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10 International Journal of Research in Pure and Applied Physics 2013; 3(2): 10-15
ISSN 2278 -134X
Original Article
STRUCTURE-MAKING AND BREAKING BEHAVIOUR OF SOME
BASE AMINO ACIDS IN AQUEOUS DIMETHYL SULPHOXIDE (DMSO)
MEDIUM AT VARYING TEMPERATURES *S.THIRUMARAN,
1GENE GEORGE,
1G.NAGESWARI AND
2 K.SATHISH
*Department of Physics (DDE), Annamalai University, Annamalai nagar- 608 002 1Department of Physics, T.B.M.L College, Poraiyar,
2Department of Physics, Research & Development Centre, Bharathiar University, Coimbatore
E-Mail: thirumaran64@gmail.com
Received 27 March 2013; accepted 06 April 2013
Abstract
The present study aims for the structure-making and structure-breaking behaviour of some amino acids in aqueous
dimethyl sulphoxide (DMSO) solution at 303.15, 308.15 and 313.15K. Experimental values of density, viscosity and
ultrasonic velocities were carried out on the ternary mixtures of water +dimethyl sulphoxide (DMSO) + amino acids,
namely (L- arginine, L- lysine, and L-histidine at 303.15, 308.15 and 313.15K. For this, binary solvent mixture
(water+DMSO) was prepared at two molarities.(Say, at 0.0M and 0.4M). The related and relevant parameters correlated to
the present study such as adiabatic compressibility (),apparent molar compressibility (K), apparent molar volume (V),
limiting apparent molar compressibility (0
Kφ ), limiting apparent molar volume (0
Vφ ) and their associated constants (SK,
SV), partial transfer volume (∆0
Vφ ) from water to aqueous solution and viscosity B-Coefficient of Jones-Dole equations
were meticulously evaluated to substantiate the present aim of this study.
© 2013 Universal Research Publications. All rights reserved
1 INTRODUCTION
Ultrasonic study on the amino acids with aqueous solution
of electrolytes and non-electrolytes provides useful
information in understanding the behaviour of intra-
molecular and intermolecular associations, complex
formation and related structural changes in liquid systems.
For the past two decades, a considerable study has been
carried out to investigate the hydration of proteins through
volumetric and ultrasonic measurements, since these
properties are sensitive to the degree and nature of
hydration.[1,2] Due to the complex molecular structure of
proteins, direct study is somewhat difficult. Therefore, an
useful approach is to study simpler model compounds, such
as amino acids which are building blocks of proteins. The
investigation of volumetric and thermodynamic properties
of amino acids and peptides in aqueous and mixed aqueous
solvents[3,4] threw more light on the molecular interactions
such as hydrogen bonding, ion-ion, ion-solvent solute-
solvent etc., and eventually exhibited the behaviour of
structure-making and breaking of studied amino acids in
the solvent mixture.
More recent studies on salt solutions exploring the large
effect on the structure and properties of proteins including
their solubility, denaturation, distribution into subunits, and
the activity of enzymes.[5,6]
Proteins are complex
molecules and their behavior in solutions are governed by
combination of many specific interactions.[7,8] have
revealed that the presence of an electrolyte drastically
affects the behavior of amino acids in solutions which can
be used for their separation and purification. Several
investigations so far carried out by the different workers on
the partial molar volumes, adiabatic compressibilities, the
heat capacity and Gibb’s free energy of transfer volume
and the Viscosity-B coefficient measurements on amino
acids with organic salts such as CaCl2, Na2SO4, KSCN and
NH4Cl solution.[9,10]
Amino acids are molecules containing an amine group, a
carboxylic acid group and a side chain that varies between
different amino acids with the general formula
NH2CHRCOOH, where R is an organic substituent. Polar
amino acids have R groups that do not ionize in solution
but are quite soluble in water due to their polar character.
They are also known as hydrophilic, or "water loving"
amino acids. Proteins are formed by polymerizing
monomers that are known as amino acids, because they
contain an amino (-NH2) and a carboxylic acid (-CO2 H)
Available online at http://www.urpjournals.com
International Journal of Research in Pure and Applied Physics
Universal Research Publications. All rights reserved
11 International Journal of Research in Pure and Applied Physics 2013; 3(2): 10-15
Table -1 Values of Density ( ), Viscosity ( ) and Ultrasonic Velocity ( U ) of L –arginine, L –lysine and L -Histidine in aqueous DMSO at 303.15 K, 308.15 and 313.15 K for
Table - 2 Values of Adiabatic Compressibility ( ), Apparent Molar Compressibility (K) and Apparent Molar Volume ( V ) of L –arginine, L - lysine and L - histidine in aqueous DMSO at 303.15 K, 308.15 and 313.15 K for
functional group. With the exception of the amino acid,
proline, which is a secondary amine, the amino acids used
to synthesize proteins are primary amines with the
following general formula
H2N – CH – CO
2H
R
The chemistry of amino acids is complicated by the fact
that the –NH2 group is a base and the –CO2 H 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.
H3N+ – CH – CO
2
R
H2N – CH – CO
2H
R
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).
There was two type of amino acids, namely Polar, Non-
Polar. In polar, we get another two branches namely (i)
polar /acidic; (ii) polar / basic. General formula for these,
12 International Journal of Research in Pure and Applied Physics 2013; 3(2): 10-15
Table - 3 Values of Limiting Apparent Molar Compressibility ( 0
K ) Limiting Apparent molar volume ( 0
V ), Constant SK and SV of amino acids at 303.15 , 308.15and 313.15 K
Table - 4 Values of A and B Parameters of Jones - Dole Equation and Partial transfer volume ( 0
V ) of amino acids at 303.15 K, 308.15 K and 313.15 K for
Non-polar: R = HM CH3, alkyl groups, aromatic,
O
Polar: R = CH2OH – CH2SH. – CH2C-NH2
The following amino acids were taken for the present study
are
i. L-arginine
ii. L-lysine, and
iii. L-histidine,
2. EXPERIMENTAL DETAILS
The chemicals used in this present study were procured
from the reputed companies with minimum assay of 99.9%.
Freshly prepared doubly distilled water (sp. conductivity -
10-6
ohm-1
cm-1
) was used for preparing the solutions at
different concentrations. Aqueous solutions were prepared
and used on the day they were prepared. The required
quantity of binary mixture [water + dimethylsulphoxide
(DMSO)] was prepared under different molarities(M) say
at 0.0M and 0.4M.The aimno acids taken for study
L-arginine, L-lysine and L-histidine were added to this
binary solvent under different molarities. The required
quantity of amino acids for a given molarity were added in
the binary mixture and similar procedure was adapted for
other amino acids. The chemicals were weighed in an
electronic digital balance (SHIMADZU AX-200, Japan
Make) with a least count of 0.0001g. The density was
determined using a specific gravity bottle by relative
measurement method with an accuracy of ±0.01kgm-3
. An
Ostwald’s viscometer of 10ml capacity was used for the
viscosity measurement. Efflux time was determined using a
digital chronometer within ±0.01s. An Ultrasonic
Interferometer having the fixed frequency of 2MHz (Mittal
Enterprises, New Delhi-Model: F-81) with an overall
accuracy of 2ms-1
has been used for velocity measurement.
An electronically digital operated constant temperature bath
(RAAGA Industries, Chennai) has been used to circulate
water through the double walled measuring cell made
up of steel containing the experimental solution at desired
temperature, whose accuracy is maintained at ± 0.1K.
3 RESULTS AND DISCUSSION
The observed experimental values of density (ρ), viscosity (η)
and ultrasonic velocity (U) of amino acids in aqueous dimethyl
sulphoxide (DMSO) at varying molarities, say (0M and 0.4M)
at a varying temperatures of 303.15, 308.15 and 313.15K
were calculated. The relevant parameters such as adiabatic
compressibility (),apparent molar compressibility (K),
apparent molar volume (V), limiting apparent molar
compressibility (0
Kφ ), limiting apparent molar volume
(0
Vφ ) and their associated constants (SK, SV), partial
transfer volume (∆0
Vφ ) from water to aqueous solution
and viscosity B-Coefficient of Jones-Dole equations for
the amino acids in aqueous dimethyl sulphoxide (DMSO)
at 303.15, 308.15 and 313.15K are listed in Tables 1 –
4. Similarly Figs 1& 2 represent the variation of molarity
with limiting apparent molar compressibility and volume
and Partial Transfer volume respectively at 303.15, 308.15
and 313.15K.
In the present study, the values of density, viscosity and
ultrasonic velocity (From Table 1) of mixture increases
with increase of molar concentration of amino acids and the
same, except ultrasonic velocity decrease with rise of
temperature. The increasing trend of ultrasonic velocity in
the mixtures suggests a moderate strong electrolytic nature
in which the solutes (amino acids) tend to attract the
solvent (aqueous dimethyl sulphoxide) molecules.[5,11]
Further, the increasing values of density with the increasing
molar concentration of amino acids suggesting an enhanced
molecular association in the solution. i.e., existence of
molecular interaction between solute (amino acids) and
solvent (aqueous dimethyl sulphoxide) molecules. The
above causes may also be attributed for an increase in
ultrasonic velocity in the mixtures. Such an increase
in ultrasonic velocity in these solutions may be attributed to
13 International Journal of Research in Pure and Applied Physics 2013; 3(2): 10-15
Fig. 1 Variation of Limitting Apparent molar volume ( фv
0 ) of
L-Arginine, L-Lysine and L-Histidine in aqueous DMSO with
0.4M at 303.15,308.15and 313.15k
Fig-1(a) Variation of Limiting Apparent Molar
Compressibility (фk0) of L-Arginine, L-Lysine and
L-Histidine in aqueous DMSO with 0.4M at 303.15,
308.15and 313.15k.
the cohesion brought about the ionic hydration.
It can be qualitatively ascribed that, when the amino acids
are dissolved in water + dimethyl sulphoxide mixtures, the
cations S=O and the anions O- are formed. Whenever an
increase of amino acid concentration in the solution, the
water molecules are attached to the ions strongly to the
electrostatic forces, resulting in greater cohesion in the
solution. Such an increased cohesion observed in these
solutions may also be due to the water structure
enhancement brought about by the increased electro-
-striction in the presence of dimethyl sulpholxide. The
electrostriction effect which brings about the shrinkage in
volume of solvent caused by the Zwitterionic portion of the
amino acids is increased in mixed solvent. Similar effect
was reported by earlier workers. [12,13]
Fig-2 Variation of Partial Transfer Volume (∆фv
0) of
L-Arginine, L-Lysine and L-Histidine in aqueous DMSO
with 0.4M at 303.15, 308.15 and 313.15k.
Interestingly, the dimethyl sulphoxide (DMSO), strongly
associated due to highly polar S=O group in the molecule
and has large dipole moment and dielectric constant
(µ = 3.96D and € = 46.68 at 298K). In the present study,
the side-chains of the acidic amino acids remain fully
deprotonated, which makes the acidic amino acids studies
are considered to exist mainly as zwitterions in aqueous
DMSO.
The perusal of Table 2 illustrates the variation of adiabatic
compressibility () with molar concentration of amino
acids. The values of in all the amino acids systems
exhibit a decreasing trend with increasing molar
concentration of amino acids as well as rise temperature.
The compressibility’s values are larger in L-arginine
14 International Journal of Research in Pure and Applied Physics 2013; 3(2): 10-15
system, comparing other amino acid systems, which
suggest that the molecular association is greater in L-
arginine than that of other amino acid systems. It is known
that amino acid molecules in the neutral solution exist in
dipolar form and then have stronger interaction with the
surrounding water molecules. The increasing electro-
-strictive compression of water around the molecules may
lead to a larger decrease in the compressibility of the
solutions.
The following observations are noticed from Table 2 on
apparent molar compressibility (φK) and apparent molar
volume (φV) of L-arginine, L-lysine and L-histidine in
aqueous DMSO solution at 303.15, 308.15 and 313.15K.
The values of the (φK) and (φV) are all negative over the
entire molarity range of amino acids. It is interesting to
note that,
i. The values of φK are found to be increased with
increasing molarity of all three the amino acids systems.
ii. However, L-arginine system exhibits an increasing
trend with increase of temperature, whereas, the other
two amino acid systems such as L-lysine and L-
histidine exhibit the decreasing trend.
iii. Secondly, the φV values also increase with increasing
molarity of amino acids in all the systems studied, but
however, the same decrease with the increase of
temperature.
iv. The values obtained from φK and φV in the present study
which clearly indicate a moderate molecular association
such as solute-solvent interactions being taking place.
v. Also, these values of apparent molar compressibility
(φK) as well as apparent molar volume (φV) which
observed in the present amino acid systems reflect
electrostriction and hydrophilic interactions occurring in
these systems, thereby indicating the presence of solute-
solvent and ion-solvent interactions.
vi. Eventually, the values φK and φV are larger in L-arginine
system comparing the other amino acids systems,
suggesting that the molecular association is more
pronounced in that system.
vii. This clearly establishes that the L-arginine is an
effective structure – maker in the solvent mixture
comparing other amino acids.
All the above observations clearly suggest that the negative
values of φK and φV in all systems indicate the presence of
ion-solvent interactions. Further, the negative values of
(φK) indicate electrostrictive salvation of ions.[14] It also
be noted here that the observed increasing behavior of φk
may pointing towards the existence of strong ion-solvent
interaction in all the systems studied. Since more number of
water molecules are available at lower concentration of
aqueous DMSO, the chances for the penetration of solute
molecules into the solvent molecules are highly favored.
Further, the increasing trends of apparent molar
compressibility (φK) and apparent molar volume (φV) of
amino acid systems with increase in molarity and thereby
confirming the presence of strong solute-solvent as well as
ion-solvent interactions in the amino acid systems.
The evaluated parameter Limiting apparent molar
compressibility (k0) which provide information’s
regarding the ion-solvent interactions and its related
constant (SK) of the ion-ion interactions in the
solution, which are systematically tabulated in Table 3. It
is noticed that the K0 values are negative in all the systems
and decrease with increase of temperature, except in L-
arginine system, where a reverse trend is observed.
Appreciable negative values of K0 for all the systems
which suggest that the existence of ion-solvent interactions.
The large values of K0 are found in L-arginine system than
other amino acid systems. Its related constant (SK ) whose
values are positive and they increase with elevation of
temperature.(except in L-arginine system) Such positive
values of SK indicate the strengthening of ion-ion
interactions and suggesting the structure-making effect of
the amino acids. The larger values of K0 in L-arginine
system once again confirming its structure-making
tendency in the solvent mixture. The perusal of Table 3
represents the values of limiting apparent molar volume V0
exhibiting negative values in all the three amino acid
systems and decrease with increase of temperature, except
L-arginine in which an increasing trend is observed. This
may enhance the electrostriction of water molecules
resulting in increase of V0 is due to enhancement in
electrostriction at the terminal groups of amino acids,
which will lead to decreasing of interaction between these
polar ends and ions. The increase of V0
values in aqueous
DMSO solution may also be attributed to hydrophobocity/
polar character of the side chains of the amino acids.
Hence, such an decreasing values of V0 in aqueous DMSO
solution clearly indicate the strengthening of ion-solvent
interaction. It is evident from the Table 3 that the values of
SV are positive in all the three amino acid systems and also
increase with increase of temperature. Such a possession of
positive values of SV clearly indicates the presence of
strong ion-ion interactions in the solution.
The Co-sphere overlap model (Gurney, 1953)[15] can be
utilized to rationalize the transfer volume values in terms of
solute-co solute interactions. The overlap of co-solute ions
and amino acids comes into play because of interactions
between (i) the charged ends of amino acids and ions of the
co-solute such as DMSO, (ii) the hydrophobic parts of the
amino acids and co-solute ions, and the charged
ends/hydrophilic parts of amino acids and hydrophobic
parts of the co-solutes; and (iii) the hydrophilic parts of the
amino acids and hydrophobic parts of ions of co-solutes.
According to this model, ion-ion interactions result in
positive transfer volume values, whereas, ion-hydrophobic
and hydrophilic group interactions result in negative
transfer volume values. The present investigation from
Fig 2, observes that all the values of partial transfer volume
(∆ϕ0) exhibit positive values and decrease with elevation
of temperature. Hence, it is very obvious that ion-ion
interactions are dominating in the present systems. The
overall positive values of transfer volume which are
observed for the amino acids can be due to the effect [16]
of the interactions of the ions (S=O) and the
zwitterionic group of the amino acids.
Viscosity is another important parameter in understanding
the structure as well as molecular interactions occurring in
the solutions. One can notice from Table 1, that the values
15 International Journal of Research in Pure and Applied Physics 2013; 3(2): 10-15
of viscosity increases with increase in solute concentration
in all the systems. This increasing trend indicates the
existence of molecular interactions occurring in these
systems. In order to shed more light on this, the role of
viscosity B-coefficient has also been obtained. From Table
4, it is observed that the values of A are almost positive and
B-coefficients are positive in Systems I & II and it is
negative for system-III. Since A is a measure of ionic
interaction which may be presumed that there is a strong
ion-ion interactions present in the liquid mixtures.
The behaviour of B-coefficient in all the amino acids
suggests the existence of strong solute-solvent interaction.
Similar trends of interaction studies studied for other amino
acids in aqueous sodium acetate solution have been
reported earlier, which supports the present investigation
clearly establishes that the behaviour of B values in these
systems suggest that the existence of strong solute-solvent
interactions. Hence, it can be concluded that the
molecular interactions between the amino acids
in the present investigation are of the order:mL-arginine
> L-lysine > L-histidine
4 CONCLUSION
In the present system of mixtures, L-arginine has been
identified as a strong structure-maker in aqueous DMSO
solution over the other two amino acids. The partial transfer
volume studies predict that ion–ion interactions
are prevailing in the present system of mixtures thereby
establishing that strong ion-ion interactions are existing in the
present study. The present investigation also observes
that a strong solute-solvent interactions in the present
systems of liquid mixtures. The molecular interactions
among the amino acids with solvent mixture from this
study are of the order: L-arginine > L-lysine > L-histidine.
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Source of support: Nil; Conflict of interest: None declared
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