10862
J . Phys. Chem. 1993, 97, 10862-10867
Transfer Energetics of Some DNA and RNA Bases in Aqueous Mixtures of Urea and Glycerol+ Sonali Canguly and Kiron K. Kudu’ Physical Chemistry Laboratories, Jadavpur University, Calcutta 700 032, India Received: April 28, 1993; In Final Form: July 9, 1993”
Standard free energies (AGOt) and entropies ( A S O t ) of transfer of some DNA and RNA bases, viz., uracil (U), thymine (T), cytosine (C), and adenine (A) from water to aqueous mixtures of urea (UH) and glycerol (GL) have been evaluated from solubility measurements a t different temperatures. The observed AGO, and TMot vs composition profiles are complicated because of the interaction effects. Elimination of the cavity effect obtained by using scaled particle theory and effects due to dipoledipole, dipoleinduced dipole, and dispersion interactions yielded the corresponding effects as guided by the hydrophilic and hydrophobic sites of the solutes with the components of the solvent mixtures as compared to that in water. In the case of transfer entropies however, the corresponding interaction effects are further complicated by the effect of the relative structuredness of solvents. However, the overall AGO behavior, reflecting increased stabilization of DNA-RNA bases, leads us to conclude that both U H and GL induce denaturation of double-stranded nucleic acid helices.
Introduction The two strands of a DNA double helix are held together by intramolecular H bonds between nucleic acid base pairs1v2(Figure 1). Denaturation of this nucleic acid helix is reported to occur in aqueous urea (UH),3-8similar to p r o t e i n ~ , with ~ J ~ the lowering of denaturation temperature Tm,especially at higher concentrations due to separation of the two strands of the double helix. Jenrett and Greenstein: from considerable lowering of viscosity of DNA solution, and Rice and Doty,6 from effective decrease in Tm,and Blout, from lowering of molar absorptivity, arrived at the conclusion that aqueous UH is a nucleic acid denaturant. But despite all this evidence, the forces responsible for denaturation are yet to be fully understood. Moreover, unlike proteins,11-14conflicting reports are available regarding the role of glycerol (GL)8Js-18on the stability of nucleic acids. Zimmerman et al.I5from X-ray diffraction study of DNA fibers, and Lee et a1.16 from energetics of unwinding using twodimensional gel electrophoresis technique, concluded that GL, like other polyols, causes unwinding of double-stranded DNA helix due to dehydration leading to conformational change. Blout and Asadourians from molar absorptivity measurements were of the opinion that aqueous GL is without any denaturing effect on DNA helix. So, on the basis of the fact that the nucleic acid base units are largely responsible for conformationalstability of doublestranded DNA helix in water, it is expected that the knowledge of relative solvation of the linking blocks of nucleic acid strands, viz., the nucleic acid ‘bases”, should be of particular interest not only in aqueous UH but in aqueous GL as well. This is because that may reflect the stability of the nucleic acid structure in these two cosolvent systems as compared to water and also impart important information toward better understanding of denaturation mechanism. With that end in view, in the present paper we are reporting the transfer free energies (AGO,) and entropies (Mot) at 25 OC of some DNA and RNA bases, namely, uracil (U), thymine (T), cytosine (C), and adenine (A) from water to a series of aqueous mixtures of UH and GL as determined from solubility measurements at five equidistant temperatures ranging from 15-35 OC. After eliminating several effects due to cavity formation, dipoleaipole, dipole-induced dipole, and dispersion interactions based on tentative computations, the results have been discussed
Thymine
Cytosine
Adenine
Guanine
Figure 1. DNA structure.
in terms of hydrophilic and hydrophobic hydration and in the case of transfer entropiesin terms of relative solvent structuredness as well. Experimental Section The nucleic acid bases U (grade U-0750), T (grade T-0376), C (grade C-3506), and A (grade A-8626) were obtained from Sigma and were used without further pretreatment. The purities of all the chemicals were checked by UV s p e c t r ~ s c o p y and ~~-~~ were found to lie within 98-99%. UH (A.R., S.D) was used after drying under vacuum for 48 h. Purification of GL (A.R., S.D) was similar to that described earlier.21 Water used was triply distilled and C02 free. The cosolvents were prepared by mixing weighed amounts of either UH or GL and water. The method adopted for the measurement of saturated solubilities at each temperature was the same as that described p r ~ v i o u s l y . ~ ~ - ~ ~ Aliquots of each of the saturated solutions were withdrawn at 2-day intervals, diluted with water and the absorbances were measured with a Perkin Elmer Lambda 3A UV-vis spectrophotometer. The required A, and molar extinction coefficients were taken from l i t e r a t ~ r e . Saturation ~ ~ , ~ ~ was generallyattained within 7-8 days. The average uncertainties involved in the measured solubilities were about t l Z . Results
t Book of Abstracts: International Conference on Thermodynamics of
Solution and Biological Systems, New Delhi, India, Jan 3-6, 1993. 0 Abstract published in Advance ACS Abstracts, September 1, 1993.
0022-3654/93/2097- 10862%04.00/0
The observed molar solubilities (S, mol dm-’) of U, T, C, and A at different temperatures in water, UH-water, and GL-water 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10863
Transfer Energetics
TABLE I: Solubilities (S in mol dm3) of U, T, C, and A in Water and Aqueous Mixtures of UH and GL at Different Temperatures compound 15 20 25 30 35 o c Water 0.0209 0.0261 0.03 13 0.0388 0.0170 U 0.0334 0.0456 0.0234 0.0277 T 0.0207 0.0908 0.1406 0.0550 0.0682 0.0479 C 0.0185 0.0197 0.0142 0.0171 0.01 11 A 11.52 wt 5% U H U
0.0725 0.0835 0.1541 0.0228
0.0513 0.0530 0.1041 0.0156
U T C A
0.0302 0.0359 0.1085 0.0165
20.31 wt % U H 0.0386 0.0440 0.0492 0.0629 0.1266 0.1526 0.0171 0.0187
0.0488 0.07 15 0.2065 0.0203
0.0536 0.0799 0.2906 0.0238
U
0.0262 0.0308 0.1582 0.0175
29.64 wt 0.0321 0.0334 0.2399 0.0192
%UH 0.0359 0.0368 0.2786 0.021 1
0.0387 0.0457 0.2968 0.0227
0.0416 0.0735 0.3122 0.0245
0.0185 0.0279 0.2054 0.0206
36.83 wt % U H 0.0247 0.0308 0.0312 0.0356 0.2512 0.2868 0.0217 0.0229
0.0355 0.0419 0.3064 0.0244
0.0396 0.0619 0.3227 0.0268
C A
0.0198 0.0217 0.0603 0.0132
10 wt % GL 0.0230 0.0283 0.0252 0.0346 0.0724 0.0887 0.0153 0.0175
0.0342 0.0366 0.1077 0.0199
0.0418 0.0460 0.1472 0.0225
U T C A
0.0212 0.0253 0.0694 0.0163
30 wt % GL 0.0255 0.0303 0.0283 0.0333 0.0867 0.1087 0.0188 0.0209
0.0370 0.0393 0.1300 0.0224
0.0444 0.0496 0.1530 0.0239
U C A
0.0231 0.0277 0.1049 0.0259
50 wt % GL 0.0272 0.0323 0.0321 0.0380 0.1205 0.1440 0.0277 0.0309
0.0380 0.0441 0.1694 0.0359
0.0480 0.0520 0.2103 0.0458
U T C A
0.0266 0.0298 0.1618 0.0349
70 wt % GL 0.0301 0.0346 0.0341 0.0396 0.1690 0.1829 0.0368 0.0414
0.041 1 0.0459 0.2061 0.0480
0.051 1 0.0565 0.2672 0.0610
C A
T C A
U
T C A
U
T
T
0.0603 0.0662 0.1243 0.0177
0.060 i 0.0768 0.1388 0.0198
0.0362 0.0394 0.0838 0.0137
T
mixtures are given in Table'I. Assuming the degree of ionization of these biomolecules to be zero in water and mixed solvents since the pKa of these solutes are all above 9 and the effect of activity coefficient factor to be negligibly small as described elsewhere,23 the free energy of solutions (AGO,) of each solute was computed on the molar scale by the relation
AGO, = -RT In S The values at different temperatures in each solvent were fitted by the method of least squares to an equation of the form
AGO, = a
+ bT + cT In T
(2)
where Tis theabsolute temperature. Thevalues ofthe coefficients a, b, and c are presented in Table 11. These reproduce the experimental data to within f0.03 kJ mol-'. The standard free energies (AGO,) and entropies (So1) of transfer from water to the cosolvents were computed at 25 OC on mole fraction scale using the relations 3 and 4, where T = 298.15 K, CY, = -d In pw/dT,CY, = -d In p,/dT; M,p, and -d In p/dT stand for molar mass, density in kg dm-3 and coefficient of thermal expansion of the solvents respectively. The subscripts refers to solvent mixtures and w to water. The required values for p and CY at 298.15 K are
TABLE II: Coefficients a, b, and c and Transfer Energetics ( A p t and TAR', in kJ mol-') of U, T, C, and A from Water to Aqueous Mixtures of UH and CL at 298.15 K (Mole Fraction Scale) ~~
compound
a/kJ mol-I
b/kJ mol-I K-'
c/kJ mol-l K-'
U T C A
-5.20 -530.41 -88 3.32 574.60
U C A
703.60 601.34 347.1 1 -29.75
11.52 wt % U H -15.3248 2.279 595 -12.9633 1.925 197 -7.3581 1.090 173 1.0572 -0.167 177
-2.25 -2.30 -1.55 -0.21
-3.95 1.015 -15.80 -1.46
U T C A
393.06s 623.22 -653.66 -3 15.13
20.31 wt % U H -8.4172 1.250 526 -13.4246 1.993 346 15.3996 -2.315 314 7.3704 -1.102 303
-1.54 -2.30 -2.25 -0.53
-8.44 2.505 -0.60 -6.38
U T C A
357.09 -1 179.56 999.05 56.08
29.64 wt % U H -7.6833 1.143 192 27.1 155 -4.059 909 -21.9972 3.274 539 -0.99075 0.146 519
-1.16 -1.09 -3.83 -0.91
-12.83 3.25 -12.95 -7.16
U T C A
522.86 -772.77 421.71 -126.55
36.83 wt % U H -11.1890 1.661 121 17.9278 -2.686 757 -9.1502 1.359 564 3.0573 -0.456 595
-0.9 1 -1.11 -4.01 -1.26
-1.75 0.635 -19.19 -9.64
U T C A
-129.46 -221.08 -349.59 50.02
10 wt % GL 3.4731 -0.528 143 5.5271 -0.834 820 8.4926 -1.281 210 -0.7151 0.101 984
-0.36 -0.38 -0.74 -0.21
-1.91 -0.57 -6.42 -0.67
-38.725 -335.80 181.63 227.33
30 wt 1.4207 8.0468 -3.5027 -4.8110
5% GL
U T C A
-0.94 -0.95 -1.62 -1.01
-1.99 -2.92 -8.65 -5.66
U T C A
-201.74 -42.82 -197.92 -5 17.06
50 wt 5.0734 1.4327 4.9510 12.0440
% GL -0.766 -0.221 -0.749 -1.804
663 466 609 413
-1.515 -1.76 -2.783 -2.48
-1.80 -3.68 -1 1.09 3.02
U T C A
-306.14 -215.26 -616.94 -528.88
70 wt 7.3642 5.3110 14.2172 12.3024
% GL -1.107 389 -0.800712 -2.129 643 -1.843 267
-2.37 -2.52 -4.07 -3.92
-3.78 -2.68 -17.39 4.21
TASot
AGOt
Water
T
0.7274 12.4964 20.6227 -12.4805
-0.119 260 -1.875 789 -3.095 658 1.858 190
-0.221 466 -1.209 654 -0.51 1 089 0.716 227
AGO, = ,AGO, - ,AGO, = (a, - a,)
+ (6, - b,)T +
(c, - c,)T In T - RT In M,p,/M,p,
(3)
ASo,= ,ASo, - ,ASo,= ( b , - b,) + (cw- c,)(l + In T ) + R In M,Pw/MwP, + R T b , - a,) (4) known from l i t e r a t ~ r e . ~ AGO, ' . ~ ~ and T A S O , values are given in Table 11. The standard deviations in AGO, and Sot, ascalculated by Please's are f0.05 kJ mol-' and *2 J K-l mol-', respectively.
Discussion
Free Energies of Transfer. Figures 2 and 3 show the variation of AGO, (solid lines) of U, T, C, and A with mol % of UH-water and GL-water cosolvent systems, respectively. While these profiles for aqueous solutions of GL are increasingly negative, indicating increasing stabilization of the nucleic acid bases with composition, those for aqueous UH are less negative and are
10864 The Journal of Physical Chemistry, Vol. 97, No. 41, 1993
Ganguly and Kundu non-ionic species i may be taken to be composed of AG',(i) = AGot,cav(i)4- AGot,dip(i) -k AG',i,d(i)
4-
AGot,disp(i) + AGot,H,H,H(i)
I
I
I
0
5
10
I
I
15
20
I
m o l % UH
Figure 2. Variation of AGt (-), AGt.int(- - -) and A G t , H , H a ( - -) (inset) of DNA and RNA bases with mol % UH at 298.15 K.
I 0
1
I
I
10
20
30
1
mol% G L
F i p e 3 . Variationof AGO, (-), AGot,int (- - -), A G O ~ , H ~ H(-~ H-) (inset) of DNA and RNA bases with mol 3'% GL at 298.15 K.
more complicated as the profiles of U and T pass through a mixture at low compositions. This may be chiefly because of various interaction effects as guided by cavity formation, dispersion, dipole-dipole and dipole-induced dipole interactions and the effects of hydrophobic and hydrophilic hydration. First, AGot,,,v(i) which denotes free energy change accompanying appropriate cavity formation for the accommodation of any species i in a mixed solvent relative to that in the reference solvent water, was tentatively calculated in light of the scaled particle theory (SPT).27As the standard free energy of transfer due to chemical interactions, AG',(int), which is free from any cavity effect, is a better indicator of the true picture of the interactions between the solute and solvent.molecules, particularly for the large-size molecule^,^^-^^ attempts were made to investigate the AGot(int)xomposition profiles (broken lines) in Figures 2 and 3 for those DNA-RNA base units in UH-water and GLwater mixtures, respectively. While the increasingly negative trends of these composition profiles in aqueous U H can be attributed to the combined effects of hydrophobic-hydrophilic hydration and pronounced dipole4ipole and amide-amide interactions similar to proteins,25the increasingly positive trends of the composition profiles in GL-water mixtures indicate the decrease in the solute-solvent interactions relative to water, However, to understand the relative behavior of these DNA and R N A bases in the two cosolvent systems used for the present study in terms of hydrophilic-hydrophobic hydration effects, elimination of other interactions is in order. As indicated earlier,2412sthe observed free energy of transfer AGot(i) of any
(5)
where AGot,disp(i)denotes free energy change due to dipoledipole interaction, AGot,i,d(i)that for dipole-induced dipole interaction, AGot,disp(i)due to dispersion interaction, and AGot,HIHbH(i)due to hydrophilic-hydrophobic hydration effect. We tentatively calculated dipole-dipole, dipole-induced dipole, and dispersion interaction effects using the appropriate formulations as depicted in Marcus' monograph29and earlier paper^.^^*^* The hard-sphere diameters of U, T, C, and A taken to be 0.513,0.620,0.570,and 0.630 nm, respectively, have been tentatively estimated from the structural geometry of the molecules taking the appropriate bond lengths and bond angles from Pauling's monograph.30 The values for AGot,av(i),AGot,dip(i),AG't-ind(i), and AGotpiSp(i)are presented in Table 111. These when subtracted from ACot(i)values yielded the AGot,HIHbH(i)effect. Figures 2 and 3 (inset) illustrate AGot,HIHbH(i)-composition profiles in aqueous mixtures of UH and GL, respectively. As has been observed earlier,24*25,31s32 organic cosolvents in general reduce the hydrophobic hydration. So, it is expected that in both the solvent systems, hydrophobic hydration for the nucleic acid bases is less as compared to water. In other words, both UH-water and GL-water cosolvents are hydrophobic hydration reducers. But while hydrophilic hydration effect gets added with hydrophobic hydration effect in aqueous UH, it opposes hydrophobic hydration in GL-water cosolvent system. Due to decrease in hydrophobic interaction or solvophilic solvation between apolar sites of these DNA and R N A bases and aqueous UH,31 there will be a net decrement in hydrophilichydrophobic hydration in UH-water cosolvent system. Furthermore, as aqueous U H is more polar than water and can undergo peptide-peptide interaction^^^ with the solutes under study, hydrophilic hydration will also decrease. Moreover, on the basis of the fact that hydrophilic hydration is guided by the basicity of the solute and acidity of the solvent and UH-water being less acidic than water due to the formation of intercomponent H-bonded complex (Figure 4A), the hydrophilic hydration induced in aqueous U H will be much less relative to water. Consequently, there will be a net decrement in hydrophilic as well as hydrophobic hydration effects in aqueous U H and possibly that explains why AG',,H,H,H(i)xomposition profiles (Figure 2, inset) show an upward trend right from the water-rich region. Notably, AG't,H,H,H(i) values increase in the order A < T < U C C. This relative order depends on the hydrophobic and hydrophilic sites present in these solutes. The lower value of AGot,HIHbH(i)for A can be attributed to the more hydrophobic character of A7 compared with that of the pyrimidine bases U, T, and C. The additional -CH3 group in T is responsible for the reduction of AGot,HIHbH(i)value compared to that for U, particularly at higher compositions. Further, since C has predominant hydrophilic character compared to other nucleic acid bases, it has the greatest AGot,HIHbH(i)values. In GL-water mixtures also the AGot,H,H,H(i)xomposition profiles (Figure 3, inset) are the result of amalgamated effects of hydrophilic and hydrophobic hydration. At higher compositions of the cosolvent, a downward trend is observed. This is possibly due to the large acidity effect of the cosolvent G L relative to water due to cooperative H bonding (Figure4B), thereby inducing more hydrophilic hydration as compared to water. C i s the most basid4 among these solutes and has the lowest AGot,H,Hd(i)value, while A and T, which are much less basic,34 possess the highest AG't,H,H,H(i) values at higher compositions. Thus, the observed results are seemingly the opposing effects of hydrophobic and hydrophilic hydrations. Entropies of Transfer. The solid lines in Figures 5 and 6 show the ThSot-composition profiles in UH-water and GL-water
The Journal of Physical Chemistry, Vol. 97, No. 41, 1993 10865
Transfer Energetics
Scale)'
U
T
C A
37.0 51.5 44.4 53.0
U
T C A U
T C A
U
T C A U
T C A U
T C A U
T C A U
T C A
5.2 7.4 6.3 7.6
13.5 19.1 16.4 19.7
1.4 1.9 1.7 2.0
39.7 55.2 47.6 56.7
11.8 16.7 14.3 17.2
2.5 3.5 3.0 3.5
41.0 57.1 49.2 58.7
11.4 16.1 13.8 16.6
42.2 58.6 50.6 60.3
38.5 53.5 46.2 55.1
11.52 wt 96 UH; a = 531 X 10-6 X K-I; V, = 19.05 cm3 mol-' -26.5 -3.0 -1.9 27.8 6.9 -3 1.7 -11.5 -2.3 -1.7 11.3 9.8 -13.7 -42.4 -3.4 -1.7 44.2 8.4 -50.7 -6.5 -2.3 -2.0 8.6 10.1 -7.7
-0.7 -0.5 -0.8 -0.5
-0.5 -0.4 -0.4 -0.5
22.1 5.8 27.7 -2.9
20.31 wt % UH; a = 435 X 10-6 K-I; V, = 19.96 cm3 mol-I -46.8 -5.3 -3.2 51.3 4.1 -54.2 -20.3 -4.0 -3.0 21.5 5.9 -23.5 -75.0 -6.0 -2.9 78.7 5.1 -86.8 -11.4 -4.1 -3.4 14.9 6.2 -13.2
-0.9 -0.7 -1.1 -0.7
-0.7 -0.6 -0.6 -0.7
43.3 21.4 82.8 2.0
3.6 5.2 4.4 5.3
29.64 wt % UH; a = 390 X 10-6 K-I; V, = 21.08 cm3 mol-' -68.1 -7.5 -4.4 75.2 2.6 -77.7 -29.7 -5.7 -4.2 33.3 3.5 -33.8 -109.4 -8.5 -4.1 113.8 3.1 -124.6 -16.7 -6.0 -4.8 21.3 3.7 -19.0
-1.2 -0.8 -1.3 -0.9
-0.8 -0.7 -0.7 -0.9
64.3 35.1 110.6 9.9
11.2 15.8 13.6 16.3
4.7 6.6 5.7 6.8
36.83 wt % UH; a = 361 X 10-6 K-I; V, = 22.09 cm3 mol-l -84.2 -9.2 -5.2 93.0 1.3 -95.1 -36.7 -7.1 -5.0 41.1 1.8 -41.4 -135.3 -10.4 -4.9 140.9 1.6 -152.7 -20.7 -7.4 -5.8 25.8 1.9 -23.3
-1.3 -0.9 -1.5 -1.0
-0.9 -0.8 -0.8 -0.9
94.3 41.9 134.2 13.7
36.9 51.3 44.3 52.8
8.3 11.7 10.0 12.0
-0.3 -0.4 -0.3 -0.4
10 wt % GL; a = 345 X 10-6 K-1; 0.1 -0.4 -2.0 0.0 -0.2 -1.7 0.0 -0.6 -1.8 0.0 -0.1 -2.1
0.0 -0.1 -0.1 0.0
-0.1
0.0
-0.3 -0.2 -0.2 -0.3
-4.9 -5.0 -10.0 -5.2
36.7 51.0 44.0 52.4
10.0 14.1 12.1 14.5
-0.8 -1.0 -0.9 -1.1
30 wt % GL; a = 398 X 10-6 K-I; 0.9 -1.0 -5.7 0.2 -0.4 -5.4 -0.6 1.0 -5.3 0.1 -0.2 -6.2
0.7 0.1 0.7 0.0
-0.2 -0.1 -0.3 -0.1
-0.9 -0.8 -0.9 -1 .o
-7.2 -9.8 -14.9 -12.6
36.4 50.0 43.6 51.8
13.2 18.6 15.9 19.1
-1.6 -2.5 -1.8 -2.2
3.4 1.o 4.5 0.6
-0.4 -0.2 -0.6 -0.2
-2.1 -1.9 -1.9 -2.1
-12.3 -16.3 -24.5 -9.0
35.5 47.5 42.5 50.4
13.3 18.7 16.0 19.2
-3.1 -5.6 -3.5 -4.2
10.8 3.8 15.5 2.1
-0.5 -0.2 -1.2 -0.2
-2.6 -2.3 -2.4 -2.7
-22.7 -22.0 -42.5 -10.8
X
1OLB/esu
V, = 22.18 cm3 mol-l 5.7 5.6 5.7 7.7 4.4 6.7 6.4 8.0 50 wt % GL; a = 497 X 10-6 K-I; V, = 26.82 cm3 mol-' 3.8 -1.0 -8.3 5.6 9.6 -0.4 1.2 -8.0 7.9 13.7 5.2 -2.1 -7.8 3.7 11.4 0.0 0.7 -9.2 8.2 13.7 70 wt % GL; a = 478 X 106 K-1; V, = 35.00 cm3 mol-' 11.0 0.0 -7.6 -2.7 11.2 4.0 0.3 -8.0 6.8 17.0 15.9 -1.2 -7.8 -7.5 13.2 2.2 0.7 -9.3 6.8 15.8
solvent/solute
M
b/nm
p X lO'*/esu
a X 1024/cm-3
HzO
18.02 60.06 92.10 112.10
0.276b O.44lb 0.494b 0.513c
1.834d 4.56od 2.6706 5.18oC
1.456d 5.146d 8.140d 9.463d
UH GL U
V, = 19.19 cm3 mol-' 2.2 3.4 1.9 4.7 2.0 4.0 2.4 4.8
solvent/solute
M
b/nm
T C
126.10 111.10 135.10
0.62oC 0.570f 0.63oC
A
0.0 -0.1
3.95w 7.1OOc 3 .OOP
X
102'/cm-3
1 1.304d 9.97odJ 13.35 3dJ
a VI = M,/q,; MI = [ W'/M1 t Wz/M2]-I X 100, where Wl and W2 are mass of water and organic solvent, respectively, M Iand M2 are molecular at 25 OC at each solvent composition were known ref 37; isothermal weights of water and organic solvent, respectively; the required density data (e) = ( n -~l ) ~M / ( n ~ ~2 ) p 4 u N ~= 3 R / 4 u N ~ , expansibility (a)= -d In p,/dT; dipole moment = p ; hard-sphere diameter = b (nm); polarizability where nb refractive index, M =: mol wt, p = density and R = molar refraction. Reference 29. Reference 30. Reference 40. Reference 41. Reference 42.
(a)
mixtures, respectively. However, a clear understanding of the solvent effects on the entropy change for the nucleic acid bases is difficult to obtain from these profiles. So, as in the case of free energy, the cavity effect AS0,,,(i) were calculated using SPT formulations.27 Also, the dipole+Iipole, dipole-induced dipole, and dispersion effects Sot,dip(i), sot,ind(i) and uot,disp(i) respectively have been computed using formulation as described earlier.28.29 First, on deducing TASo,,,(i) from TASot(i), we obtain the profiles for TASot,int(i)arising from chemical type interactions and structural changes. Significantly enough, these TASot,int(i)-composition profiles show a pattern similar to that observed in the case of amino acids like glycine (G), diglycine (DG), and triglycine (TG)z5 in these two aquo-organic cosolvents. In the case of aqueous UH, the observed initial downward trends
+
(Figure 5 ) of the TASot,int(i)
