Interactions of some amino acids and glycine peptides with aqueous sodium dodecyl sulfate and cetyltrimethylammonium bromide at T=298.15 K: a volumetric approach

Abstract

The apparent molar volumes Vφ of glycine, alanine, valine, leucine, and lysine have been determined in aqueous solutions of 0.05, 0.5, 1.0 mol · kg−1 sodium dodecyl sulfate (SDS) and 1.0 mol · kg−1 cetyltrimethylammonium bromide (CTAB) by density measurements at T=298.15 K. The apparent molar volumes have also been determined for diglycine and triglycine in 1 mol · kg−1 SDS and CTAB solutions. These data have been used to calculate the infinite dilution apparent molar volumes V20 for the amino acids and peptides in aqueous SDS and CTAB and the standard partial molar volumes of transfer (ΔtrV2,m0) of the amino acids and peptides to these aqueous surfactant solutions. The linear correlation of V20 for a homologous series of amino acids has been utilized to calculate the contribution of the charged end groups (NH3+, COO−), CH2 group and other alkyl chains of the amino acids to V20. The results on the partial molar volumes of transfer from water to aqueous SDS and CTAB have been interpreted in terms of ion–ion, ion–polar and hydrophobic–hydrophobic group interactions. The volume of transfer data suggests that ion–ion or ion–hydrophilic group interactions of the amino acids and peptides are stronger with SDS compared to those with CTAB. Comparison of the hydration numbers of amino acids calculated in the present studies with those in other solvents from literature shows that these numbers are almost the same at 1 mol · kg−1 level of the cosolvent/cosolute. Increasing molality of the cosolvent/cosolute beyond 1 mol · kg−1 lowers the hydration number of the amino acids due to increased interactions with the solvent and reduced electrostriction.

Full text

Page 1

Interactions of some amino acids and glycine peptides
with aqueous sodium dodecyl sulfate and
cetyltrimethylammonium bromide at T ¼ 298:15 K:
a volumetric approach
Sreelekha K. Singh, Agnita Kundu, Nand Kishore*
Department of Chemistry, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India
Abstract
The apparent molar volumes V/of glycine, alanine, valine, leucine, and lysine have been determined in aqueous solutions of 0.05,
0.5, 1.0 mol?kg?1sodium dodecyl sulfate (SDS) and 1.0 mol?kg?1cetyltrimethylammonium bromide (CTAB) by density mea-
surements at T ¼ 298:15 K. The apparent molar volumes have also been determined for diglycine and triglycine in 1 mol?kg?1SDS
and CTAB solutions. These data have been used to calculate the infinite dilution apparent molar volumes V0
and peptides in aqueous SDS and CTAB and the standard partial molar volumes of transfer (DtrV0
peptides to these aqueous surfactant solutions. The linear correlation of V0
to calculate the contribution of the charged end groups (NHþ
The results on the partial molar volumes of transfer from water to aqueous SDS and CTAB have been interpreted in terms of
ion–ion, ion–polar and hydrophobic–hydrophobic group interactions. The volume of transfer data suggests that ion–ion or ion–
hydrophilic group interactions of the amino acids and peptides are stronger with SDS compared to those with CTAB. Comparison
of the hydration numbers of amino acids calculated in the present studies with those in other solvents from literature shows that
these numbers are almost the same at 1 mol?kg?1level of the cosolvent/cosolute. Increasing molality of the cosolvent/cosolute
beyond 1 mol?kg?1lowers the hydration number of the amino acids due to increased interactions with the solvent and reduced
electrostriction.
2for the amino acids
2;m) of the amino acids and
2for a homologous series of amino acids has been utilized
3, COO?), CH2group and other alkyl chains of the amino acids to V0
2.
Keywords: Partial molar volume; a-Amino acids; Peptides; Hydration number; Sodium dodecyl sulfate; Cetyltrimethylammonium bromide
1. Introduction
Surfactants are extensively employed in pharmaceu-
tical [1,2] and biotechnological processes [3,4]. From a
technological perspective, protein–surfactant interac-
tions are important because they modulate the functional
properties of proteins. Surfactant–protein interactions
have been studied using conductivity [5,6], chromatog-
raphy [5], axisymmetric drop shape analysis [7], FTIR
[8–10], circular dichroism [11,12], fluorescence [13,14],
and direct calorimetry [15–18]. It has been proposed that
surfactants may interact with proteins directly by com-
peting for oil–water or air–water interfaces [19,20] and
by binding to them thereby leading to substantial chan-
ges in the protein conformation [21,22]. Alterations in
the molecular characteristics of globular proteins due to
their interactions with surfactants may lead to changes
in their ability to bind other molecules; i.e. they may self-
associate and be absorbed at interfaces thus altering their
functional characteristics. However, several details in the
mode of interaction of the surfactants with the proteins
remain unanswered. Therefore it is very important to
understand the origin and nature of protein–surfactant
interactions both qualitatively and quantitatively. In

Page 2

order to understand the fine details, the interactions of
the building blocks of the protein with surfactants must
be studied owing to the complex structural organization
of the biological macromolecules. To the best of our
knowledge, no report is available in the literature on the
volumetric properties of the amino acids and peptides in
aqueous surfactant solutions. In this paper we report the
partial molar volumes of transfer of some amino acids
and glycine peptides from water to aqueous sodium do-
decyl sulfate (SDS) and cetyltrimethylammonium bro-
mide (CTAB) solutions and discuss the interactions
operating in these systems including their effect on the
hydration number of these solutes. Among various
physical parameters, partial molar volume has been
recognized as a quantity that is sensitive to structural
changes occurring in solutions.
2. Experimental
The amino acids and peptides were procured from
Sigma Chemical Company, USA. Sodium dodecyl sul-
fate and cetyltrimethylammonium bromide were of ex-
trapure analytical reagent grade purchased from Sisco
Research Laboratories, India, and Spectrochem Ltd.,
India, respectively. Purity of all the chemicals as re-
ported by the vendors is listed in table 1. All the amino
acids and peptides were dried over P2O5 in a vacuum
dessicator. The moisture content of the amino acids and
peptides was determined using a Karl Fisher Titrator
(Systronics, India). Appropriate corrections were ap-
plied wherever necessary. Water used for making the
solutions of amino acids and peptides was double dis-
tilled and deionized by passing it through a Cole–Parmer
Barnstead mixed-bed ion exchange resin column fol-
lowed by degassing. Solution densities were measured on
a vibrating tube digital densimeter (model DA-210 from
Kyoto Electronics, Japan) details of which has been
described elsewhere [23]. The temperature of the den-
simeter cell was maintained by circulating water from a
Julabo constant temperature circulation bath. This ar-
rangement gave a temperature stability of ?0.01 K. The
vibrational period of the densimeter tube containing the
solution of interest was measured three times each. The
reproducibility in the density measurements on the av-
erage was ?3?10?6g?cm?3. The calibration of the den-
simeter was performed every day and accuracy checked
by measuring the densities of aqueous sodium chloride
solutions, the result of which were found to be in ex-
cellent agreement with the literature [24] with a maxi-
mum difference of 0.03 cm3?mol?1in the values of V/.
3. Results and discussion
The apparent molar volumes (V/) were calculated
from the measured density data using the following
equation:
V/¼ ðM=qÞ ? ðq ? q0Þ ? 103=ðm ? q ? q0Þ;
where M is the molar mass of the solute in g?mol?1, m is
the molality of the solute in mol?kg?1in surfactant–
water mixtures, q and q0are the densities of the ternary
system and reference solvent (desired molality of aque-
ous SDS or CTAB), respectively, in g?cm?3. The results
of the density measurements are given in table 2. In the
cases where molality dependence of V/was found to be
either negligible or having no definite trend, the partial
molar volume at infinite dilution, V0
taking an average of all the data points, the standard
deviations pertain to the mean value. In all other cases
the standard partial molar volumes were obtained by
least-squares fitting to the following equation:
ð1Þ
2was evaluated by
V/¼ V0
Here, Svis the experimental slope, which is sometimes
considered to be volumetric pairwise interaction coeffi-
cient [25,26]. The standard partial molar volumes along
with the Svvalues are given in table 3.
The values of V0
2, which are, in all cases, positive in
both SDS and CTAB at all the molalities, show an in-
crease with an increase in the molality of SDS. In each
solvent system, the values of V0
variation with the number of carbon atoms in the alkyl
2þ Svm:
ð2Þ
2(table 3) showed a linear
TABLE 1
Compounds used in this study with their empirical formula, molar mass (Mr), source (S¼Sigma Chemical Company, USA, SRL¼Sisco Research
Laboratories, India), mass fraction moisture contents (w), and their mole fraction purity (x) as reported by the vendors
CompoundFormula
Mr=g ? mol?1
75.07
89.09
131.2
117.1
182.6
132.1
189.2
288.38
364.46
w
Source
x
Glycine
L L-alanine
L L-leucine
L L-valine
L L-lysine monohydrochloride
Glycyl–glycine
Glycyl–glycyl–glycine
Sodium dodecyl sulfate (SDS)
Cetyltrimethylammonium bromide (CTAB)
C2H5NO2
C3H7NO2
C6H13N2
C5H11NO2
C6H14N2O2
C4H8N2O3
C6H11N3O4
C12H25NaO4S
C19H42BrN
0.0012
0.0018
0.0008
0.0007
0.0419
0.0058
0.0062
S
S
S
S
S
S
S
SRL
Spectrochem
>0.99
>0.99
>0.98
>0.98
>0.98
>0.98
>0.99
>0.99
>0.98
8

Page 3

TABLE 2
Densities q and apparent molar volumes V/of amino acids and pep-
tides in aqueous surfactants at T ¼ 298:15 K at various molalities m
and in the presence of cosolutes
m
q
V/
(mol?kg?1) (g?cm?3) (cm3?mol?1)
Cosolute: SDS (m ¼ 0:05 mol?kg?1)
Glycine
0.999103
1.003265
1.006398
1.018092
1.024388
1.026122
1.034433
1.037234
1.046474
1.056212
Valine
0.999151
1.000510
1.000548
1.000971
1.001140
1.001577
1.001959
1.002104
Lysine hydrochloride
0.999134
1.001809
1.001954
1.002831
1.003446
1.003614
1.003959
1.005051
Alanine
0.999078
1.004265
1.007592
1.009752
1.012015
1.015526
1.016451
1.020859
1.021639
1.024838
Leucine
0.999133
0.999830
0.999871
1.000127
1.000450
1.000557
1.000828
1.000931
1.001164
1.001476
0.00000
0.13291
0.23423
0.62570
0.84427
0.90571
1.20892
1.31433
1.67028
2.06759
43.59
43.62
43.89
44.02
44.06
44.29
44.38
44.61
44.90
0.00000
0.05260
0.05424
0.06951
0.07635
0.09316
0.10800
0.11371
91.19
91.27
90.80
90.93
90.89
90.90
90.92
0.00000
0.04874
0.05129
0.06738
0.07848
0.08150
0.08830
0.10847
127.43
127.33
127.33
127.17
127.12
127.41
127.36
0.00000
0.18435
0.30592
0.38601
0.47107
0.60585
0.64221
0.81574
0.84778
0.97852
60.66
60.77
60.82
60.87
60.97
61.01
61.09
61.13
61.22
0.00000
0.03048
0.03241
0.04362
0.05767
0.06260
0.07411
0.07802
0.08895
0.10248
108.34
108.44
108.37
108.30
108.37
108.22
108.04
108.22
108.15
Cosolute: SDS (m ¼ 0:5 mol?kg?1)
Glycine
1.014574
1.021628
1.024954
1.031031
1.038318
0.00000
0.23282
0.34633
0.55413
0.82020
44.25
44.42
44.42
44.82
TABLE 2 (continued)
m
q
V/
(mol?kg?1)
0.99978
1.20958
1.46411
1.67413
2.32496
(g?cm?3)
1.043280
1.048892
1.055339
1.060419
1.075625
(cm3?mol?1)
44.83
44.91
45.13
45.34
45.73
Valine
0.00000
0.02851
0.04202
0.06029
0.07035
0.08425
0.09090
0.10401
0.13788
1.014514
1.015186
1.015494
1.015938
1.016166
1.016503
1.016656
1.016951
1.017731
Lysine hydrochloride
1.014581
1.015387
1.016639
1.017134
1.017775
1.018163
1.018761
1.019374
1.019468
1.020052
1.020969
Alanine
1.014573
1.019865
1.022000
1.024302
1.028490
1.031050
1.033489
1.035896
Leucine
1.014570
1.014921
1.015061
1.015402
1.015613
1.015683
1.016008
1.016142
1.016352
1.016708
92.47
92.67
92.35
92.45
92.31
92.33
92.44
92.46
0.00000
0.01523
0.03938
0.04879
0.06120
0.06838
0.08000
0.09206
0.09358
0.10467
0.12202
128.47
128.94
128.82
128.86
128.63
128.69
128.79
128.62
128.51
128.31
0.00000
0.19758
0.27846
0.37125
0.53471
0.63676
0.73474
0.83688
61.47
61.45
61.76
61.68
61.67
61.65
61.76
0.00000
0.01838
0.02524
0.04271
0.05368
0.05707
0.07505
0.08068
0.09233
0.11127
110.33
110.36
110.29
110.33
110.25
110.51
110.22
110.37
110.42
Cosolute: SDS (m ¼ 1:0 mol?kg?1)
Glycine
1.027423
1.034377
1.040979
1.045914
1.047326
1.053268
1.069292
1.073368
Valine
1.027460
1.028417
1.028696
1.029121
0.00000
0.24150
0.47517
0.65541
0.70253
0.92413
1.56220
1.73892
45.48
45.44
45.52
45.35
45.43
45.81
45.98
0.00000
0.04227
0.05514
0.07323
92.44
92.63
92.34
9

Page 4

TABLE 2 (continued)
m
q
V/
(mol?kg?1)
0.07740
0.09600
0.10470
0.10860
0.18850
(g?cm?3)
1.029195
1.029608
1.029796
1.029882
1.031645
Lysine hydrochloride
1.027425
1.028699
1.028818
1.028064
1.031052
1.031235
1.031635
1.031796
1.032598
Diglycine
1.027430
1.030835
1.030841
1.032530
1.032541
1.034583
1.034593
1.035215
1.035230
1.036251
1.036269
1.036604
1.036622
Alanine
1.027428
1.030088
1.031962
1.035017
1.037731
1.040690
1.044363
1.048667
1.049403
1.050472
Leucine
1.027474
1.028383
1.028535
1.028632
1.028876
1.028925
1.029322
1.029453
Triglycine
1.027435
1.031078
1.032274
1.033162
1.034188
1.034350
1.034723
1.034794
1.036380
1.036483
1.037206
(cm3?mol?1)
92.58
92.57
92.62
92.63
92.56
0.00000
0.02509
0.02735
0.03226
0.07153
0.07583
0.08372
0.08703
0.10247
129.46
129.31
129.38
129.24
129.65
129.55
129.60
129.25
0.00000
0.06653
0.06658
0.09979
0.09987
0.13998
0.14008
0.15349
0.15360
0.17352
0.17365
0.18124
0.18138
79.83
79.77
79.79
79.70
79.61
79.58
79.92
79.86
79.73
79.67
79.91
79.85
0.00000
0.10370
0.17647
0.30272
0.40702
0.52947
0.70192
0.87239
0.90883
0.95193
62.25
62.10
62.50
62.11
62.18
62.82
62.36
62.47
62.38
0.00000
0.05281
0.06240
0.06702
0.08316
0.08504
0.10782
0.11602
111.29
111.47
111.20
111.57
111.37
111.26
111.32
0.00000
0.05070
0.06740
0.07987
0.09403
0.09630
0.10203
0.10334
0.12577
0.12681
0.13742
115.66
115.59
115.58
115.35
115.36
115.66
115.86
115.76
115.54
115.69
TABLE 2 (continued)
m
q
V/
(mol?kg?1) (g?cm?3)(cm3?mol?1)
Cosolute: CTAB (m ¼ 1:0 mol?kg?1)
Glycine
1.004430
1.009117
1.009688
1.010427
1.014824
1.014852
1.017075
1.019795
1.022651
1.022748
1.023296
Valine
1.004432
1.005393
1.006589
1.007866
1.008535
1.008754
1.009126
1.009191
1.009717
Lysine hydrochloride
1.004420
1.005731
1.007411
1.008164
1.008485
1.008677
1.009727
1.009807
1.010511
1.010643
Alanine
1.004435
1.007644
1.010721
1.013036
1.014155
1.015697
1.017365
1.019917
Leucine
1.004427
1.004960
1.005134
1.005409
1.005516
1.005731
1.005760
1.006198
1.006273
1.006633
Diglycine
1.004430
1.007260
1.007306
1.008386
1.009525
1.010002
0.00000
0.15114
0.17050
0.19641
0.29582
0.33987
0.41706
0.50826
0.60775
0.61150
0.62900
43.80
43.94
44.21
43.97
43.89
44.13
44.10
44.22
44.24
44.18
0.00000
0.02377
0.05358
0.08578
0.10324
0.10898
0.11794
0.11993
0.13239
90.47
90.52
90.63
90.86
90.92
90.75
90.86
90.57
0.00000
0.02383
0.05477
0.06851
0.07446
0.07814
0.09768
0.09884
0.11227
0.11474
127.07
127.28
127.15
127.16
127.25
127.27
127.09
127.24
127.25
0.00000
0.11436
0.22609
0.31245
0.35452
0.41231
0.47731
0.57593
60.69
60.76
60.89
60.93
60.94
61.06
61.11
0.00000
0.02356
0.03106
0.04302
0.04951
0.05754
0.05916
0.07870
0.08238
0.09885
108.15
107.98
107.90
108.70
108.02
108.15
108.12
108.21
108.26
0.00000
0.05185
0.05267
0.07285
0.09387
0.10264
77.20
77.17
77.39
77.32
77.28
10

Page 5

chain of the amino acids; the average correlation coeffi-
cient is 0.99953. Similar linear correlations have been
observed earlier for homologous series of x-amino acids
in aqueous potassium thiocyanate solution [27] and a-
aminoacids[28,29].Thislinearvariationisrepresentedby
V0
2¼ V0
where nc is the number of carbon atoms in the alkyl
chain of the a-amino acids, V0
2ðNHþ
3;COO?Þ þ ncV0
2ðCH2Þ;
ð3Þ
2(NHþ
3, COO?) and
V0
2(CH2) are the zwitterionic end groups and the meth-
ylene group contribution to V0
of V0
2(CH2), calculated by a least-
square regression analysis are listed in table 4. Since the
alkyl chains of homologous series of the a-amino acids
studied in this work are CH2-(Gly), CH3CH-(Ala),
CH3CH3CHCH-(Val) and CH3CH3CHCH2CH-(Leu),
the value of V0
2(CH2) obtained by this procedure
characterize the mean contribution of CH– and CH3–
groups to V0
Hakin et al. [30,31], the contribution of the other alkyl
chain of the a-amino acids reported in table 4. were
calculated as follows:
2, respectively. The values
2(NHþ
3, COO?) and V0
2of the a-amino acids. As suggested by
V0
2ðCH3Þ ¼ 1:5V0
2ðCH2Þ;
ð4Þ
V0
2ðCHÞ ¼ 0:5V0
2ðCH2Þ:
ð5Þ
TABLE 2 (continued)
m
q
V/
(mol?kg?1)
0.10430
0.13373
0.16199
(g?cm?3)
1.010086
1.011675
1.013186
(cm3?mol?1)
77.33
77.26
77.27
TABLE 3
Volumetric parameters of amino acids and peptides in aqueous SDS and CTAB solutions at T ¼ 298:15 Ka;b
Water
SDSCTAB
(m ¼ 0:05
mol?kg?1)
(m ¼ 0:5
mol?kg?1)
(m ¼ 1:0
mol?kg?1)
(m ¼ 1:0
mol?kg?1)
Amino acids
Glycine
V0
Sv/(cm3?mol?2?kg)
R
2/(cm3?mol?1) 43.14(0.06)c
0.86(0.09)
0.97
43.46(0.01)
0.69(0.01)
1.0
44.13(0.05)
0.70(0.04)
0.99
45.57(0.23) 44.07(0.16)
Alanine
V0
Sv/(cm3?mol?2?kg)
R
2/(cm3?mol?1)60.43(0.04)c
0.73(0.06)
0.99
60.55(0.01)
0.68(0.02)
1.0
61.63(0.13)62.35(0.23)60.57(0.03)
0.96(0.07)
0.99
Valine
V0
2/(cm3?mol?1)
V0
90.39(0.14)c
90.99(0.17)92.44(0.11) 92.55(0.10)90.70(0.17)
Leucine
2/(cm3?mol?1)107.72(0.24)c
108.27(0.13) 110.34(0.09)111.35(0.13) 108.17(0.23)
Lysine
Hydrochloride
V0
2/(cm3?mol?1)
V0
Sv/(cm3?mol?2?kg)
R
125.90(0.30)d
127.31(0.12) 128.66(0.20)129.43(0.16)127.20(0.08)
Diglycine
2/(cm3?mol?1)76.43(0.15)c
1.84(0.09)
0.99
79.77(0.11)77.28(0.07)
Triglycine
V0
2/(cm3?mol?1)112.51(0.23)c
115.61(0.16)
aR, regression coefficient of the linear fit.
bEntries in the parentheses are standard deviations.
cRef. [55].
dRef. [56].
TABLE 4
Contributions of zwitter ionic group (NHþ
solution at T ¼ 298:15 K
Group
3, COO?), CH2group and the other alkyl chains to partial molar volume (V0
2) in aqueous SDS and CTAB
V0
2/(cm3?mol?1)
WaterSDSCTAB
(m ¼ 0:05 mol?kg?1)
27.80(0.95)
16.00(0.28)
32.00(0.40)
64.00(0.40)
80.00(0.50)
(m ¼ 0:5 mol?kg?1)
28.17(1.11)
16.32(0.33)
32.64(0.40)
65.28(0.40)
81.60(0.50)
(m ¼ 1:0 mol?kg?1)
29.43(1.50)
16.18(0.44)
32.36(0.50)
64.72(0.40)
80.90(0.50)
(m ¼ 1:0 mol?kg?1)
28.38(0.99)
15.83(0.29)
31.66(0.40)
63.32(0.40)
79.15(0.55)
NHþ
CH2–
CH3CH–
CH3CH3CHCH–
CH3CH3CHCH2CH–
3, COO?
27.68(1.12)
15.91(0.33)
31.82(0.40)
63.64(0.40)
79.55(0.50)
Values in the parentheses are standard deviation values. Average correlation coefficient: 0.99953.
11