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)

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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