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