volumetric, viscometric, and refractive index behaviour of α-amino acids and their groups’...

$: Volumetric, viscometric, and refractive index behaviour of α-amino acids and their groups’ contribution in aqueous d-glucose solution at different temperatures$
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J. Chem. Thermodynamics 38 (2006) 136–143

Volumetric, viscometric, and refractive index behaviourof a-amino acids and their groups� contribution in aqueous

D-glucose solution at different temperatures

Anwar Ali *, Soghra Hyder, Saba Sabir, Dinesh Chand, Anil Kumar Nain

Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi 110 025, India

Received 13 January 2005; received in revised from 7 April 2005; accepted 8 April 2005

Available online 6 June 2005

Abstract

Density, q, viscosity, g, and refractive index, nD measurements have been carried out on a-amino acids, glycine, DL-alanine,

L-serine, and DL-valine (0.1 to 0.5) M in 0.2 M aqueous D-glucose solution at T = (298.15, 303.15, 308.15, and 313.15) K. These mea-

surements have been performed to evaluate some important parameters, viz, apparent molar volume, /v, limiting apparent molar

volume, /�v transfer volume, /�

vðtrÞ, viscosity A and B-coefficients of Jones–Dole equation, variation of B with temperature, dB/dT,

free energy of activation per mole of solvent, Dl�#1 , and solute, Dl�#

2 , respectively, and molar refractive index, RD. These parameters

have been interpreted in terms of solute–solute and solute–solvent interactions and structure making/breaking ability of solutes in

the given solution. In addition to this, /�v, B-coefficient and Dl�#

2 , have been split into group contributions NHþ3 ; COO�; and CH2

of the amino acids using their linear correlation with number of carbon atoms in the alkyl chain of the amino acids.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Amino acids; Glucose; Partial molar volume; B-coefficient; Transfer volume; Groups� contribution

1. Introduction

The present work is a continuation of our research

program [1–3], on the physicochemical studies of aminoacids (AAs) in mixed aqueous solution and the effect of

these compounds on water structure. Such study helps

in a better understanding of the interactions occurring

between amino acid molecules and entities present in

mixed aqueous medium in the living cells. Although a

lot of attention has been given to the behaviour of

AAs in different salt-water mixed solvents [4–8], very

few studies have been carried out on AAs in (carbohy-drate + water) mixtures [9–11], probably due to the

0021-9614/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jct.2005.04.011

* Corresponding author. Tel.: +91 11 26981717x3257; fax: +91 11

26980229.

E-mail addresses: [email protected], [email protected]

(A. Ali).

complex nature of their interactions. Information has

been obtained about the interactions between carbohy-

drates and proteins from X-ray crystallography

[12,13], n.m.r. spectra, computer calculations [14], chro-matography data [15], and kinetic studies [16,17]. How-

ever, thermodynamic studies of these compounds in

solutions are rare.

Carbohydrates located at cell surfaces, are important

as receptors for the bioactive structures of harmones,

enzymes, viruses, antibodies, etc. [18]. Therefore, the

study of carbohydrate-protein interactions is very

important for immunology, biosynthesis, pharmacol-ogy, and medicine [9]. It is widely recognised that sugars

(as cosolutes) help in stabilizing biological macromole-

cules [19]. This action is performed either due to direct

interactions between them and/or through alteration of

the water structure [20–22]. These considerations led

us to undertake the study of amino acids (glycine

mailto:[email protected]

mailto:[email protected]

A. Ali et al. / J. Chem. Thermodynamics 38 (2006) 136–143 137

(Gly), DL-alanine (Ala), L-serine (Ser), and DL-valine

(Val)) in concentration range (0.1 to 0.5) M in 0.2 M

aqueous D-glucose solution at different temperatures.

Interactions of AAs in aqueous D-glucose solution and

the temperature dependence of these interactions play

a key role in understanding the biochemical processesin living cells.

In this paper, we report the densities, q, viscosities, g,and refractive indices, nD of (0.1, 0.2, 0.3, 0.4, and 0.5)

M Gly, Ala, Ser, and Val in 0.2 M aqueous D-glucose

solution at T = (298.15, 303.15, 308.15, and 313.15) K.

Various physical parameters like apparent molar vol-

ume, /v, limiting apparent molar volume, /�v transfer

volume, /�vðtrÞ, the viscosity A and B-coefficients derived

from Jones–Dole [23] equation, free energies of activa-

tion of viscous flow, Dl�#1 and Dl�#

2 per mole of solvent

and solutes, respectively, were calculated from the den-

sity and viscosity data. nD values were used to compute

molar refractivity, RD of the ternary mixtures under

study. All these parameters are discussed in terms of sol-

ute–solvent and solute–solute interactions occurring in

the (AA + glucose + water) systems and the structuremaking/breaking tendency of the solutes in the given

solvent.

In addition to this, the contributions of the charged

end groups ðNHþ3 ;COO�Þ and CH2 groups of the AAs

to, /�v, B-coefficient and Dl�#

2 have also been deter-

mined. The various contributory factors to the magni-

tude of polar /�v, B-coefficient and Dl�#

2 are discussed.

2. Experimental

Glycine (mass fraction >0.997, Merck), DL-alanine

(mass fraction >0.990, Loba-Chemie), L-serine (mass

fraction >0.985, Thomas Baker), and DL-valine (mass

fraction >0.990, Loba-Chemie) were used after recrys-

tallization from (ethanol + water) mixture and driedover P2O5 in a desiccator for 72 h. before use. D-Glucose

(mass fraction >0.990, Merck) was used as such without

any pretreatment. Aqueous glucose solution of 0.2 M

concentration was prepared using double distilled deion-

ized water and was used as solvent to prepare (0.1, 0.2,

0.3, 0.4, and 0.5) M amino acid solutions, which were

stored in special air tight bottles.Exposure of the solu-

tions to air was kept to a minimum to avoid contamina-tion and evaporation. The weighings were done on a

Precisa XB 220 A (Swiss make) electronic balance with

a precision of ±0.1 mg. The densities and viscosities of

the solutions were measured using a pycnometer and

Ubbelohde-type suspended level viscometer, respec-

tively, the method of which is given in our earlier work

[1–3]. The accuracies in q and g measurements were

found to be ±0.01 kg Æ m�3 and 3 Æ 10�6 N Æ m�2 Æ s,respectively. Refractive indices were measured with the

help of a thermostated Abbe-refractometer after cali-

brating it with distilled water and toluene at known tem-

peratures. g and nD measurements were carried out in

triplicate. The error in nD measurements was less than

±0.001 units. The temperature of the solutions was

maintained in an electronically controlled water bath

(JULABO, Germany) having a precision of ±0.02 K.

3. Results and discussion

The experimental values of q, g, and nD of 0.2 M

aqueous D-glucose solution and of (0.1, 0.2, 0.3, 0.4,

and 0.5) M amino acids in aqueous D-glucose solution

at T = (298.15, 303.15, 308.15, and 313.15) K are listedin table 1. The apparent molar volumes, /v were calcu-

lated from q data using the following relation:

/v ¼ ðq0 � qÞ=Cq0 þMq0; ð1Þwhere q and q0 are the densities of solution and solvent

(0.2 M aqueous D-glucose), respectively, C is the molar

concentration of amino acid and M is its molar mass.The calculated values of /v are included in table 2.

From the /v data, /v vs. C1/2 curves have been drawn

and the plots were found to be linear in the concentra-

tion range studied in all cases with positive slopes, as

represented by the following equation [24]:

/v ¼ /�v þ S�

vC1=2. ð2Þ

The intercept /�v which is the limiting apparent molar

volume of the solute (equal to the partial molar volume

at infinite dilution, �V �2) is obtained by the least-squares

fitting of /v values to the above equation. These values

along with the experimental slope, S�v are reported in

table 3. The volume behaviour of a solute at infinite

dilution is satisfactorily represented by /�v which is inde-

pendent of the solute–solute interactions and provides

information concerning solute–solvent interactions.

Table 3 reveals that the values of /�v are large positive

for all the ternary mixtures at all the temperatures inves-

tigated, suggesting the presence of strong solute–solvent

interactions in these mediums. Further, /�v values

increase as the size of alkyl group in the amino acids in-

creases from Gly to Val. The hydration behaviour ofamino acids can be examined by considering the follow-

ing interactions [25–30]: (a) the terminal groups of

zwitterions of AAs, NHþ3 and COO� are hydrated in

an electrostatic manner whereas, hydration of interven-

ing backbone depends on its nature which may be

hydrophobic, hydrophilic or amphiphilic; (b) electro-

striction of NHþ3 group is greater than COO� group

by a factor of �10; and (c) the overlap of hydration co-spheres of terminal (NHþ

3 and COO� groups) and of

adjacent groups results in volume change. /�v increases

due to the reduction in the electrostriction at the termi-

nals whereas it decreases due to disruption of side group

hydration by that of the charged end. A pronounced

TABLE 1

Values of density, q, viscosity, g, and refractive index, nD, of amino

acids, glycine, DL-alanine, L-serine, and DL-valine in aqueous D-glucose

at T = (298.15, 303.15, 308.15, and 313.15) K

C/(mol Æ l�1) 298.15 K 303.15 K 308.15 K 313.15 K

Glycine + aq. D-glucose

q/(kg Æ m�3)

0.0 1010.3 1008.7 1007.1 1005.3

0.1 1013.6 1011.9 1010.3 1008.4

0.2 1016.7 1015.0 1013.3 1011.4

0.3 1019.8 1018.1 1016.3 1014.4

0.4 1022.8 1021.1 1019.3 1017.4

0.5 1025.8 1024.1 1022.3 1020.4

10�3 Æ g/(N Æ m�2 Æ s)0.0 1.01 0.89 0.80 0.70

0.1 1.00 0.88 0.79 0.71

0.2 1.01 0.90 0.81 0.72

0.3 1.03 0.92 0.83 0.73

0.4 1.04 0.93 0.84 0.75

0.5 1.07 0.95 0.86 0.76

nD0.0 1.3360 1.3350 1.3349 1.3340

0.1 1.3372 1.3368 1.3361 1.3357

0.2 1.3381 1.3378 1.3372 1.3365

0.3 1.3398 1.3390 1.3387 1.3382

0.4 1.3410 1.3402 1.3398 1.3390

0.5 1.3420 1.3418 1.3413 1.3410

DL-Alanine + aq. D-glucose

q/(kg Æ m�3)

0.0 1010.3 1008.7 1007.1 1005.3

0.1 1013.8 1012.1 1010.4 1008.5

0.2 1016.6 1014.8 1013.0 1011.1

0.3 1018.9 1017.1 1015.3 1013.3

0.4 1021.0 1019.2 1017.3 1015.3

0.5 1022.9 1021.1 1019.2 1017.1

10�3 Æ g/(N Æ m�2 Æ s)0.0 1.01 0.89 0.80 0.70

0.1 1.02 0.90 0.81 0.71

0.2 1.04 0.93 0.83 0.74

0.3 1.07 0.95 0.85 0.75

0.4 1.10 0.97 0.87 0.77

0.5 1.12 0.99 0.89 0.79

nD0.0 1.3360 1.3350 1.3349 1.3340

0.1 1.3389 1.3379 1.3369 1.3360

0.2 1.3395 1.3387 1.3380 1.3372

0.3 1.3410 1.3400 1.3392 1.3385

0.4 1.3421 1.3413 1.3408 1.3399

0.5 1.3445 1.3425 1.3416 1.3411

L-Serine + aq. D-glucose

q/(kg Æ m�3)

0.0 1010.3 1008.7 1007.1 1005.3

0.1 1015.1 1013.4 1011.7 1009.8

0.2 1019.6 1017.8 1016.0 1014.0

0.3 1023.9 1022.1 1020.2 1018.1

0.4 1028.1 1026.3 1024.3 1022.2

0.5 1032.3 1030.4 1028.4 1026.2

TABLE 1 (continued)

C/(mol Æ l�1) 298.15 K 303.15 K 308.15

K

313.15 K

10�3 Æ g/(N Æ m�2 Æ s)0.0 1.01 0.89 0.80 0.70

0.1 1.05 0.92 0.81 0.71

0.2 1.08 0.94 0.84 0.73

0.3 1.12 0.97 0.86 0.75

0.4 1.15 1.00 0.88 0.77

0.5 1.17 1.02 0.90 0.78

nD0.0 1.3360 1.3350 1.3349 1.3340

0.1 1.3391 1.3383 1.3379 1.3372

0.2 1.3412 1.3406 1.3400 1.3397

0.3 1.3426 1.3420 1.3419 1.3413

0.4 1.3443 1.3440 1.3435 1.3431

0.5 1.3465 1.3459 1.3459 1.3454

DL-Valine + aq. D-glucose

q/(kg Æ m�3)

0.0 1010.3 1008.7 1007.1 1005.3

0.1 1013.0 1011.3 1009.6 1007.8

0.2 1015.5 1013.8 1012.0 1010.1

0.3 1018.0 1016.2 1014.3 1012.4

0.4 1020.4 1018.6 1016.6 1014.7

0.5 1022.8 1021.0 1018.9 1017.0

10�3 Æ g/(N Æ m�2 Æ s)0.0 1.01 0.89 0.80 0.70

0.1 1.04 0.92 0.83 0.74

0.2 1.08 0.97 0.88 0.78

0.3 1.14 1.01 0.92 0.82

0.4 1.18 1.06 0.96 0.86

0.5 1.22 1.09 1.00 0.89

nD0.0 1.3360 1.3350 1.3349 1.3340

0.1 1.3397 1.3381 1.3375 1.3369

0.2 1.3410 1.3401 1.3391 1.3385

0.3 1.3430 1.3428 1.3411 1.3402

0.4 1.3450 1.3442 1.3431 1.3428

0.5 1.3480 1.3460 1.3453 1.3445

138 A. Ali et al. / J. Chem. Thermodynamics 38 (2006) 136–143

increase in /�v from Gly to Val (table 3) may be attrib-

uted to the increased hydrophobicity/non-polar charac-

ter of the side chain as the H atom of Gly is replaced

by a hydrophobic group –CH3 in Ala and by a more

hydrophobic group –CH(CH3)2 in Val. As a conse-

quence of (b), relatively freer N-terminal in Gly would

cause the largest volume contraction followed by Ala

and least by Val in which the N-terminal is highlyshielded for electrostriction. Similar increase in /�

v with

increasing side chain length from Gly to Val have been

reported by Banipal and Kapoor [31], in aqueous solu-

tion. It may also be noted that for all the amino acids

in aqueous glucose solution, /�v increases with rise in

temperature. The increase in /�v with temperature may

be due to release of some solvent molecules from the

loose solvation layers of the solutes in solution [32].

TABLE 2

Values of apparent molar volume, /v, for amino acids, glycine, DL-

alanine, L-serine, and DL-valine in aqueous D-glucose at T = (298.15,

303.15, 308.15, and 313.15) K

C/(mol Æ l�1) 10�5 Æ /v/(m3 Æ mol�1)

298.15 K 303.15 K 308.15 K 313.15 K


0.1 4.16 4.27 4.33 4.38

0.2 4.26 4.32 4.38 4.43

0.3 4.30 4.34 4.41 4.45

0.4 4.34 4.37 4.43 4.46

0.5 4.36 4.39 4.44 4.46


0.1 5.35 5.46 5.57 5.68

0.2 5.70 5.80 5.92 5.98

0.3 5.98 6.06 6.13 6.21

0.4 6.17 6.23 6.31 6.38

0.5 6.32 6.37 6.44 6.52


0.1 5.65 5.76 5.87 5.98

0.2 5.80 5.91 6.02 6.13

0.3 5.92 5.99 6.10 6.21

0.4 6.00 6.06 6.17 6.25

0.5 6.05 6.12 6.20 6.30


0.1 8.92 9.04 9.15 9.22

0.2 9.02 9.09 9.20 9.267

0.3 9.06 9.14 9.25 9.30

0.4 9.10 9.16 9.27 9.32

0.5 9.12 9.18 9.29 9.33


The experimental S�v values (table 3) for all the amino

acids are found to be positive but smaller than /�v values,

suggesting that solute–solute interactions are weaker

than solute–solvent interactions in the systems under

study. For all the amino acids studied, S�v decreases with

temperature. This indicates the decreased solute–solute

interaction with rise in temperature. Generally speaking,

the types of interactions occurring between the AA mol-ecules and D-glucose molecules can be classified as

follows:

(i) Hydrophilic-ionic interactions between the OH

group of the carbohydrate and the zwitterionic

centres of the AAs.

(ii) Hydrophobic–hydrophobic interactions between

the non-polar side groups of the carbohydratemolecules and the AAs.

(iii) Hydrophilic–hydrophilic interactions between the

OH group of the carbohydrate molecules and the

OH group of the AAs.

(iv) Hydrophilic–hydrophobic interactions between

the OH group of the carbohydrate molecules and

the non-polar side group of the AAs.

Similar types of interactions were also suggested by

Huaji et al. [10], in their study of enthalpic behaviour

of Gly, Ala, and Ser in aqueous saccharides at

T = 298.15 K.

The transfer volumes, /�vðtrÞ of the AAs from aque-

ous glucose to aqueous solution were calculated using

the following equation:

/�vðtrÞ ¼ /�

vðaq: glucoseÞ � /�vðaqÞ; ð3Þ

and are included in table 3. The /�vðaqÞ at T = (298.15,

308.15, and 313.15) K were taken from the literature

[26,31,33,34]. The /�vðtrÞ are found to be negative for

all the AAs� solutions at all studied temperatures. As po-

sitive /�vðtrÞ values are reported for Gly in higher con-

centrations of glucose in water by other workers

[11,35], we may conclude that /�vðtrÞ decreases with de-

crease in concentration of glucose and ultimately gives

negative values in our case. The increased concentra-

tions of glucose lead to the greater hydrophilic-ionic

and hydrophilic–hydrophilic interactions that are not

compounded by hydrophilic–hydrophobic interactions.

Increase in /�vðtrÞ with increase in glucose concentration

has also been reported by Changwei et al. [36] in argi-

nine + aqueous glucose solutions.It is of interest to examine the contribution of

side-chain residues (R = CH3, CH2OH, and CH(CH3)2)

on the a-C of AAs to the partial molar volumes [37],

and transfer volumes [38], using the following

equations:

/�vðRÞ ¼ /�

v ðsubstituted AAÞ � /�vðGlyÞ; ð4Þ

/�vðtrÞðRÞ ¼ /�

vðtrÞ ðsubstituted AAÞ � /�vðtrÞðGlyÞ; ð5Þ

where /�vðRÞ is the contribution of the R group to the

partial molar volume, /�v and /�

vðtrÞðRÞ is the contribu-tion of the R group to the transfer volume /�

vðtrÞ and are

given in table 3. In this calculation it is assumed that vol-ume contribution of the H atom in Gly can be neglected.

The results of /�vðRÞ reveal that the contribution of R

group is significant and positive for all the AAs and they

increase with increase in side-chain from CH3 to

CH(CH3)2. The /�vðtrÞðRÞ are all negative and increase

as we move from Ala to Val. The introduction of a

CH3 group in Ala and CH(CH3)2 group in Val provides

additional tendency of hydrophilic–hydrophobic (type(iv)) as well as hydrophobic–hydrophobic (type (ii))

interactions, while the replacement of one H atom of

Ala by OH group in Ser adds to hydrophilic–hydro-

philic interactions (type (iii)). This may be supported

by the work of Quingwang et al. [38], on the ternary sys-

tems glycine, L-alanine, and L-serine in (ethylene gly-

col + water) mixtures, in which similar results have

been reported.The viscosity A and B coefficients for the AAs in

aqueous D-glucose solutions were calculated from the

Jones–Dole equation [23,24]

gr ¼ g=go ¼ 1þ AC1=2 þ BC; ð6Þ

TABLE 3

Values of limiting apparent molar volume, /�v, experimental slope, S�

v, /�vðwaterÞ, volume transfer, /�

vðtrÞ, and side chain contribution to /�v, and /�

vðtrÞ,for amino acids, glycine, DL-alanine, L-serine, and DL-valine in aqueous D-glucose solution at T = (298.15, 303.15, 308.15, and 313.15) K

298.15 K 303.15 K 308.15 K 313.15 K


10�5 � /�v=ðm3 �mol�1Þ 4.02 4.18 4.24 4.33

10�6 � S�v=ðm3 �mol�3=2 � l1=2Þ 4.95 2.98 2.83 1.98

10�5 � /�vðwaterÞ=ðm3 �mol�1Þ 4.32a 4.38 4.40b

10�5 � /�vðtrÞ=ðm3 �mol�1Þ �0.30 �0.14 �0.07


10�5 � /�v=ðm3 �mol�1Þ 4.57 4.74 4.89 5.01

10�6 � S�v=ðm3 �mol�3=2 � l1=2Þ 25.07 23.40 22.37 21.50

10�5 � /�vðwaterÞ=ðm3 �mol�1Þ 6.05a 6.10a 6.12c

10�5 � /�vðtrÞ=ðm3 �mol�1Þ �1.47 �1.21 �1.11

10�5 � /�vðCH3Þ=ðm3 �mol�1Þ 0.55 0.56 0.64 0.68

10�5 � /�vðtrÞðCH3Þ=ðm3 �mol�1Þ �1.17 �1.07 �3.28


10�5 � /�v=ðm3 �mol�1Þ 5.33 5.48 5.61 5.75

10�6 � S�v=ðm3 �mol�3=2 � l1=2Þ 10.33 9.01 8.65 8.03

10�5 � /�vðwaterÞ=ðm3 �mol�1Þ 6.06a 6.12a 6.17c

10�5 � /�vðtrÞ=ðm3 �mol�1Þ �0.73 �0.50 �0.42

10�5 � /�vðCH2OHÞ=ðm3 �mol�1Þ 1.31 1.31 1.37 1.41

10�5 � /�vðtrÞðCH2OHÞ=ðm3 �mol�1Þ �0.42 �0.30 �3.96


10�5 � /�v=ðm3 �mol�1Þ 8.78 8.92 9.04 9.13

10�6 � S�v=ðm3 �mol�3=2 � l1=2Þ 4.95 3.68 3.68 2.84

10�5 � /�vðwaterÞ=ðm3 �mol�1Þ 9.09a 9.15a 9.17d

10�5 � /�vðtrÞ=ðm3 �mol�1Þ �0.32 �0.12 �0.03

10�5 � /�vCHðCH3Þ2=ðm3 �mol�1Þ 4.76 4.74 4.79 4.80

10�5 � /�vðtrÞCHðCH3Þ2=ðm3 �mol�1Þ �0.02 �0.06 �0.03

a Data taken from reference [31].b Data taken from reference [33].c Data taken from reference [26].d Data taken from reference [34].


where gr is the relative viscosity, g and go are the viscos-ities of the solution and the solvent (aqueous D-glucose),

respectively. A is determined by the ionic attraction the-

ory of Falkenhagen–Vernon and therefore also called

Falkenhagen coefficient [24], B or Jones–Dole coefficient

is an empirical constant determined by solute–solvent

and solvent–solvent interactions. A and B were obtained

from the intercepts and slopes of the plots of (gr � 1)/C1/2

vs. C1/2.

Eyring and co-workers [39], proposed that the free

energy of activation of viscous flow per mole of solvent,

Dl�#1 could be calculated from the following equation:

g0 ¼ ðhNA=�V�1Þ expðDl

�#1 =RT Þ; ð7Þ

where h; NA; and �V �1 are the Planck�s constant, Avoga-

dro�s number and partial molar volume of the solvent,

respectively. On rearranging we get

Dl�#1 ¼ RT lnðgo �V

�1=hNAÞ. ð8Þ

Feakins et al. [40], showed that if equations (6) and (8)

are obeyed, then

B ¼ ð�V �1 � �V �

2Þ þ �V �1 ðDl�#

2 � Dl�#1 Þ=RT

� �; ð9Þ

where �V �2 is the partial molar volume ð/�

vÞ of the solute

(AAs). It can be rearranged as follows:

Dl�#2 ¼ Dl�#

1 þ ðRT =V �1Þ B� ð�V �

1 � �V �2Þ

� �. ð10Þ

The values of A; B; Dl�#1 and Dl�#

2 are included in table

4. It is observed from table 4 that the larger and positive

values of B-coefficient compared to A-coefficient sup-

port the behaviour of /�v and S�

v, respectively, both sug-

gesting stronger solute–solvent interactions as comparedto solute–solute interactions. The variation of B with

temperature, dB/dT provides direct evidence regarding

structure-making or breaking ability of the solute in

the solution. Figure 1 depicts that B for Gly decreases

with rise in temperature whereas for Ser and Val it in-

creases with increase in temperature. Since dB/dT is

negative for structure-maker and positive for structure-

breaker [41], we can classify Gly as structure-makerand Ser and Val as structure-breaker in aqueous D-

glucose solutions. Ser having a polar OH group is in-

volved in hydrogen bonding with water, resulting in a

more pronounced breaking up of solvent structure.

The B value for Ala increases up to T = 303.15 K and

then decreases at higher temperatures. This might be

TABLE 4

Values of A and B coefficients of Jones–Dole equation, free energy of activation for the solvent, Dl�#1 and solute, Dl�#

2 for amino acids, glycine, DL-

alanine, L-serine, and DL-valine in aqueous D-glucose at T = (298.15, 303.15, 308.15, and 313.15) K

298.15 K 303.15 K 308.15 K 313.15 K


10�2 Æ A/(dm3/2 Æmol�1/2) �12.65 �9.51 �7.01 �4.92

10�1 Æ B/(dm3 Æ mol�1) 3.01 2.76 2.65 2.57

Dl�#1 =ðkJ �mol�1Þ 9.51 9.36 9.23 9.05

Dl�#2 =ðkJ �mol�1Þ 52.95 50.28 49.33 48.73


10�2 Æ A/(dm3/2 Æmol�1/2) �6.14 �5.20 �3.20 �1.83

10�1 Æ B/(dm3 Æ mol�1) 3.26 3.27 3.07 3.05

Dl�#1 =ðkJ �mol�1Þ 9.51 9.36 9.23 9.05

Dl�#2 =ðkJ �mol�1Þ 57.04 58.09 56.01 56.39


10�2 Æ A/(dm3/2 Æmol�1/2) 4.53 1.25 �2.17 �5.87

10�1 Æ B/(dm3 Æ mol�1) 2.77 2.90 3.13 3.33

Dl�#1 =ðkJ �mol�1Þ 9.51 9.36 9.23 9.05

Dl�#2 =ðkJ �mol�1Þ 51.44 53.97 57.81 61.43


10�2 Æ A/(dm3/2 Æmol�1/2) �4.57 �3.07 �1.21 �1.26

10�1 Æ B/(dm3 Æ mol�1) 5.04 5.18 5.46 5.46

Dl�#1 =ðkJ �mol�1Þ 9.51 9.36 9.23 9.05

Dl�#2 =ðkJ �mol�1Þ 86.62 89.85 94.83 96.04

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

293 298 303 308 313 318

T/ K

10-1

. B/(

dm ·3

mol

-1)

FIGURE 1. Variation of B against temperatures T/K for the amino

acids {(r) glycine, (j) DL-alanine, (m) L-serine, and (——) DL-valine}

in aqueous glucose.


due to decrease in solvation at higher temperatures. This

behaviour suggests enhancing of water structure by Ala

at lower temperature and disruption of liquid structure

at higher temperatures.

It is evident from table 4 that Dl�#2 values are positive

and much larger than Dl�#1 , suggesting that the interac-

tions between solute (amino acids) and solvent

(water + D-glucose) molecules in the ground state are

stronger than in the transition state. Thus, the solvation

of the solute in the transition state is unfavourable in

free energy terms. Further, Dl�#2 increases from Gly to

Val at a given temperature, indicating that the solvation

of amino acid molecules becomes increasingly unfavour-

able as the hydrophobicity or the number of carbon

atoms of the side chain increases from Gly to Val. Thus,

the results inferred from Dl�#2 are consistent with those

inferred from /�v.

3.1. Amino acids group contributions

A more useful discussion of the volumetric and

viscometric data can be achieved by separating the

contribution of ðNHþ3 ;COO�Þ and ðCH2Þ groups to

/�v, B-coefficient, and Dl�#

2 values of amino acids. It

is observed that /�v values of amino acids vary linearly

with the number of C atoms in their alkyl chains at

given temperatures. This linear relation can be repre-

sented by

/�v ¼ /�

vðNHþ3 ;COO�Þ þ nc/

�vðCH2Þ; ð11Þ

where nc is the number of C atoms in the alkyl chain of

the AAs. The alkyl chain of the homologous series ofa-amino acids investigated in this work are: CH2–

(Gly), CH3CH– (Ala), and CH3CH3CHCH– (Val). As

proposed by Hakin et al. [42,43]

/�vðCH3Þ ¼ 1.5/�

vðCH2Þ; ð12Þ

/�vðCHÞ ¼ 0.5/�

vðCH2Þ. ð13Þ

A linear regression analysis of /�v values at any given

temperature using equation (11) gives /�v of zwitterionic

end group ðNHþ3 ;COO�Þ and /�

v of methylene group

(CH2) contributions. The OH group contribution of

Ser was obtained by

3.80

3.85

3.90

3.95

4.00

4.05

4.10

0 0.1 0.2 0.3 0.4 0.5

C / (mol . l-1)

10-6

.R

D/(

m3 ·m

ol-1

)

FIGURE 2. Variation of molar refractive index, RD against concen-

tration, C for the amino acids {(r) glycine, (j) DL-alanine, (m)

L-serine, and (——) DL-valine} in aqueous glucose at T = 303.15 K.


/�vðOHÞ ¼ /�

vðSerÞ � /�vðNHþ

3 ;COO�Þ � /�vðCH2Þ � /�

vðCHÞ.ð14Þ

These results are listed in table 5. It may be pointed out

that /�vðCH2Þ value reported here includes the mean con-

tribution of CH and CH3 groups to /�v of the a-amino

acids. Table 5 reveals that values of /�vðNHþ

3 ;COO�Þin aqueous D-glucose are larger that those of

/�vðCH2Þ and /�

vðOHÞ. These results indicate that the

interactions of various groups of AAs with D-glucose

and water molecules increase in the following order:

OH < CH2 < ðNHþ3 ;COO�Þ.

Similar trends were reported in the ternary mixtures of

a-amino acids in aqueous CaCl2 solutions [44].As for /�

v the B-coefficient and Dl�#2 of AAs also vary

linearly with nc

B ¼ BðNHþ3 ;COO�Þ þ ncBðCH2Þ; ð15Þ

Dl�#2 ¼ Dl�#

2 ðNHþ3 ;COO�Þ þ ncDl

�#2 ðCH2Þ. ð16Þ

The regression of B and Dl�#2 data using equations (15)

and (16), respectively, gives B ðNHþ3 ;COO�Þ, B (CH2),

Dl�#2 ðNHþ

3 ;COO�Þ, and Dl�#2 ðCH2Þ values as respec-

tive contributions of ðNHþ3 ;COO�Þ and the CH2 groups

and are included in table 5. Similar linear correlation has

been found for some AAs in aqueous potassium thiocy-

anate [7], urea [45], and sodium butyrate [46]. The neg-

ative dB/dT values for ðNHþ3 ;COO�Þ groups (table 5)

confirm that these charged end groups are structure-

makers while CH2 groups with positive dB/dT values

are structure-breakers. In Gly due to single CH2 group,

the structure-making effect of the charged groups is

more pronounced, making it a structure-maker in aque-

ous glucose solution. In case of other AAs the stabiliza-

tion effect of charged groups is overcome owing to the

increase in alkyl chain and we observe a positive dB/dT as a result of net structure-breaking effect.

The experimental refractive index presented in table 1

shows an increasing trend with increasing concentra-

TABLE 5

Contribution of ðNHþ3 ;COO�Þ, (OH), and (CH2) groups to the

limiting apparent molar volume, /�v and of ðNHþ

3 ;COO�Þ and ðCH2Þgroups to B-coefficients and Dl�#

2 , of the amino acids glycine,

DL-alanine, L-serine, and DL-valine in aqueous D-glucose solution at

T = (298.15, 303.15, 308.15, and 313.15) K

298.15 K 303.15 K 308.15 K 313.15 K

10�5 � /�v=ðm3 �mol�1Þ

ðNHþ3 ;COO�Þ 1.92 2.09 2.17 2.27

(CH2) 1.66 1.65 1.67 1.67

(OH) 0.92 0.92 0.94 0.98

10�1 Æ B/(dm3 Æ mol�1)

ðNHþ3 ;COO�Þ 0.21 0.18 0.15 0.14

(CH2) 0.07 0.08 0.10 0.10

Dl�#2 =ðkJ �mol�1ÞðNHþ

3 ;COO�Þ 38.16 34.40 29.92 28.91

(CH2) 11.73 13.57 15.77 16.35

tions of AAs. This behaviour is consistent with the re-

sults which show the effect of AAs� concentration onthe apparent molar volume, indicating that the refrac-

tive index is directly related to the interactions in the

solution. nD data were used to calculate molar refractiv-

ity, RD, using Lorentz–Lorenz equation:

RD ¼ ðn2D � 1Þ=ðn2D þ 2Þ� � X3

i¼1

xiMi=q

!; ð17Þ

where xi and Mi are the mole fraction and molecular

weight of the ith component of the mixture. The variation

of RD at T = 303.15 K, as a function of AAs concentra-

tion, is depicted graphically in figure 2. Single temperature

plots have been given because there is not much variationinRD with temperature. The plots in figure 2 are found to

increase linearly with increasing amount of solute for all

the AAs. Since RD is directly proportional to the molecu-

lar polarizability, figure 2 reveals that the overall polariz-

ability of the four systems under study increases with

increasing amount of AAs in the mixture.

Acknowledgements

S.H. and A.K.N. are thankful to Council of Scientific

and Industrial Research and Department of Science and

Technology, New Delhi for the award of Research

Associateship and Young Scientist Fast Track Fellow-

ship, respectively.

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