Physicochemical Properties of Amino Acids in Aqueous Caffeine Solution at 25, 30, 35 and 40 °C

Chinese Journal of Chemistry, 2006, 24, 1547—1553 Full Paper

* E-mail: [email protected] Received December 17, 2004; revised and accepted August 24, 2006.

© 2006 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Physicochemical Properties of Amino Acids in Aqueous Caffeine Solution at 25, 30, 35 and 40 ℃

ALI, A.* SABIR, S. SHAHJAHAN HYDER, S.

Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi-110025, India

Density, viscosity, and refractive index, for glycine, DL-alanine, L-serine and DL-valine have been determined in aqueous solution of 0.05 mol/kg caffeine as a function of amino acid (AA) concentration at 25, 30, 35, and 40 ℃. The density data have been used to compute apparent molar volume. The partial molar volume (limiting apparent molar volume) was obtained by applying the Masson’s equation. The viscosity data have been analyzed by means of Jones-Dole equation. The values of Falkenhagen coefficient and Jones-Dole coefficient thus obtained are used to interpret the solute-solute and solute-solvent interactions, respectively. Hydration number was also computed. The transition-state theory was applied to obtain the activation parameters of viscous flow, i.e., free energy of activation per mole of solvent, and solute. The enthalpy and entropy of activation of viscous flow were computed for the sys-tem. Refractive index was used to calculate molar refractivity of the mixtures. The results have been interpreted in the light of various interactions occurring between the components of the mixtures under applied experimental con-ditions.

Keywords amino acid, caffeine, Falkenhagen coefficient, Jones-Dole coefficient, molecular interactions

Introduction

Amino acids are among the simplest biomolecules that contain intramolecular hydrogen bonds and they serve as building blocks of more complex peptides and proteins.1 In aqueous medium amino acids exist as di-polar ions manifesting a unique hydration behavior which is linked to the vital biological phenomenon. Due to this linkage, the study of amino acids is considered important in unfolding the role of dipolar ions in the living systems.2 Several biological processes like fever, hypothermia, anaesthesia,3,4 etc. involve expansion and contraction of protein molecules resulting from tem-perature and pressure variation in living systems. Study of these processes requires fundamental information about volumetric and related properties of proteins. Amino acids are models well suited for the estimation of such properties.

Caffeine is a plant alkaloid obtained from coffee, tea and kola nuts. It is a heart stimulant, diuretic and is ex-tensively used in medicine,5 most commonly in head-ache medications. It is an addictive drug and affects the central nervous system.

In view of the above mentioned practical importance of the compounds selected for the present study, we ex-tend here our earlier study6 to the systematic study on amino acids, glycine, DL-alanine, L-serine and DL-valine in aqueous caffeine in the temperature range 25—40 ℃, which covers body temperature also. A survey of literature indicates that no work has been done on these systems from the point of view of their

thermodynamic behavior. Although studies on amino acids in aqueous solutions have been carried out by many workers,7-9 no attention has been paid to the in-teractions of amino acids with caffeine drug in aqueous medium.

In the present article we report the densities, ρ, vis-cosities, η and refractive indices, nD of solutions of gly-cine (Gly), DL-alanine (Ala), L-serine (Ser) and DL-valine (Val) as a function of amino acid concentra-tion in 0.05 mol/kg aqueous caffeine solution at tem-peratures 25, 30, 35 and 40 . Using these experime℃ n-tal data, the various parameters computed are apparent molar volume, φV, limiting apparent molar volume, 0

Vφ , relative viscosity, ηrel, Falkenhagen coefficient,10 A, Jones-Dole coefficient,11 B, hydration number, Hn, tem-perature derivative of B, δB/δT and free energies of ac-tivation of viscous flow, 0#

1µ∆ and 0#2µ∆ per mole of

solvent and solute, respectively. nD values were used to calculate molar refractive index, RD of the different sol-utes in aqueous caffeine solution. All these functions offer a convenient method to study the thermodynamic properties of amino acids (AAs) in various solvents, not easily obtained by other means.

Experimental

Materials

Amino acids glycine (Merck, 99.7%), DL-alanine (Merck, 99%), L-serine (Thomas Baker, 98.5%) and DL-valine (Loba Chemie, 99%) were used after recrys-

1548 Chin. J. Chem., 2006, Vol. 24, No. 11 ALI et al.


tallization from ethanol-water mixtures and drying in vacuum over P2O5 at room temperature for at least 72 h. Caffeine (S.d fine, 98.5%—101%) was used without any pretreatment. The water used was deionized and doubly distilled. First 0.05 mol/kg aqueous caffeine so-lution was prepared and this was used to prepare AA solutions of different concentrations. The mixtures were prepared by mass in a dry box using Precisa XB 220 A electronic balance precise up to 1.0×10－4 g. They were stored in special airtight bottles.

Instrumental

The densities of aqueous caffeine solution and AAs in this solution were measured using a single-capillary pycnometer made of Borosil glass having a bulb capac-ity of 8×10－6 m3. The capillary of pycnometer had graduated marks with a uniform bore and a well fitting glass cap. The marks on the capillary were calibrated by using doubly distilled water. The position of the liquid level in the capillary was noted after keeping the pycnometer for about 25 min in an electronically con-trolled water bath (Julabo Labrotechnic, GMBH Ger-many) (±0.02 ). The density of pure water at calibr℃ a-tion temperature was taken from literature.12 The vis-cosities of solutions were measured using a Cannon Ubbelohde type viscometer. An electronic digital stop-watch with a readability of ±0.01 s was used for flow time measurements. The measured density and viscosity were accurate to 0.01 kg•m－3 and 3×10－6 N•m－2•s, respectively. Refractive indices of the solutions were measured using a thermostatically controlled Abbe re-fractometer (Metrex, India). Calibration of the instru-ment was done by measuring the nD of doubly distilled water and toluene at known temperatures.13 The sample mixtures were directly injected into the prism assembly of the instrument by means of an airtight hypodermic syringe. When the liquid mixtures attained constant temperature, the nD measurements were made. A mini-mum of 3 readings were taken for each composition and the average value was considered in all calculations. The error in nD data was less than ±0.001.

Results

The experimental values of ρ, η and nD of Gly, Ala, Ser, and Val in aqueous caffeine over the concentration range of 0.10 to 0.52 mol/kg at 25, 30, 35, and 40 are ℃

given in Table 1. ρ data were used to calculate φV from the relation:

φV＝M/ρ－1000(ρ－ρ0)/mρρ0 (1)

where ρ and ρ0 are the densities of solution (amino acids in aqueous caffeine) and of aqueous caffeine, respec-tively, m is the molality of amino acid, and M is its mo-lar mass. φV values are given in Table 2 and are graphi-cally shown in Figure 1.

The partial molar volume or the limiting apparent molar volume, 0

Vφ of solute (AA) was derived from the Masson’s equation:14

Figure 1 Plots of φV versus concentration, m of AAs, glycine, DL-alanine, L-serine and DL-valine in aqueous caffeine solution at 30 ℃.

φV＝0Vφ ＋

*VS C1/2 (2)

where *VS is the experimental slope of the plots φV vs.

C1/2. The values of 0Vφ and *

VS are summarized in Table 3.

The entire viscosity data were analyzed in terms of the Jones-Dole11 viscosity equation:

η/ηo＝1＋AC1/2＋BC (3)

where η/ηo＝ηr, the relative viscosity, η and ηo are vis-cosities of ternary solutions and the aqueous caffeine, respectively. A and B are the characteristics of the solute and solvent. A represents the contribution from sol-ute-solute interactions and B is known to depend on the size of the solute particle and on the interactions be-tween solute and solvent. They were obtained by a least-squares treatment as intercept and slope of the lin-ear plots of (η/η0－1)/C1/2 vs. C1/2. For a dilute solution of unsolvated spherical colloidal suspensions, Einstein derived the relation

ηr＝1＋2.5φ (4)

where φ is the volume fraction of the solute.15

For amino acids Eq. (4) becomes

ηr＝1＋2.5VhC (5)

where Vh is the hydrodynamic volume. Desnoyers and Perrson16 have assumed Vh to be the partial molar vol-ume, 0

Vφ of the unsolvated solute particle in a contin-uum solvent. Thus, the value of B/ 0

Vφ is the hydration number, Hn, and lies between 0 to 2.5 for unsolvated species and has higher values for solvated species. The values of Hn are given in Table 3. The variation of B with temperature is depicted graphically in Figure 2.

According to the transition state treatment of relative viscosity of electrolytic and non-electrolytic solutions, as suggested by Feakins et al.17 there lies some relation between B-coefficient and molar volume as given in the equation:

B＝( 01V －

02V )/1000＋ 0

1V [( 0#2µ∆ －

0#1µ∆ )/RT]/1000

(6)

Amino acid Chin. J. Chem., 2006 Vol. 24 No. 11 1549


Table 1 Values of density, ρ (kg•m－3), viscosity, η (10－3 N•m－2•s) and refractive index, nD, of amino acids, glycine, DL-alanine, L-serine and DL-valine in aqueous caffeine at 25, 30, 35 and 40 ℃

25 ℃ 30 ℃ 35 ℃ 40 ℃

m/ (mol•kg－1) ρ η nD ρ η nD ρ η nD ρ η nD

0.00 999.9 0.9681 1.3345 998.1 0.8613 1.3340 996.4 0.7801 1.3336 994.6 0.6839 1.3334

0.10 1002.4 0.9728 1.3350 1000.7 0.8729 1.3345 999.1 0.7967 1.3341 997.5 0.7046 1.3335

0.20 1005.3 0.9926 1.3360 1003.7 0.8906 1.3357 1002.2 0.8164 1.3355 1000.6 0.7215 1.3351

0.30 1008.4 1.0130 1.3375 1006.8 0.9105 1.3370 1005.4 0.8362 1.3361 1003.8 0.7398 1.3359

0.41 1011.6 1.0321 1.3382 1010.1 0.9303 1.3379 1008.8 0.8553 1.3370 1007.2 0.7597 1.3368

Glycine ＋

aq. caffeine

0.51 1015.1 1.0548 1.3421 1013.7 0.9534 1.3405 1012.3 0.8777 1.3400 1010.7 0.7810 1.3398

0.00 999.9 0.9681 1.3345 998.1 0.8613 1.3340 996.4 0.7801 1.3336 994.6 0.6839 1.3334

0.10 1001.5 0.9987 1.3360 999.8 0.8923 1.3355 998.2 0.8123 1.3349 996.5 0.7146 1.3342

0.20 1004.2 1.0282 1.3378 1002.6 0.9205 1.3375 1001.1 0.8398 1.3371 999.5 0.7391 1.3365

0.31 1007.3 1.0581 1.3391 1005.7 0.9464 1.3389 1004.2 0.8638 1.3383 1002.6 0.7602 1.3380

0.41 1010.5 1.0838 1.3405 1008.9 0.9698 1.3401 1007.4 0.8844 1.3399 1005.9 0.7802 1.3392

DL-Alanine＋

aq. caffeine

0.52 1013.8 1.1056 1.3430 1012.2 0.9925 1.3427 1010.8 0.9081 1.3422 1009.3 0.8009 1.3418

0.00 999.9 0.9681 1.3345 998.1 0.8613 1.3340 996.4 0.7801 1.3336 994.6 0.6839 1.3334

0.10 1003.5 0.9807 1.3365 1001.8 0.8791 1.3362 1000.2 0.8026 1.3359 998.5 0.7066 1.3357

0.20 1007.5 1.0063 1.3379 1005.9 0.9041 1.3377 1004.4 0.8252 1.3375 1002.8 0.7282 1.3372

0.31 1011.8 1.0364 1.3400 1010.3 0.9311 1.3398 1008.9 0.8517 1.3396 1007.3 0.7514 1.3394

0.41 1016.3 1.0699 1.3413 1014.8 0.9604 1.3411 1013.5 0.8786 1.3410 1012.0 0.7761 1.3408

L-Serine ＋

aq. caffeine

0.52 1021.0 1.1078 1.3431 1019.6 0.9945 1.3430 1018.2 0.9097 1.3429 1016.8 0.8023 1.3428

0.00 999.9 0.9681 1.3345 998.1 0.8613 1.3340 996.4 0.7801 1.3336 994.6 0.6839 1.3334

0.10 1001.9 1.0046 1.3356 1000.2 0.8807 1.3352 998.6 0.7885 1.3350 996.9 0.6843 1.3348

0.20 1004.1 1.0555 1.3379 1002.5 0.9186 1.3372 1001.0 0.8205 1.3368 999.4 0.7082 1.3363

0.31 1006.4 1.1077 1.3398 1004.9 0.9613 1.3391 1003.5 0.8546 1.3385 1002.0 0.7312 1.3381

0.42 1008.9 1.1614 1.3421 1007.5 1.0026 1.3415 1006.1 0.8886 1.3411 1004.7 0.7616 1.3408

DL-Valine＋

aq. caffeine

0.52 1011.5 1.2288 1.3445 1010.2 1.0588 1.3439 1008.8 0.9320 1.3433 1007.5 0.7919 1.3428

Figure 2 Plots of B versus temperature, for the amino acids, glycine, DL-alanine, L-serine and DL-valine in aqueous caffeine solution at 30 ℃.

where 01V and 0

2V (＝ 0Vφ ) are the partial molar vol-

umes of the solvent (aqueous caffeine) and solute at infinite dilution, respectively. 0#

1µ∆ is the free energy of activation per mole of solvent and is given by the relation:

0#1µ∆ ＝RTln(ηo

01V /hNA) (7)

where h is the Planck’s constant and NA is the

Avogadro’s number. 0#2µ∆ is the contribution per mole

of solute to the free energy of activation for viscous flow of the solution. Eq. (6) can also be rearranged as

0#2µ∆ ＝

0#1µ∆ ＋(RT/ 0

1V )[1000B－( 01V －

02V )] (8)

The values of A, B, 0#1µ∆ and 0#

2µ∆ obtained from Eqs. (3), (7) and (8) are recorded in Table 3. The enthal-pies, ∆H* and entropies, ∆S* of activation of viscous flow can be obtained from the free energy of activation using Eqs. (9) and (10) as

∆S*＝－d(∆G*)/dT (9)

∆G*＝∆H*

－T∆S* (10)

∆G* is calculated from the equation

∆G*＝n1

0#1µ∆ ＋n2

0#2µ∆ (11)

The values of ∆H* and ∆S* have been deduced from



Table 2 Values of apparent molar volume, φV, for amino acids, glycine, DL-alanine, L-serine and DL-valine in aqueous caffeine at 25, 30, 35 and 40 ℃

φV/(10－5 m3•mol－1)

m/ (mol•kg－1) 25 ℃ 30 ℃ 35 ℃ 40 ℃

0.10 5.008 4.916 4.824 4.632

0.20 4.808 4.716 4.624 4.532

0.30 4.674 4.616 4.523 4.464

0.41 4.583 4.516 4.423 4.381

Glycine ＋

aq. caffeine

0.51 4.467 4.395 4.345 4.311

0.10 7.310 7.223 7.135 7.047

0.20 6.760 6.672 6.583 6.494

0.31 6.443 6.388 6.332 6.276

0.41 6.260 6.221 6.1813 6.117

DL-Alanine ＋

aq. caffeine

0.52 6.130 6.101 6.051 6.001

0.10 6.910 6.822 6.733 6.645

0.20 6.710 6.622 6.533 6.444

0.31 6.543 6.455 6.365 6.310

0.41 6.410 6.346 6.257 6.192

L-Serine ＋

aq. caffeine

0.52 6.290 6.221 6.171 6.102

0.10 9.716 9.633 9.550 9.466

0.20 9.616 9.533 9.449 9.366

0.31 9.549 9.466 9.382 9.299

0.42 9.466 9.383 9.324 9.240

DL-Valine ＋

aq. caffeine

0.52 9.396 9.313 9.268 9.185

linear plots of ∆G* vs. T as intercept and slope, respec-tively, and are tabulated in Table 4. The plots of ∆G* vs. temperature at 0.20 mol/kg amino acid are shown graphically in Figure 3. The nD data were used to calcu-late molar refractivity of the mixtures under study using the Lorenz-Lorentz equation:

RD＝[( 2Dn －1)/( 2

Dn ＋2)](3

1i i

i

x M∑＝

/ρ) (12)

where xi and Mi are respectively, molar fraction and molecular weight of the ith component of the mixture. The calculated values of RD in the temperature range from 25 to 40 are summarized i℃ n Table 5 and plotted at one single temperature, i.e., at 30 in Figure 4. ℃

Discussion

It is evident from Table 2 that φV values for AAs in aqueous caffeine are positive and decrease regularly with increase in AAs concentration as well as with rise in temperature, indicating that solute-solvent interac-tions decrease both with increase in concentration and temperature. The 0

Vφ values depicted in Table 3 are positive and decrease with rise in temperature in all the

Table 3 Values of limiting apparent molar volume, 0Vφ (10－5

m3•mol－1), 0V(water)φ (10－5 m3•mol－1), volume transfer, 0

vtrφ (10－5 m3•mol－1), experimental slope, *

VS (10－5 m3•mol－3/2•L1/2), A (10－2 dm3/2•mol－1/2) and B (10－1 dm3•mol－1) coefficients of Jones-Dole equation, hydration number, Hn (103), free energy of activation for the solvent, 0 #

1µ∆ (kJ•mol－1) and solute 0 #2µ∆

(kJ•mol－1) for amino acids, glycine, DL-alanine, L-serine and DL-valine in aqueous caffeine at 25, 30, 35 and 40 ℃

25 ℃ 30 ℃ 35 ℃ 40 ℃

Glycine＋aq. caffeine 0Vφ 5.426 5.314 5.191 4.896 0V(water)φ 4.324a — 4.379a 4.400b 0vtrφ 1.102 — 0.812 0.496 *VS －1.355 －1.286 －1.214 －0.816

A －4.999 －4.327 －1.761 －0.308

B 2.418 2.672 2.680 2.780

Hn 4.457 5.029 5.163 5.679 0 #1µ∆ 9.379 9.246 9.150 8.960 0 #2µ∆ 47.294 51.084 51.538 52.964

DL-Alanine＋aq. caffeine 0Vφ 8.180 8.032 7.898 7.774 0V(water)φ 6.049a — 6.101a 6.120c 0vtrφ 2.131 — 1.797 1.654 *VS －3.018 －2.851 －2.720 －2.617

A 2.395 3.899 5.782 6.882

B 2.501 2.457 2.396 2.375

Hn 3.057 3.059 3.034 3.054 0 #1µ∆ 9.379 9.246 9.150 8.960 0 #2µ∆ 52.176 51.872 51.351 51.284

L-Serine＋aq. caffeine 0Vφ 7.416 7.304 7.184 7.075 0V(water)φ 6.060a — 6.150a 6.170c 0vtrφ 1.356 — 1.034 0.905 *VS －1.591 －1.530 －1.457 －1.389

A －8.831 －5.494 －2.433 －0.636

B 3.965 3.700 3.489 3.395

Hn 5.347 5.065 4.857 4.799 0 #1µ∆ 9.379 9.246 9.150 8.960 0 #2µ∆ 71.107 68.061 65.705 64.833

DL-Valine＋aq. caffeine 0Vφ 9.979 9.897 9.772 9.689 0V(water)φ 9.098a — 9.155a 9.167d 0vtrφ 0.881 — 0.617 0.522 *VS －0.811 －0.813 －0.712 －0.713

A －7.687 －1.197 －1.449 －1.640

B 6.101 5.925 5.676 5.267

Hn 6.114 5.987 5.808 5.453 0 #1µ∆ 9.379 9.246 9.150 8.960 0 #2µ∆ 103.740 102.470 100.060 95.351

a Data taken from reference 18. b Data taken from reference 19. c Data taken from reference 20. d Data taken from reference 21.



Table 4 Values of enthalpy, ∆H*, entropy, ∆S* of activation of viscous flow and correlation factor, f of amino acids, glycine, DL- alanine, L-serine and DL-valine in aqueous caffeine

m/(mol•kg－1) ∆H*/(kJ•mol－1) ∆S*/(kJ•mol－1) f

0.00 0.872 0.001 0.991

0.10 －4.727 －0.034 0.926

0.20 －10.326 －0.069 0.929

0.30 －15.925 －0.103 0.930

0.41 －21.524 －0.138 0.930

Glycine ＋

aq. caffeine

0.51 －27.122 －0.173 0.931

0.00 0.872 0.001 0.991

0.10 7.995 0.008 0.981

0.20 15.118 0.014 0.975

0.31 22.241 0.021 0.972

0.41 29.364 0.027 0.971

DL-Alanine ＋

aq. caffeine

0.52 36.487 0.033 0.970

0.00 0.872 0.001 0.991

0.10 10.713 0.023 0.978

0.20 20.553 0.044 0.976

0.31 30.394 0.065 0.976

0.41 40.234 0.086 0.975

L-Serine ＋

aq. caffeine

0.52 50.075 0.107 0.975

0.00 0.872 0.001 0.991

0.10 27.762 0.057 0.964

0.20 54.653 0.112 0.963

0.31 81.543 0.167 0.963

0.42 108.433 0.222 0.963

DL-Valine ＋

aq. caffeine

0.52 135.323 0.277 0.963

Figure 3 Plots of ∆G* versus temperature at 0.20 mol/kg amino acids, glycine, DL-alanine, L-serine and DL-valine in aqueous caffeine solution.

systems under study. On comparing the 0Vφ values of

AAs in caffeine-water system with those of AAs in pure water (taken from literature18-21) it is found that the former ones are higher than latter ones resulting in posi-tive transfer volume, 0

vtrφ . This is attributed to the fact that caffeine penetrates the three dimensional water structure. Also it is well known22 that zwitterionic groups of AAs induce a considerable volume contraction

Table 5 Values of molar refractive index, RD of amino acids, glycine, DL-alanine, L-serine and DL-valine in aqueous caffeine at 25, 30, 35 and 40 ℃

RD/(10－6 m3•mol－1)

m/ (mol•kg－1) 25 ℃ 30 ℃ 35 ℃ 40 ℃

0.00 3.751 3.753 3.755 3.760

0.10 3.768 3.770 3.772 3.772

0.20 3.789 3.792 3.796 3.798

0.30 3.814 3.815 3.812 3.816

0.41 3.831 3.834 3.830 3.834

Glycine ＋

aq. caffeine

0.51 3.880 3.869 3.869 3.873

0.00 3.751 3.753 3.755 3.760

0.10 3.787 3.789 3.789 3.788

0.20 3.823 3.826 3.827 3.827

0.31 3.851 3.856 3.855 3.859

0.41 3.881 3.883 3.887 3.886

DL-Alanine ＋

aq. caffeine

0.52 3.922 3.925 3.926 3.928

0.00 3.751 3.753 3.755 3.760

0.10 3.791 3.794 3.797 3.802

0.20 3.823 3.827 3.831 3.834

0.31 3.862 3.866 3.869 3.873

0.41 3.892 3.896 3.900 3.904

L-Serine ＋

aq. caffeine

0.52 3.926 3.931 3.935 3.940

0.00 3.751 3.753 3.755 3.760

0.10 3.792 3.795 3.799 3.803

0.20 3.846 3.845 3.847 3.848

0.31 3.895 3.894 3.894 3.895

0.42 3.949 3.949 3.950 3.953

DL-Valine ＋

aq. caffeine

0.52 4.004 4.004 4.003 4.003

Figure 4 Plots of RD versus concentration, m of amino acids, glycine, DL-alanine, L-serine and DL-valine in aqueous caffeine solution at 30 ℃.

in peripheral solvent because of electrostrictive effect. This electrostrictive effect of AAs is diminished on ad-dition of caffeine because the zwitterionic groups are shielded by caffeine molecules. Thus the AAs will be unable to exert their maximal effects in this regard. Similar conclusion was drawn by others for AAs in aqueous alkali chloride solutions.23

A glance at the molecular structure of caffeine



(Scheme 1) reveals several possible sites it can interact with zwitterions of AAs. The lone pair of electrons on the two O atoms and one on N atom may interact with the 3NH＋ of AAs. At the same time the (＋)ve charge of CH3 groups (due to positive inductive effect of C) may interact with the COO－ terminal of the AAs.

Scheme 1 The structure of caffeine (1,3,7-trimethyl xanthine)

On comparing the 0Vφ values of the four AAs under

study they are found to increase in the order

Gly＜Ser＜Ala＜Val

thus, indicating the trend of solute-solvent interaction. Keeping in view the comparative size of side groups in alanine (—CH3) and serine (—CH2OH) a lower value of

0Vφ for Ala was expected. On the contrary, small values

for Ser seem to represent the shrinkage due to H-bonding between the —OH of serine and surround-ing solvent molecules, an interaction missing in case of Ala. Similar conclusion was drawn by Iqbal et al.2

The *VS values (Table 3) are all negative and in-

crease with increase in temperature revealing weak sol-ute-solute interactions with increase with temperature. The trends in *

VS values are

Ala＜Ser＜Gly＜Val

i.e., they are opposite to that of 0Vφ , except for Val,

which shows maximum solute-solute as well as sol-ute-solvent interactions. Gly and Ala have smallest side chains, i.e., a proton and a methyl group, respectively, so their backbone conformations are dominated by in-tramolecular H-bonding.1 Ser shows stronger intra-molecular H-bonding due to —OH group. The isopro-pyl side chain of Val, however, has greater steric hin-drance, and, hence, weaker intramolecular H-bonding. Thus, weak self-association explains its stronger inter-action with cosolute caffeine as well as with water. The values of A, B, 0#

1µ∆ , 0#2µ∆ [derived from Eqs. (3), (7)

and (8)] and Hn are given in Table 3. It is interesting to note that the larger positive values of B coefficients as compared to A coefficients support the behavior of 0

Vφ and *

VS , respectively, both suggesting stronger sol-ute-solvent interactions as compared to solute-solute interactions. B decreases, whereas, A increases with temperature, thereby, supporting the earlier conjecture that solute-solvent interactions decrease and sol-ute-solute interactions increase with rise in temperature, except in Gly where a reverse trend is observed. Table 3 also depicts that 0#

2µ∆ are positive and larger than 0#1µ∆ , suggesting that the formation of transition state

is less favored in the presence of AAs and that all AAs behave as structure-makers/promoters in aqueous caf-

feine at the studied temperatures. According to Feakins et al.,17 the magnitude of 0#

2µ∆ reveals the struc-ture-making ability of the solute. Results shown in Table 3 reflect that Val contributes maximum in struc-ture-making, while, Gly the least. The order of struc-ture-making ability comes out to be

Gly＜Ala＜Ser＜Val

which is similar to the increasing order of size of the AAs under study.

In the present study the values of Hn (Table 3) are found to be larger than 2.5 for all the AAs, showing a distinct hydration. Hn decreases with temperature em-phasizing lesser hydration at higher temperatures. However, Gly shows the opposite trend.

The sign of δB/δT is another indicator of struc-ture-making or breaking ability of the solute.24 It is ob-served from Figure 2 that for almost all the four AAs, δB/δT is negative, thus classifying them as struc-ture-promoters in caffeine-water mixture. The slope of B vs. T is less pronounced in case of Gly and Ala and is maximum for Val, thus, supporting the result inferred from 0#

2µ∆ that Val has maximum structure-promoting ability in aqueous caffeine. Similar behavior was shown in system valine＋urea＋water25 and alanine, valine, leucine＋sodium acetate＋water.26

The analysis of solute activation parameters ∆H* and ∆S* is presented in Table 4. ∆H* and ∆S* clearly indicate a macroscopic relaxation process. It is observed from Table 4 that ∆H* values are large and positive and ∆S* has small positive values. Both ∆H* and ∆S* increase positively with increase in concentration. Both are greatly influenced by concentration, increasing posi-tively with increase in concentration. However, Gly shows decrease in ∆H* and ∆S* with rise in concentra-tion. The values of ∆H* are comparable for Ala, Ser and Val, whereas, they differ having much lower values for Gly. This suggests that the formation of the activated species necessary for the viscous flow appears quite easy in case of Gly as compared to other three AAs. Positive ∆H* was also reported for the system urea＋water＋ammonium sulphate27 and adenosine＋water＋dioxane mixtures.28

The study of variation of RD with mixture composi-tion gives information on the interaction in the system.29 The values of RD (Figure 4) increase almost linearly with increasing amount of AAs in aqueous caffeine. RD being directly proportional to the molecular polarizabil-ity, Figure 4 suggests an increase in overall polarizabil-ity of all the ternary systems under study with increas-ing amount of AAs in the mixtures. The increasing magnitude of RD on moving from Gly to Val is in ac-cordance with the conclusions drawn from other results that interactions between components in Gly＋caffeine＋water are the least and those between the components in Val＋caffeine＋water are the strongest. There is not much variation in RD with temperature for the present systems.



Conclusion

The positive values of 0Vφ and reverse trends in

*VS , for all the amino acids studied in aqueous caffeine

solution suggest the presence of strong solute-solvent and weak solute-solute interactions in these systems, and that their values are influenced by the side groups in these amino acids. Positive 0

vtrφ values indicate that electrostrictive effect of AAs is diminished on addition of caffeine because it shields the zwitterionic groups of AAs from water and the shielding effect becomes more and more as the side groups of AAs increase in size as we move from Gly to Val. The values of B and A coeffi-cients of the present AAs also support the behaviors of

0Vφ and

*VS . For all the AAs 0#

2µ∆ ＞0#1µ∆ , suggest-

ing that the formation of transition state is less favored in the presence of AAs and all the AAs behave as struc-ture-makers in aqueous caffeine solution. The decrease in Hn with temperature is attributed to the decreased hydration of AAs. The values of ∆H* and ∆S* suggest that the formation of activated species necessary for viscous flow is easy for Gly as compared to the rest three AAs. Almost linear increase in RD with concentra-tion for all the AAs studied is attributed to the increase in overall polarizability of the systems as the concentra-tion of AAs in the system increases.

Acknowledgement

SH is thankful to CSIR (New Delhi) for financial support in the form of Research Associateship.

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Documents

Physicochemical Properties of Amino Acids in Aqueous Caffeine Solution at 25, 30, 35 and 40 °C