Thermodynamics of the interactions of some amino acids and peptides with dodecyltrimethylammonium bromide and tetradecyltrimethylammonium bromide

J. Chem. Thermodynamics 70 (2014) 182–189

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Thermodynamics of the interactions of some amino acids and peptides with dodecyltrimethylammonium bromide and tetradecyltrimethylammonium bromide Paurnima Talele, Nand Kishore ⇑ Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India

a r t i c l e

i n f o

Article history: Received 19 July 2013 Received in revised form 17 October 2013 Accepted 4 November 2013 Available online 11 November 2013 Keywords: Amino acids Peptides Partial molar volume Partial molar compressibility Enthalpy of dilution Transfer thermodynamic properties

a b s t r a c t The values of apparent molar volume V 2;/ and apparent molar adiabatic compressibility K S;2;/ of amino acids glycine, L-alanine, DL-a-amino-n-butyric acid, L-valine, L-leucine and peptides glycyl-glycine, glycyl-glycyl-glycine and glycyl-leucine have been determined in aqueous solutions of cationic surfactants dodecyltrimethylammonium bromide (DTAB) and tetradecyltrimethylammonium bromide (TTAB) by means of density and sound velocity measurements. The heat evolved or absorbed (q) during the course of interactions of amino acids and peptides with the aqueous solutions of surfactants were determined by isothermal titration calorimetry at T = 298.15 K. The values of standard partial molar volume V 02;m and standard partial molar adiabatic compressibility K 0s;2;m at infinite dilution were calculated from the values of V 2;/ and K S;2;/ . Similarly the values of limiting enthalpies of dilution (Ddil H0 ) of the amino acids/peptides were calculated from heat evolved or absorbed during calorimetric experiments. The standard partial molar quantities of transfer from water to aqueous surfactant solutions have been used to identify the interactions of amino acids and peptides with surfactants in terms of ionic–ionic, ionic– hydrophobic and hydrophobic–hydrophobic group interactions. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Protein–surfactant interactions have been a focus of studies for a long time [1–4]. It is known that protein–surfactant interactions play very important role in industrial, biological, pharmaceutical, and cosmetic applications [5,6]. Surfactants are used for protein molecular weight determination [7], membrane protein solubilization [8], and crystallization [9]. Binding of surfactants to proteins alters intermolecular forces which maintain the secondary and tertiary structure, thereby producing conformational changes [10,11]. Surfactants can interact directly or indirectly with the proteins through different physicochemical mechanisms such as electrostatic or hydrophobic interactions [12–14]. Surfactants may either bind to protein or initiate its unfolding or only bind and retain its tertiary structure intact [15]. The conformational stabilization of the protein in surfactants is related to the nature of solute-solute and solute-solvent interactions. Therefore a detailed understanding of the interactions of the intact proteins and the constituents of proteins with surfactants is essential. Studies on the interactions of model compounds such as amino acids and peptides with surfactants can help in understanding the fine details of protein– surfactant interactions [16–20]. ⇑ Corresponding author. Fax: +91 022 2576 7157. E-mail address: [email protected] (N. Kishore). 0021-9614/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jct.2013.11.001

In this work, we have investigated the effect of the cationic surfactants tetradecyltrimethylammonium bromide (TTAB, cmc3.7  103 mol dm3) [21] and dodecyltrimethylammonium bromide (DTAB, cmc-15  103 mol dm3) [22] on some amino acids and peptides using densimetry, sound velocity and calorimetric measurements. Volumetric measurements can be used to investigate the hydration properties of charged (oppositely charged amino and carboxyl termini), polar (a peptide group), and non-polar (a methylene group) groups of a-amino acids [23]. Post micellar concentrations of surfactants TTAB and DTAB have been selected and various physico-chemical parameters, such as partial molar volume, partial molar adiabatic compressibility, and enthalpy of dilution have been evaluated. The corresponding transfer properties have been discussed in terms of various types of intermolecular interactions, to understand the origin and nature of amino acids/peptides–surfactant interactions both qualitatively and quantitatively.

2. Experimental 2.1. Materials The amino acids glycine, L-alanine, DL-a-amino-n-butyric acid (a-ABA), L-valine, L-leucine and peptides glycyl-glycine,

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glycyl-glycyl-glycine, glycyl-leucine were procured from Sigma Aldrich Co., USA. The surfactants tetradecyltrimethylammonium bromide (TTAB) and dodecyltrimethylammonium bromide (DTAB) were procured from Tokyo Chemical Industry Co., Japan. All the amino acids and peptides were dried over P2O5 for at least 72 h and used without further purification. Moisture contents were determined by Karl Fischer analysis and the dry weights of these samples were corrected wherever required. The relative molar mass, moisture content, and purity of these compounds as listed by the vendors are given in table 1. All the solutions were prepared in water that was double distilled, deionized using a Cole-Parmer Barnstead mixedbed ion exchange resin column and then degassed. All of the mass determinations were done on a Sartorius BP 211D digital balance which had readability of 0.01 mg.

liberated or absorbed at different molalities of amino acids or peptides in the solutions. The limiting heat of dilution (Ddil H0 ) of the aqueous amino acid and peptide solution was calculated by fitting the values of the measured heat (q) to the following equation,

2.2. Methods

The value of apparent molar volume (V 2;/ ) of amino acids and peptides in aqueous surfactant solutions was determined from the measured density q using equation (2). The value of isentropic compressibility of the aqueous solution (jS ) was calculated from the speed of sound, u, using the Newtonian–Laplace equation (3). The value of apparent molar adiabatic compressibility, K S;2;/ was determined by using equation (4).

Density and sound velocity measurements The densities and sound velocities of solutions were measured using digital density and sound velocity analyser DSA 5000 purchased from Anton Paar GmbH, Austria. The instrument determines two independent physical properties with the same sample as it is equipped with a density cell and sound velocity cell thus combining the oscillating U-tube method with highly accurate measurement of sound velocity. The temperature of the measurements was stable to within 0.01 K. The chemical calibration of the densimeter was performed by measuring the values of apparent molar volume and apparent molar adiabatic compressibility of aqueous sodium chloride solutions at different values of molality and comparing with the literature values [24], which had excellent agreement. The maximum uncertainties in the density and sound velocity measurements were observed to be 3  106 g  cm3 and 0.03 m  s1, respectively. Isothermal titration calorimetry Isothermal titration calorimetric measurements were performed at T = 298.15 K on Nano ITC (TA Instruments, New Castle, DE, USA). All the solutions were thoroughly degassed prior to the experiments. Titrations were carried out using 250 lL syringe filled with aqueous amino acids or peptides solutions with a stirring speed of 250 rpm in all the experiments. The sample cell of 950 lL capacity was filled with 0.1 mol  kg1 aqueous surfactant solutions and the reference cell was filled with water. Titrations of aqueous amino acids and peptides were performed using 0.2 mol  kg1 of glycine, alanine, a-amino-n-butyric acid, 0.05 mol  kg1 valine, leucine, glycyl-leucine, and 0.025 mol  kg1 glycyl-glycine and glycyl-glycyl-glycine. Titrations consisted of 25 consecutive injections of aqueous amino acids or peptides with 20 s duration each and 240 s interval between every successive injection. The ITC experiments provided a set of values of the heat

q ¼ Ddil H0 þ mSV :

ð1Þ

Here m is the molality of the solution and SV is the empirical slope. The data which showed linear molality dependence were fitted to the above equation. In other cases, the data points were fitted to an appropriate polynomial equation to obtain the values of Ddil H0 . 3. Results and discussion

V 2;/ ¼

jS ¼

M

q



ðq  qo Þ  103 ; mqqo

ð2Þ

1 ; u2 q

K S;2;/ ¼

ð3Þ

jS M 1000ðj0s q  jS q0 Þ þ mqq0 q

ð4Þ

Here M is the molar mass of the solute in g  mol1, m is the molality of the solution in mol  kg1, qo is the density of water or the reference solvent in units of g  cm3. The j0S is the isentropic compressibility of water or the reference solvent at T = 298.15 K. In so far as the isentropic compressibility is dependent on the ultrasonic speed, the latter may be considered as a thermodynamic property in the sole case when a negligible amount of ultrasonic absorption of the acoustic waves of low frequency and of low amplitude is observed. The values of measured density (q), apparent molar volume (V2,/), speed of sound (u), isentropic compressibility (jS), and apparent molar adiabatic compressibility (Ks,2,/) of amino acids and peptides at different molalities at T = 298.15 K, in 0.1 mol  dm3 aqueous DTAB and TTAB surfactant solutions are given in tables S1 and S2, respectively. The values of the measured thermodynamic quantities have been plotted as a function of molality in figures 1–4. In the case where the values of V 2;/ were found to be molality dependent, the values of standard partial

TABLE 1 Compounds used in this study with their Molecular formula, Molar mass (M), Source (S = Sigma Aldrich Co, USA, SRL = Sisco Research Laboratories, India, F = Fluka Analytical, Japan, TCI= Tokyo Chemical Industry Co, Japan), CAS number, mass fraction moisture content (w) and their mole fraction purity (x) as reported by the vendors. Compound

Molecular formula

Glycine

C2H5NO2 C3H7NO2

75.07 89.09

L-valine

C4H9NO2 C5H11NO2

L-leucine

Glycyl-glycine glycyl-glycyl-glycine glycyl-leucine DTAB TTAB

L-alanine

DL-a-amino-n-butyric acid

M

Source

CAS No.

w

mass fraction purity

S S

56-40-6 302-72-7

0.0004 0.0006

P0.99 P0.99

103.12 117.15

SRL S

2835-81-6 72-18-4

0.0004 0.0006

=0.99 P0.98

C6H13NO2

131.12

S

61-90-5

0.0007

P0.98

C4H8N2O3 C6H11N3O4 C8H16N2O3 C15H34BrN C17H38BrN

132.12 189.17 188.23 308.35 336.40

F S S TCI TCI

556-50-3 556-33-2 869-19-2 1119-94-4 1119-97-7

0.0009 0.0010 0.0008 0.0010 0.0015

P0.99 P0.99 >0.99 >0.98 >0.98

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P. Talele, N. Kishore / J. Chem. Thermodynamics 70 (2014) 182–189 110

110

A (DTAB)

100

90 -1

V2,φ/(cm .mol )

80 70

80 70

3

3

-1

V2,φ/(cm .mol )

90

B (TTAB)

100

60 50 40

60 50 40

Glycine Alanine DL-α-amino-n-butyric acid L-Valine L-Leucine

30 20 10 0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Glycine Alanine DL-α-amino-n-butyric acid L-Valine L-Leucine

30 20 10 0.00

0.45

-1

0.05

0.10

0.15

m/mol kg

0.20

0.25

0.30

0.35

0.40

-1

m/mol kg

FIGURE 1. Apparent molar volume (V 2;/ ) of glycine ( ), alanine ( ), DL–a-amino-n-butyric acid ( ), L-valine ( ), and L-leucine ( ) plotted against molality (m) of the amino acid in 0.1 mol  dm3 dodecyltrimethylammonium bromide (A, DTAB), and 0.1 mol  dm3 tetradecyltrimethylammonium bromide (B, TTAB) solutions at T = 298.15 K.

-2.2 -2.4

-1

10 Ks,2,φ/(cm .mol .pa )

-2.6

-1

-2.6

-1

-1

10 Ks,2,φ/(cm .mol .pa )

-2.4

-2.8

3

3

-2.8

-3.0

-8

-8

-3.0

Glycine Alanine DL-α-amino-n-butyric acid L-Valine L-Leucine

-3.2

A (DTAB)

-3.4 0.0

0.1

0.2

0.3

B(TTAB)

-3.4 0.00

0.4

Glycine Alanine DL-α-amino-n-butyric acid L-Valine L-Leucine

-3.2

0.05

0.10

-1

0.15

0.20

0.25

0.30

0.35

0.40

-1

m/mol kg

m/mol kg

FIGURE 2. Apparent molar adiabatic compressibility (K S;2;/ ) of glycine ( ), alanine ( ), DL-a-amino-n-butyric acid ( ), L-valine ( ), and L-leucine ( ) plotted against molality (m) of the amino acid in 0.1 mol  dm3 dodecyltrimethylammonium bromide (A, DTAB), and 0.1 mol  dm3 tetradecyltrimethylammonium bromide (B, TTAB) solutions at T = 298.15 K.

160

170 160

A(DTAB)

B(TTAB)

150

150

140 130

-1

V2,φ/(cm .mol )

130 120

120

3

3

-1

V2,φ/(cm .mol )

140

110 100 90

110 100

80

90 70

Gly-gly Gly-gly-gly Gly-leu

60 50 0.01

0.02

0.03

0.04 -1

m/mol kg

0.05

Gly-gly Gly-gly-gly Gly-leu

80 70 0.01

0.02

0.03

0.04

0.05

-1

m/mol kg

FIGURE 3. Apparent molar volume (V 2;/ ) of glycyl-glycine ( ), glycyl-glycyl-glycine ( ), and glycyl-leucine ( ) plotted against molality (m) of the peptide in 0.1 mol  dm3 dodecyltrimethylammonium bromide (A, DTAB), and 0.1 mol  dm3 tetradecyltrimethylammonium bromide (B, TTAB) solutions at T = 298.15 K.

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-3.5 -3.6

-3.6

-3.7

10 Ks,2,φ/(cm .mol .pa )

-3.8 -1

-3.9

-4.1

-4.2

3

-4.2 -4.3 -4.4 -4.5

-4.4 -4.6

-8

3 -8

-4.0

-1

-4.0

-1

-1

10 Ks,2,φ/(cm .mol .pa )

-3.8

-4.6 -4.7 -4.8

Gly-gly Gly-gly-gly Gly-leu

A(DTAB)

-4.9

-4.8 -5.0

Gly-gly Gly-gly-gly Gly-leu

B(TTAB)

-5.2

-5.0 0.01

0.02

0.03

0.04

0.01

0.05

0.02

0.03

0.04

0.05

-1

-1

m/mol kg

m/mol kg

FIGURE 4. Apparent molar adiabatic compressibility (K S;2;/ ) of glycyl-glycine ( ), glycyl-glycyl-glycine ( ), and glycyl-leucine ( ) plotted against molality (m) of the peptide in 0.1 mol  dm3 dodecyltrimethylammonium bromide (A, DTAB), and 0.1 mol  dm3 tetradecyltrimethylammonium bromide (B, TTAB) solutions at T = 298.15 K.

TABLE 2 Standard partial molar volume (V 02;m ) of amino acids/peptides in 0.1 mol  dm3 aqueous dodecyltrimethylammonium bromide (DTAB) and tetradecyltrimethylammonium bromide (TTAB) surfactant solutions at T = 298.15 K and corresponding partial molar volume of transfer (Dtr V 02;m ) of amino acids from water to these aqueous solutions. Amino acid/peptides

Glycine Alanine DL-ABA L-Valine L-Leucine Gly-gly Gly-gly-gly Gly-leu a

Watera

0.1 mol  dm3 DTAB

V 02;m cm3  mol1

V 02;m cm3  mol1

0.1 mol  dm3 TTAB

Dtr V 02:m cm3  mol1

V 02;m cm3  mol1

Dtr V 02:m cm3  mol1

43.14 ± 0.06 60.43 ± 0.04 75.51 ± 0.02 90.39 ± 0.14

43.07 ± 0.06 60.28 ± 0.07 75.61 ± 0.07 90.01 ± 0.12

0.07 ± 0.08 0.15 ± 0.08 0.10 ± 0.07 0.38 ± 0.18

43.09 ± 0.05 60.11 ± 0.03 75.28 ± 0.02 90.28 ± 0.12

0.05 ± 0.08 0.32 ± 0.05 0.23 ± 0.03 0.11 ± 0.18

107.72 ± 0.24

106.79 ± 0.09

0.93 ± 0.25

107.70 ± 0.18

0.02 ± 0.03

76.23 ± 0.07 111.81 ± 0.01 139.69 ± 0.07

75.89 ± 0.06 110.47 ± 0.20 139.07 ± 0.15

0.34 ± 0.09 1.34 ± 0.20 0.62 ± 0.16

76.33 ± 0.09 110.24 ± 0.16 138.57 ± 0.08

0.10 ± 0.11 1.57 ± 0.16 1.12 ± 0.10

Refs. [27,36].

molar volumes at infinite dilution, (V 02;m ) were obtained by least square fitting of the data points to the following equation

V 2;/ ¼ V 02;m þ SV  m:

ð5Þ

TABLE 3  Contributions of zwitterionic group (NHþ 3 , COO ), CH2 group and other alkyl chains to infinite dilution standard partial molar volumes V 02;m in 0.1 mol  dm3 aqueous dodecyltrimethylammonium bromide (DTAB) and tetradecyltrimethylammonium bromide (TTAB) surfactant solutions at T = 298.15 K.

Here Sm is the empirical slope which is sometimes considered to be volumetric pair-wise interaction coefficient [25,26]. In other case where molality dependence of V 2;/ was found to have no definite trend or negligible trend was obtained by taking an average of all data points. The values of standard partial molar volume and transfer volume of amino acids from water to aqueous surfactant solutions are given in table 2. 3.1. Contributions of zwitterionic end groups, methylene groups and other alkyl chains to standard partial molar volume V 02;m

Groups

 NHþ 3 ; COO CH2CH3CHCH3CH2CHCH3CH2CHCHCH3CH2CHCH2CHa

V 02;m /(cm3  mol1) Watera

0.1 mol  dm3 DTAB

0.1 mol  dm3 TTAB

27.68 ± 1.12 15.91 ± 0.33 31.82 ± 0.40 47.73 ± 0.40 63.64 ± 0.40 79.55 ± 0.50

28.01 ± 0.88 15.72 ± 0.27 31.44 ± 0.38 47.16 ± 0.46 55.02 ± 0.54 70.74 ± 0.60

27.46 ± 0.77 15.95 ± 0.23 31.90 ± 0.32 47.85 ± 0.39 55.83 ± 0.46 71.78 ± 0.51

Ref. [29].

V 02;m

The values of for all the five amino acids studied in both 0.1 mol dm3 DTAB and TTAB are positive and show linear variation with number of carbon atoms in the alkyl side chain of the amino acid. Since the standard partial molar volume V 02;m of an amino acid is the sum of the contributions of zwitter ionic end groups and the number of CH2 groups in it, its linear variation can be represented by the equation


V 02;m ¼ V 02;m NHþ3 ; COO þ nC V 02;m ðCH2 Þ;

ð6Þ

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

a-amino acids, and V 02;m ðNHþ3 ; COO Þ and V 02;m ðCH2 Þ are the corresponding partial molar volume contribution of the zwitterionic

end group and the methylene group to overall value of V 02;m . The val 0 ues of V 02;m ðNHþ 3 ; COO Þ and V 2;m ðCH2 Þ, and those for other alkyl chains, calculated by a least squares regression analysis, are listed in table 3. The alkyl chains of the homologous series of a-amino acids studied in this work are CH2-(glycine), CH3CH-(alanine), CH3CH2CH-(a-amino-n-butyric acid), CH3CH2CHCH-(valine) and CH3CH2CHCH2CH-(leucine). The value of V 02;m ðCH2 Þ obtained by this procedure characterizes the average contribution of the CH- and CH3-groups to V 02;m of the a-amino acids. The contribution of other alkyl chains of the a-amino acids reported in table 3 were calculated as follows [28]:

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V 02;m ðCH3 Þ ¼ 1:5V 02;m ðCH2 Þ;

ð7Þ

V 02;m ðCHÞ ¼ 0:5V 02;m ðCH2 Þ:

ð8Þ

The values of V 02;m ðCH2 Þ observed in both 0.1 mol  dm3 DTAB and 0.1 mol dm3 TTAB are nearly same as that in water. The val 0 3 ues of V 02;m ðNHþ aqueous 3 ; COO Þ, and V 2;m ðCH2 Þ in 0.1 mol dm DTAB are (28.01 ± 0.88) cm3  mol-1 and (15.72 ± 0.27) cm3  mol1, respectively. In 0.1 mol  dm3 aqueous TTAB, the values of  0 3 1 V 02;m ðNHþ 3 ; COO Þ, and V 2;m ðCH2 Þ are (27.46 ± 0.77) cm  mol and (15.95 ± 0.23) cm3  mol1, respectively. The larger values of  0 V 02;m ðNHþ 3 ; COO Þ than V 2;m ðCH2 Þ in both the cases indicate that both surfactants interact primarily with the zwitterionic groups of the amino acids and contribution of zwitterionic group diminishes with increasing alkyl chain length. It is seen that the contributions of the zwitter ionic end groups and alkyl groups to the overall standard partial molar volume of glycine, alanine and aamino butyric acid are nearly the same in 0.1 mol dm3 aqueous DTAB, 0.1 mol dm3 aqueous TTAB, and only water. However, for valine and leucine, these contributions are lesser than those observed in water. 3.2. Number of water molecules hydrated to the amino acids (Nw) in aqueous solvents The zwitter ionic end groups of the amino acids and peptides can re-orient water molecules around them resulting in electrostriction depending upon the solvent environment. The extent of electrostriction is reflected in the values of hydration number (Nw) which can be determined from the values of standard partial molar volume (V 02;m ) and the intrinsic partial molar volume V 02:m ðintÞ of the amino acids by using the following relation [29].

Nw ¼

V 02;m ðelectÞ ðV 0E  V 0B Þ

:

ð9Þ

The values of V 02;m ðelectÞ were calculated from the values of V 02;m and V 02:m ðintÞ by the procedures described earlier [30]. It was further assumed in these calculations that the difference in molar volume of the electrostricted water (V 0E ) and that of the bulk water is approximately 3.0 cm3  mol1 [31] for the electrolytes at T = 298.15 K. The values of hydration number (Nw) thus calculated in 0.1 mol dm3 aqueous DTAB and TTAB are reported in table 4. The calculated values of Nw for the amino acids in these two aqueous surfactant solutions are observed to vary in the following order: Nw (leucine) > Nw (valine) > Nw (alanine) > Nw (glycine). This trend is due to increasing alkyl chain length of amino acids, which results in increased electrostriction around the zwitter ionic end groups. The data shows that the values of Nw did not show any significant variation in aqueous 0.1 mol dm3 DTAB or aqueous 0.1 mol dm3 TTAB solutions compared to their values in water (figure 1). 3.3. Partial molar volume of transfer (Dtr V 02:m ) of amino acids from water to aqueous DTAB/TTAB The intrinsic volume of a solute and its interactions with the solvent environment determine the value of the partial molar volume of a non-electrolyte [30]. Thus the partial molar volume of transfer (Dtr V 02:m ) from water to aqueous surfactant solution is indicative of the nature of solute–solvent interactions, assuming that the volume occupied by the solute due to van der Waals volume and that due to voids and empty spaces present in the solution remain the same in both water and aqueous solution [33,34]. The values of standard partial molar volume of transfer (Dtr V 02:m ) of the amino acids from water to 0.1 mol dm3 aqueous DTAB/

TABLE 4 The number of water molecules hydrated (Nw) to amino acids in water and in aqueous solutions of 0.1 mol  dm3 aqueous dodecyltrimethylammonium bromide (DTAB) and tetradecyltrimethylammonium bromide (TTAB) surfactant solutions at T = 298.15 K. Amino acids

Watera

0.1 mol  dm3 DTAB

0.1 mol  dm3 TTAB

L-Valine

2.9 3.8 3.9

2.9 3.8 4.0

2.9 3.9 3.9

L-Leucine

5.5

5.8

5.5

Glycine Alanine

a

Hydration number (Nw)

Ref. [32].

TTAB solutions were calculated according to the following equation and are presented in table 2.

Dtr V 02:m ¼ V 02;m ½in DTAB=TTABðaqÞ  V 02;m ½in water:

ð10Þ

Based on the properties of water molecules in the hydration co-sphere, which largely depends on nature of the solute species, different types of solute–solvent interactions can be identified [35]. Above the critical micelle concentration (cmc), surfactant molecules self-aggregate to form micelles in aqueous solution [36]. The cmc values of TTAB and DTAB used in this work are (3.7  103 and 15  103) mol dm3, respectively [21,22]. The concentrations of both the surfactants used in this work are above their cmc values. The following types of interactions can occur in the ternary system of amino acid, DTAB/TTAB, and water: (a) Ion–ion/hydrophilic–hydrophilic interactions between Br of DTAB/TTAB and + NHþ 3 group of amino acids or N ACH3 group of DTAB/TTAB and  COO group of amino acids, (b) hydrophobic–hydrophobic interactions between alkyl chain (hydrophobic tail) of surfactants and hydrophobic group (non-polar parts) of amino acids, and (c) hydrophilic–hydrophobic/ion–hydrophobic interactions occurring between non-polar parts of amino acids and polar head group of DTAB/TTAB and vice versa. The first type of these interactions gives a positive value of Dtr V 02:m since these lead to reduction in electrostriction of solvent resulting in strengthening of the structure of water in the bulk. The last two types of interactions contribute negatively to Dtr V 02:m due to overall reduction in the structure of water upon the overlap of their hydration co-spheres. As we have used post micellar concentrations of DTAB/TTAB, the interactions of amino acids with these co-solvents are mainly with the micelle. The Dtr V 02:m values of amino acids from water to 0.1 mol dm3 DTAB and 0.1 mol dm3 TTAB are small. This indicates that amino acids interact with DTAB/TTAB micelles maintaining an overall balance between ionic–ionic and ionic–hydrophobic or hydrophobic–hydrophilic group interactions. A good correlation has been found between hydration number and partial molar volume of transfer of amino acids (figure 5). It is reported by NMR spectroscopy, including NMR self-diffusion experiments that spherical to rod shape transition for DTAB in aqueous micellar solution occurs at a concentration of 32.0  103 mol dm3 [37]. For TTAB, the spherical to rod shape transition occurs at 26  103 mol dm3 [38]. Therefore in this case the interactions are mainly with rod like micelles. The values of standard partial molar volume of the peptides glygly, gly-gly-gly and gly-leu are also observed to be positive. The values of Dtr V 02:m for gly-gly, gly-gly-gly and gly-leu 0.1 mol dm3 aqueous TTAB are (0.10 ± 0.11) cm3  mol1, (1.57 ± 0.16) cm3  mol1, (1.12 ± 0.10) cm3  mol1, respectively. These results suggest that introduction of peptide group does not strengthen the polar interactions, rather predominance of hydrophobic–hydrophobic or ion–hydrophobic group interactions are indicated. These

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molar compressibility due to hydration of the amino acid. Since the value of K 0s;2;m ðintÞ is very small [30], the value of K 0s;2;m can essentially be considered to represent K 0s;2;m ðelectÞ. The value of standard partial molar compressibility of transfer (Dtr K 0S;2:m ) of the amino acids from water to 0.1 mol dm3 aqueous DTAB/TTAB solutions are calculated by using equation (13) and are listed in table 5.

110 100 90 80

Property

70

Dtr K 0S;2:m ½water to DTAB=TTABðaqÞ

60

¼ K 0S;2:m ½in DTAB=TTABðaqÞ  K 0S;2;m ½in water:

50 40

ð13Þ

K 0S;2:m

30 20 10 0 +1

+2

+3

+4

+5

nC FIGURE 5. Partial molar volume (in cm3  mol1) of amino acids in DTAB ( ) and TTAB ( ); contribution of side chain to partial molar volume (in cm3  mol1) in DTAB ( ) and TTAB ( ); standard partial molar adiabatic compressibility (in 109 cm3  mol1  Pa1) of amino acids in DTAB ( ) and TTAB ( ); and hydration number of amino acids in DTAB ( ) and TTAB ( ) at T = 298.15 K vs. the number of carbon atoms (nC) in the amino acids. The concentrations of both the DTAB and TTAB are 0.1 mol  dm3 each.

observations are further strengthened by the fact that the value of Dtr V 02:m decreased by (1.52 ± 0.17) cm3  mol1 in going from glycine to diglycine. Similarly in going from leucine to gly-leu, the value of Dtr V 02:m decreased by (1.10 ± 0.10) cm3  mol1. Similar results have been observed in aqueous DTAB. 3.4. Partial molar compressibility of the amino acids in aqueous DTAB/ TTAB solutions The values of standard partial molar adiabatic compressibility (K 0s;2;m ) of amino acids reported in table 5 were obtained by fitting the values of K S;2;/ (tables S1, S2) against molality (m) by following equation

K S;2;/ ¼ K 0s;2;m þ SK m:

ð11Þ

The values of standard partial molar adiabatic compressibility can be expressed by simple model [27]

K 0s;2;m ¼ K 0s;2;m ðintÞ þ K 0s;2;m ðelectÞ:

ð12Þ

where K 0s;2;m ðintÞ is the intrinsic partial molar isentropic compressibility of the amino acid and K 0s;2;m ðelectÞ is the electrostriction partial

Here indicates compressibility of the overall solution, and Dtr K 0S;2:m indicates the contribution of electrostricted water in hydration shells to the overall compressibility of the solution. In the case of polar interactions between solute and solvent, as water moves from hydration shell into the bulk (from less ordered to more ordered), the value of Dtr K 0S;2:m is positive resulting from increased compressibility of the solution. In Contrast to this, in hydrophobic interactions, the value of Dtr K 0S;2:m is negative as water moves from more structured (bulk) to less structured (hydration shell) region, decreasing the compressibility of the solution. For all the amino acids in aqueous TTAB solution and for glycine, and alanine in aqueous DTAB solution, no significant values of Dtr K 0S;2:m are observed. These results are also indicative of an overall balance of different type of interactions. In valine and leucine, the increased negative value of Dtr K 0S;2:m indicating movement of water from bulk into the hydration shell, results in more electrostriction. This may arise due to increased hydrophobicity of two amino acids. A positive value of Dtr K 0S;2:m observed in case of a-amino butyric acid in DTAB suggests enhancement of the structure of water in the bulk due to less electrostriction. The value of Dtr K 0S;2:m changes by (3.27 ± 0.14)  109  cm3  mol1  pa1 in going from gly-gly to gly-gly-gly. This change is by (1.26 ± 0.16)  109  cm3  mol1  pa1 in going from leucine to gly-leu. Both in aqueous DTAB and TTAB solutions, the changes are also very small and do not support strengthening of additional polar interactions upon introduction of peptide groups. Figure 5 shows the values of standard partial molar volume, standard partial molar adiabatic compressibility, hydration number and side chain group contributions of amino acids in 0.1 mol dm3 DTAB and TTAB at T = 298.15 K. This figure also suggests an overall balance of different intermolecular interactions which do not lead to significant changes in these thermodynamic properties. 3.5. Enthalpies of dilution The heat liberated/absorbed (q) during the titration of the amino acids and peptides with aqueous 0.1 mol dm3 DTAB and TTAB

TABLE 5 Standard partial molar isentropic compressibilities (K 0S;2;m )a of amino acids/peptides in 0.1 mol  dm3 aqueous dodecyltrimethylammonium bromide (DTAB) and tetradecyltrimethylammonium bromide (TTAB) surfactant solutions at T = 298.15 K, and the corresponding standard partial molar isentropic compressibilities of transfer (DtrK 0s;2;m ) of amino acids from water to these aqueous solutions. Amino acid/ peptides

0.1 mol  dm3 DTAB

0.1 mol  dm3 TTAB

109 K 0S;2;m

109 K 0S;2;m

109 Dtr K 0S;2:m

109 K 0S;2;m

109 Dtr K 0S;2:m

cm3  mol1  Pa1

cm3  mol1  Pa1

cm3  mol1  Pa1

cm3  mol1  Pa1

cm3  mol1  Pa1

L-Valine

27.00 ± 0.44 25.56 ± 0.60 29.95 ± 0.30 30.62 ± 0.23

26.48 ± 0.02 25.29 ± 0.03 25.49 ±0.07 34.57 ± 0.04

0.52 ± 0.44 0.27 ± 0.60 4.46 ± 0.30 3.95 ± 0.23

26.80 ± 0.01 25.93 ± 0.01  28.06 ± 0.01 31.40 ± 0.34

0.20 ± 0.44 0.37 ± 0.60 1.89 ± 0.30 0.71 ± 0.41

L-Leucine

31.78 ± 0.56

33.89 ± 0.06

2.11 ± 0.56

29.83 ± 0.10

1.95 ± 0.57

Gly-gly Gly-gly-gly Gly-leu

40.20 ± 0.10 44.90 ± 0.01 47.58 ± 0.09

43.97 ± 0.05 44.54 ± 0.08 46.11 ± 0.05

3.77 ± 0.11 0.36 ± 0.08 1.47 ± 0.10

40.54 ± 0.04 48.51 ± 0.11 46.89 ± 0.08

0.34 ± 0.10 3.61 ± 0.10 0.69 ± 0.12

Glycine Alanine DL-ABA

a

Watera

Ref. [32,39].

188

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FIGURE 6. Representative plots of heats of dilution (q) agains molality (m) of (A) amino acids, and (B) peptides in 0.1 mol  dm3 tetradecyltrimethylammonium bromide at T = 298.15 K.

at T = 298.15 K are presented in table S3. The representative heats of dilution as a function of molality of amino acids/peptides in 0.1 mol dm3 TTAB are shown in figure 6. The enthalpy of transfer (Dtr Ddil H0 ) of aqueous amino acids/peptides from water to 0.1 mol dm3 aqueous DTAB/TTAB were calculated according to equation (14) and the values of Ddil H0 and Dtr Ddil H0 are reported in table 6.

Dtr Ddil H0 ¼ Ddil H0 ðaqueous DTAB=TTABÞ  Ddil H0 ðWaterÞ:

ð14Þ

The values of Ddil H0 (in water) have been taken from our previously published results [39,40]. The values of Ddil H0 of amino acids/peptides in 0.1 mol dm3 TTAB are positive except for a-amino butyric acid. In 0.1 mol dm3 DTAB, negative values of Ddil H0 are observed for L-valine, L-leucine and gly-gly. The exothermicity and endothermicity of enthalpy of dilution depends on different types of interactions and solvent structure making and breaking effects. Ion-polar interactions between ionic groups of amino acids/peptides and DTAB/TTAB give exothermic contribution to the value of enthalpy. The second type of interactions consisting hydrophobic interactions between hydrophobic groups of amino acids/peptides and DTAB/TTAB contribute endothermicity to overall observed heat. The third type of interactions between ions and hydrophobic groups also provide endothermic contribution to the value of Ddil H0 . Another contribution to the enthalpy of dilution arises from the influence which solute molecules exert on solvent structure. The structure making solutes provide exothermic contribution and structure breaking solutes provide endothermic contribution to the overall heat of interaction. Endothermic values obtained for Dtr Ddil H0 indicate predominance of heat required to create cavity in the solvent to accommodate the solute molecules or hydrophobic–hydrophobic interactions between amino acids/peptides and DTAB/TTAB in aqueous solution. The observed small values of Dtr V 02:m suggest a balance between polar and hydrophobic interactions in these systems. Therefore, it can be concluded that endothermic contribution to Dtr Ddil H0 predominantly arises from structure breaking effects of solvents upon addition of amino acids/peptides. However, for L-valine, L-leucine, gly-gly in presence of DTAB and glycine in presence of TTAB, Dtr Ddil H0 was found to be negative which may be due to ion-hydrophilic interactions between zwitterionic end groups and ionic head groups of surfactant molecule indicating that heat released upon electrostatic interactions is larger than that required to accommodate these solutes in the solvent. No specific trend is observed for Dtr Ddil H0 values of amino acids. However, a trend is observed for peptides in presence of DTAB/ TTAB. The endothermic contribution to the value of Dtr Ddil H0 increases in going from gly-gly to gly-gly-gly. Thus it can be asserted that with increasing number of peptide bonds the polar interactions are not strengthened which is also reflected in Dtr V 02:m values. In the literature, several volumetric studies have been carried out to understand the nature of interactions between amino acids or peptides and surfactant solutions [32,41–47]. In aqueous anionic sodium dodecyl sulfate (SDS) surfactant solutions, the amino acids or peptides are reported to interact predominantly by ionic–ionic or ionic–hydrophilic interactions [32,43–45]. Volumetric properties of amino acids or peptides in aqueous cationic cetyltrimethylammonium bromide (CTAB) have been reported [32,43–45]. It is reported [32] the interactions between amino acids and highly concentrated CTAB solution above cmc are

TABLE 6 Standard enthalpies of dilution (Ddil H0) of aqueous amino acids/peptides in 0.1 mol  dm-3 aqueous dodecyltrimethylammonium bromide (DTAB) and tetradecyltrimethylammonium bromide (TTAB) surfactant solutions at T = 298.15 K, and the corresponding limiting transfer heats of dilution (Dtr DdilH0). Amino acids/peptides

Glycine Alanine DL-ABA L-Valine

a

Watera

0.1 mol  dm3 DTAB

Ddil H0 J  mol1

Ddil H0 J  mol1

87.2 ± 3.0 45.6 ± 2.0 112.2 ± 4.0 49.4 ± 3.0

141.2 ± 3.9 82.6 ± 3.6 90.5 ± 1.4 112.1 ± 14.0

L-Leucine

67.0 ± 4.0

90.4 ± 5.1

Gly-gly Gly-gly-gly Gly-leu

13.9 ± 1.0 5.2 ± 0.5 10.3 ± 0.8

142.3 ± 18 384.8 ± 25 806.1 ± 31

Ref [39,40].

0.1 mol  dm3 TTAB

Dtr Ddil H0 J  mol1

Ddil H0 J  mol1

Dtr Ddil H0 J  mol1

54.0 ± 4.9 128.2 ± 4.1 202.7 ± 4.2 62.7 ± 14

72.4 ± 6.1 24.4 ± 2.6 35.6 ± 4.3 52.8 ± 8.7

14.8 ± 6.8 70.0 ± 3.3 76.6 ± 5.9 102.2 ± 9.2

23.41 ± 5.7

320.3 ± 9.1

387.3 ± 10.0

156.2 ± 18.1 379.6 ± 26 816.4 ± 31.2

81.1 ± 1.6 277.6 ± 8.8 651.3 ± 21.3

67.2 ± 1.9 272.4 ± 8.9 661.6 ± 21.3

P. Talele, N. Kishore / J. Chem. Thermodynamics 70 (2014) 182–189

balanced. Further, Ali et al. [43] have shown that with increasing concentration of CTAB above cmc, the strength of ionic-ionic interactions decreases. These results suggest that above cmc at high concentration of cationic surfactant, balancing of ionic-ionic and hydrophobic–hydrophobic interactions would occur. The results obtained in this work with DTAB and TTAB also suggest overall balance of interactions of amino acids with micelles of cationic surfactants and the introduction of peptide groups does not strengthen polar interactions. 4. Conclusions The thermodynamic properties of transfer Dtr V 02:m , Dtr K 0S;2:m , Dtr Ddil H0 from water to aqueous dodecyltrimethylammonium bromide and tetradecyltrimethylammonium bromide permitted an understanding of the fine details of the interactions of the amino acids and peptides with the cationic surfactant solutions. The values of Dtr V 02:m for amino acids from water to 0.1 mol  dm3 DTAB/ TTAB do not show significant variation. Hydration numbers of amino acids in surfactant solutions also did not change significantly. The transfer data suggest different modes of interactions of amino acids with DTAB/TTAB micelles maintaining an overall balance between ionic–ionic, ionic–hydrophobic and hydrophobic–hydrophobic group interactions. Negative values of transfer volume observed for peptides indicate hydrophobic–hydrophobic interactions between alkyl chain of DTAB/TTAB and hydrophobic group of peptides and ion–hydrophobic group interactions between ions of DTAB/TTAB and non-polar groups of peptides. This is also asserted from the increasing endothermic contribution to the Dtr Ddil H0 from gly-gly to gly-leu and hence it can be concluded that with increasing number of peptide bonds the polar interactions with the micelles of cationic surfactants are not strengthened. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jct.2013.11.001. References [1] A.V. Few, R.H. Ottewill, H.C. Parreira, Biochim. Biophys. Acta 18 (1955) 136– 137. [2] C. Blinkhorn, M.N. Jones, Biochem. J. 135 (1973) 547–549. [3] K.K. Andersen, P. Westh, D.E. Otzen, Langmuir 24 (2008) 399–407. [4] D. Otzen, Biochim. Biophys. Acta 1814 (2011) 562–591.

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[5] E.D. Goddard, K.P. Ananthapadmanabhan, Interactions of Surfactants with Polymers and Proteins, CRC Press, London, 1993. [6] O. Kirk, T.V. Borchert, C.C. Fuglsang, Curr. Opin. Biotechnol. 13 (2002) 345–351. [7] M. Aivaliotis, P. Samolis, E. Neofotistou, H. Remigy, A.K. Rizos, G. Tsiotic, Biochim. Biophys. Acta 1615 (2003) 69–76. [8] C. Duval-Terrié, P. Cosette, G. Molle, G. Muller, E. Dé, Protein Sci. 12 (2003) 681–689. [9] K. Weckström, FEBS Lett. 192 (1985) 220–224. [10] W. Parker, P.S. Song, Biophys. J. 61 (1992) 1435–1439. [11] C.O. Rangel-Yagui, A. Pessoa-Jr, L.C. Tavares, J. Pharm. Pharmaceut. Sci. 8 (2005) 147–163. [12] D. Kelley, D.J. McClements, Food Hydrocolloids 17 (2003) 73–85. [13] P. Misra, N. Kishore, J. Colloid Interface Sci. 354 (2011) 234–247. [14] T. Chakraborty, I. Chakraborty, S.P. Moulik, S. Ghosh, Langmuir 25 (2009) 3062–3074. [15] M.N. Jones, Chem. Soc. Rev. 21 (1992) 127–136. [16] H. Zhao, Biophys. Chem. Rev. 122 (2006) 157–183. [17] S.K. Singh, N. Kishore, J. Solution Chem. 33 (2004) 1411–1427. [18] T.V. Chalikian, Annu. Rev. Biophys. Biomol. Struct. 32 (2003) 207–235. [19] K.D. Collins, Methods 34 (2004) 300–311. [20] O. Enea, C. Jollcoeur, J. Phys. Chem. 86 (1982) 3870–3881. [21] E. Fuguet, C. Rafols, M. Roses, E. Bosch, Anal. Chim. Acta 548 (2005) 95–100. [22] W. Tong, Q. Zheng, S. Shao, Q. Lei, W. Fang, J. Chem. Eng. Data 55 (2010) 3766– 3771. [23] O. Likhodi, T.V. Chalikian, J. Am. Chem. Soc. 122 (2000) 7860–7868. [24] D.G. Archer, J. Phys. Chem. Ref. Data 21 (1992) 793–829. [25] T.E. Lesile, T.H. Lilley, Biopolymers 24 (1985) 695–710. [26] J.E. Desnoyers, Pure Appl. Chem. 54 (1982) 1469–1478. [27] F.J. Millero, A.L. Surdo, C. Shin, J. Phys. Chem. 88 (1984) 86–92. [28] J. Wang, Z. Zuo, K. Liu, Biophys. Chem. 80 (1999) 179–188. [29] E. Berlin, M.J. Pallansch, J. Phys. Chem. 72 (1968) 1887–1889. [30] F. Franks, M.A. Quickenden, D.S. Reid, B. Watson, Trans. Faraday Soc. 66 (1970) 582–589. [31] A.W. Hakin, M.M. Duke, J.L. Marty, K.E. Preuss, J. Chem. Soc. Faraday Trans. 90 (1994) 2027–2035. [32] S.K. Singh, A. Kundu, N. Kishore, J. Chem. Thermodyn. 36 (2004) 7–16. [33] F. Shahidi, P.G. Farrell, J.T. Edwards, J. Solution Chem. 5 (1976) 807–816. [34] S. Terasawa, H. Itsuki, S. Arakwa, J. Phys. Chem. 79 (1975) 2345–2351. [35] H.S. Frank, M.W. Evans, J. Chem. Phys. 13 (1945) 493–507. [36] A. Bernheim-Groswasser, R. Zana, Y. Talmon, J. Phys. Chem. B 104 (2000) 4005–4009. [37] Y.S. Lee, K.W. Woo, Bull. Korean Chem. Soc. 14 (1993) 599–602. [38] L.U. Xing-Yu, J. Yan, C. Xiao-Hong, M. Shi-Zhen, L. Mai-Li, D. You-Ru, Acta Phys. Chim. Sin. 25 (2009) 1357–1361. [39] S. Choudhary, Nand Kishore, J. Chem. Thermodyn. 52 (2012) 36–42. [40] R. Sharma, N. Kishore, J. Solution Chem. 35 (2006) 231–249. [41] A. Ali, F.A. Itoo, N.H. Ansari, Z. Naturforsch. 66a (2011) 345–352. [42] Z. Yan, Q. Zhang, W. Li, J. Wang, J. Chem. Eng. Data 55 (2010) 3560–3566. [43] A. Ali, V. Bhushan, P. Bidhuri, J. Mol. Liq. 177 (2013) 209–214. [44] A. Anwar, S. Sabir, S. Hyder, Acta Phys. Chim. Sin. 23 (2007) 1007–1012. [45] A. Ali, Shahjahan, N.H. Ansari, Russ. Chem. Bull. Int. Ed. 59 (2010) 1999–2004. [46] P.P. Misra, N. Kishore, J. Chem. Thermodyn. 54 (2012) 453–463. [47] J. Wang, Z. Yan, Y. Zhao, F. Cui, J. Chem. Eng. Data 49 (2004) 1354–1358.

JCT 13-434