at 2.5% of 1,4-dioxane, seems to be a little out of step. This difference is of the order of uncertainty in the measurement of cj)... However it may arise due to weak structure breaking effect of 1,4-dioxane in this range coupled with weak structure making propensity of DLa-alanine as compared to glycine which is a structure breaker. The negative Y \tr values ill the higher concentration range (l0, 20, 25%) of aqueous 1,4-. dioxane solutions may be resulting from the dominating effect of the hydrophobic-hydrophobic interactions which contribute negatively to Y \tr in the case of L-valine, L-Ieucine and L-phenylalanine due to the hydrophobic nature of 1,4-dioxane. The positive Y \tr value in the case of glycine and DL-a-. alanine may certainly be due to the dominating effect of charged end groups of amino acids. Sv values for L-valine in 5 and 10% aqueous 1,4- dioxane solution and for L-Ieucine in 5% aqueous 1,4- dioxane solution are negative which become positive at higher concentrations. These values indicate the 1,4-dioxane induced effect on the solute-solute interactions which have the bearing of both the increasing non-polar part of the amino acids and the dependence of the behaviour of 1,4-dioxane on the concentration in aqueous medium. This fwther strengthens the view that behaviour of 1,4-dioxane is concentration dependent in aqueous medium as change in slopes again occurs in the region of approximately 10% 1,4-dioxane (== 1m) solutions. Kozak et al. 3l have proposed a formalism based on the McMillan-Mayer theory of solutions that permit the formal separation of effect due to interactions between pairs of solute molecules and those due to interactions involving three or more solute molecules. This approach has further been discussed by Friedman & Krishnan 33 and Frank et al. 31 in order to include solute-cosolute interactions in the solvation sphere 2934 d th and used by vanous workers' to stu y e interactions of the amino acids and cosolutes in aqueous medium. According to this treatment, a thermodynamic transfer function at infinite dilution -0 e.g. V 2,tr can be expressed as - 2 3 4 V \tr = 2 V AB mb + 3 V ABB m B + 4 V ABBB m B +... where A stands for solute and B for cosolute. V AR, V ABB, V ABBB are respectively the pair, triplet, quartet interaction coefficients corresponding to a particular thermodynamic property and fib is the molality of cosolute (l,4-dioxane in present case). Y\tr data have been fitted into the above equation to obtain V AB, V ABB and V ABBB interaction coefficients which are given in Table 3. V AB is positive for glycine which. becomes negative for DL-a-alanine and decreases up. to L-Ieucine and becomes positive again for L phenylalanine. Unlike V AB, the triplet interaction coefficient V ABB is negative for glycine, and is positive for DL-a-alanine and again becomes negative and decreases continuously up to L-phenylalanine.
1018 INDIAN J CHEM, SEC. A, OCTOBER 2000 Table 3--Interaction coefficients for various amino acids at 298.15 K in aqueous I A.. dioxane solutions obtained from Eq. (4) Compo V AD V ABB V ADBB Glycine 0.1958-0.0676-0.0107 DL-a-AJanine -0.0475 0.0409-0.0043 L-Valine -0.0743-0.1930-0.0354 L.Leucine -0.1640-0.1388-0.0275 L-Phenylalanine 1.2200-0.6745-0.0888 These trends indicate that various types of interactions are contributing to different extents. It may further be seen that the positive contribution of V AB to V \tr in case of L-phenylalanine in the lower concentration region of l,4-dioxane is very prominent which may be attributed to the hydrophobic hydration of the large non-polar group, i.e., phenyl group. In case of L leucine and L.. valine positive contribution is smaller in the lower concentration region which indicates that the disruption of hydration spheres around the charged centres is more pronounced than the hydrophobic hydration. This again supports the previous view that the effect of charged end groups is dominant in the case of glycine and DL.. a.. alanine. Negative values of higher order coefficients suggest that the negative contribution increases with increase in the l,4-dioxane concentration. References I Creighton T E, Protein folding (W H Freeman & Co, New York) 1992. 2 Lapanze S, Physicochemical aspects of protein denaturation, (Wiley, New York) 1978. 3 Lilley T H, Biochemical thermodynamics, edited by M N Jones, (Elsevier, Amsterdam) 1998, Ch. I. 4 Hedwig G R & Hoiland H, J chem Thermodyn, 23 (1991) 1029; 25 (1993) 349., 5 Hedwig G,R, Pure appj Chem, 66, 3 (1994) 387. 6.Hedwig G.R & Hoiland H, Biophys Chem, 49 (1994) 175. 7 Chalikian T V, Sarvazyan A P & Breslauer K J, J phys Chem, 97 (1993) 13017. 8 Bhat R & Ahluwalia J C, J phys Chem, 99 (1985) 1099. 9 Wadi R K & Ramasami P, J chem Soc Faraday Trans, 93 (1997) 243. 10 Banipal T S & Sehgal G, Thermochim Acta, 262 (1995) 175. 11 Banipal T S & Kapoor P, J Indian chem Soc, 76 (1999) 431. 12 Havenga E & Leaist D G, J chem Soc Faraday Trans, 94 (1998) 3353. 13 Picker P, Trembley E & Jalicoeur C, J sol Chem, 3 (1974) 377. 14 Leduc P A, Fortier J C & Desnoyers J E, J phys Chem, 7 (1974) 1217. 15 Millero F J, Chem Rev, 71(1971 )147. 16 Millero F J, Lo Surdo A & Shin C, J phys Chem, 82 (1978) 784. 17 Dipola G & Belleau B, Can J Chem, 56 (1978) 1827. 18 Lark B S & Bala K, Indian J Chem, 22A (1983) 192. 19 Lark B S, Banipal T S & Bala K, Unpublished results. 20 Duke M M, Hakin A W, Mickay R M & Preuss K E, Can J Chem, 72 (1994) 1489. 21 Kiyohara 0, Perron G & Desnoyers J E, Can J Chem, 53 (1975) 2591. 22 Kay R L & Broadwater T L, Electrochem Acta, 16 (1971) 667. 23 Gancy A B, Water and aqueous solutions, edited by R A Home, (Wiley Interscience, New York) 1965. 24 Hamann S D, J phys Chem, 67 (1963) 2233. 25 Lo Surdo A, Shin C & Millero F J, J chem engg Data, 23 (1978) 197. 26 Tada Y, Ibuki K, Tsuchihashi N & Ueno M, J sol Chem, 26 (1997) 595. 27 Franks T, Quickenden M A J, Reid D S & Watson B, Trans Faraday Soc, 66 (1970) 582. 28 Shahidi F, Farrell P G & Edwards J T, J sol Chem, 5 (1976) 807. 29 Mishra A K & Ahluwalia J C, J chem Soc Faraday Trans I, 77 (1981) 1469. 30 Gurney R W, Ionic process in solutions, (McGraw Hill, New York) 1953. 31 Frank H S & Evan M W, J chem Phys, 13 (1945) 507. 32 Kozak J J, Knight W & Kauzmann W, J chem Phys 68 (1968) 675. 33 Friedman H L & Krishnan C V, Water - A comprehensive treatise,edited by F Franks, (Plenum, New York) Vol 3, Ch. I. 34 Lark B S & Bala K, Nat Acad Sci Letters, 12 (1989) 155.