Peptides in aqueous protic ionic liquid solutions: Apparent and transfer volumes at 298.15 K and at 0.1 MPa

Article abstract of DOI:10.1016/j.molliq.2020.113943

The densities of glycine based peptides namely glycyl-glycine (digly), glycine-?-valine (gly-val), glycyl-?-leucine (gly-leu) and glycyl-glycyl-glycine (triglycine/trigly) in aqueous solutions containing ~0.20 mol·kg^?1 of triethylammonium acetate [TEAA], triethylammonium glycolate [TEAG], triethylammonium pyruvate [TEAPy] and triethylammonium propionate [TEAP] protic ionic liquids (PILs) are reported at 298.15 K and at atmospheric pressure. Experimental density data obtained for these systems were used to estimate apparent molar volume (?_V), the limiting apparent molar volume (?_V⁰), partial molar volume of solute (Vˉ₂) and partial molar volume of mixed solvent (PILs + water) (Vˉ₁). The change in limiting apparent molar volume due to transfer (Δ_tr?_V⁰) of peptides from water to aqueous ionic liquid solutions were also estimated. The outcomes obtained from all these thermodynamic parameters were discussed in terms of ion–peptide interactions, ion–ion interactions, hydrophobic solvation, peptide group contributions to limiting volumes, etc.

Full text of DOI:10.1016/j.molliq.2020.113943

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Peptides in aqueous protic ionic liquid solutions: Apparent and
transfer volumes at 298.15 K and at 0.1 MPa
Kunal R. Patil, Shrikant P. Musale, Dilip H. Dagade
PII:
S0167-7322(20)32653-2
DOI:
https://doi.org/10.1016/j.molliq.2020.113943
MOLLIQ 113943
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
27 April 2020
22 July 2020
28 July 2020
Please cite this article as: K.R. Patil, S.P. Musale and D.H. Dagade, Peptides in aqueous
protic ionic liquid solutions: Apparent and transfer volumes at 298.15 K and at 0.1 MPa,
Journal of Molecular Liquids (2020), https://doi.org/10.1016/j.molliq.2020.113943
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2020 Published by Elsevier.

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Peptides in Aqueous Protic Ionic Liquid Solutions: Apparent and
Transfer Volumes at 298.15 K and at 0.1 MPa
a
a
a,
Kunal R. Patil , Shrikant P. Musale and Dilip H. Dagade *
a
Department of Chemistry, Shivaji University, Kolhapur, India- 416 004
*
Corresponding author: Email – dhd_chem@unishivaji.ac.in
ABSTRACT: The densities of glycine based peptides namely glycyl-glycine (digly), glycine-
ւ-valine (gly-val), glycyl-ւ-leucine (gly-leu) and glycyl-glycyl-glycine (triglycine/trigly) in
−
1
aqueous solutions containing ~0.20 mol·kg of triethylammonium acetate [TEAA],
triethylammonium glycolate [TEAG], triethylammonium pyruvate [TEAPy] and
triethylammonium propionate [TEAP] protic ionic liquids (PILs) are reported at 298.15 K and
at atmospheric pressure. Experimental density data obtained for these systems were used to
0
estimate apparent molar volume (V ), the limiting apparent molar volume (V ), partial molar
volume of solute (
V
2
) and partial molar volume of mixed solvent (PILs+water) (
V
1
). The
0
change in limiting apparent molar volume due to transfer (   ) of peptides from water to
tr
V
aqueous ionic liquid solutions were also estimated. The outcomes obtained from all these
thermodynamic parameters were discussed in terms of ion−peptide interactions, ion−ion
interactions, hydrophobic solvation, peptide group contributions to limiting volumes, etc.
Keywords: peptides; protic ionic liquids; partial molar volume; transfer molar volume;
hydrophobic and hydrophilic Interaction; group contributions
1
. Introduction
Structure and function of biological macromolecules and assemblies is decided by the
surrounding environment of these molecules. Stability and the native structure of such
1

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molecules is controlled by collective efforts of various non−covalent forces such as hydrogen
bonding, ionic and dipolar interactions, dispersion forces, hydrophobic interactions, etc. [1,2].
Denaturation of globular proteins in aqueous medium results due to alteration in hydrogen
bonding (H–bond) interactions, −helix, −sheets in proteins as compared to its native form
which leads to the uncoiling of protein into a disorderly shape [3−6]. Due to complex nature
of such non-covalent interactions between proteins and co-solutes/solvents, it is preferable to
investigate small building blocks such as amino acids or small peptides which help largely to
interpolate or scale up the outcomes to larger polypeptides or proteins as some structural
features and functional groups are common [7−10]. Thus, to enhance our understanding about
various ionic and/or molecular interactions existing in aqueous biomolecular solutions,
measurement and analysis of effect of additive concentrations and of temperature variation on
the thermodynamic properties helps a lot. In this line, thermodynamic and physicochemical
investigation of amino acids and peptides in aqueous and in aqueous cosolute solutions are
well documented to get fruitful information about ion−ion and ion−solvent interactions which
helps to increase our deep learning towards naturally occurring bioprocesses [7−23].
Ionic liquids (ILs) are showing notable presence in almost every scientific field of
basic and applied sciences as well as in technological innovations and in medicinal sciences
[
24−26] due to large probabilities in their designs through combinations of structurally
different types of cations and anions as per the need. Especially, bio-ionic liquids (ionic
liquids from biological resources) are gaining much attention due to their potential role in
protein chemistry as osmolytes or denaturants or stabilizer, in pharmaceutical chemistry as a
part of drugs or as solubilizing and stabilizing agents for many unstable and insoluble drugs
etc. due to less or nontoxic nature of bio-ionic liquids. To understand the potential role of
traditional ILs and such bio-ILs in protein chemistry and medicinal fields, many researchers
are engaged in investigations on molecular level understanding of interactions between
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biomolecules and ionic liquids. In this regard, thermodynamic and physicochemical properties
of amino acids and peptides in aqueous aprotic ionic liquids (APILs) are well-studied
[
(
27−48], however, a limited work is available in aqueous solutions of protic ionic liquids
PILs) [49]. Volumetric investigations can give novel data as far as the folding/defolding
phenomenon, conformational stability and ligand−binding properties of a protein are at
central place [50]. Volumetric studies of amino acids and peptides in imidazolium based
bromide ionic liquids studied by Fang [27] and Wen [40] revealed that the increased
concentration of imidazolium-based bromide ionic liquids causes dehydration effect on the
amino acids/peptides which gets enhanced with temperature increase. Furthermore, this study
revealed that the hydrophilic–hydrophilic interactions are more prominent amongst amino
acids/dipeptide and imidazolium-based bromide ILs rather than with the head groups or
hydrophobic side chains. Recently, we reported thermodynamic properties of bio-ionic liquids
using volumetric, speed of sound and osmotic vapor pressure measurements for the
ammonium based carboxylate/nitrate PILs and imidazolium based amino acids
(
[C Mim][AA] (n=3,4)) in aqueous solutions [51−58]. We observed that these bio-ionic
n
liquids gets strongly hydrated and shows the high hydration number due to cooperative
hydrogen bonding of carboxylate and amino acid anions with water structure making effect
i.e. kosmotropic effect which gets further enhanced with increase in hydrophobicity of ions.
In this work, we are reporting volumetric studies for peptides (glycyl-glycine, glycine-
ւ-valine, glycyl-ւ-leucine and glycyl-glycyl-glycine) in aqueous solutions containing ~0.2
−
1
mol·kg
glycolate [TEAG], triethylammonium pyruvate [TEAPy] and triethylammonium propionate
TEAP]) at 298.15 K. Volumetric properties such as apparent molar volume ( V ), the limiting
apparent molar volume (V ), limiting apparent transfer molar volume (   ), partial molar
protic ionic liquids (triethylammonium acetate [TEAA], triethylammonium
[

0
0
tr
V
volume of solute (
V
2
) and partial molar volume of mixed solvent (PILs+water) (
V
1
) were
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estimated using density data are reported. The group contributions method is presented and
0
used to estimate group contribution to limiting apparent molar volume (V ) for peptides in
−
1
aqueous and aqueous ~0.20 mol·kg PIL solutions. The details of measurements, data
analysis and discussion on results obtained are outlined below.
2
.
Experimental
2
.1. Materials
The glycine based peptides namely glycyl-glycine (Sigma, mass fraction >0.99),
glycine-ւ-valine (Hi-Media, mass fraction 0.98), glycyl-ւ-leucine (Hi-media, mass fraction
.98) and glycyl-glycyl-glycine i.e. triglycine (Sigma, mass fraction 0.99), triethylamine
Merck, mass fraction 0.99) and glacial acetic acid (Merck, mass fraction 0.99) were used
0
(
without further purification. Sodium chloride (NaCl) (BDH, mass fraction >0.999) and
potassium chloride (KCl) (BDH, mass fraction >0.998), were used without further
purification but dried under a vacuum at 393 K for 24 hours before use. The source, purity
method, purity and CAS number for the chemicals used are reported in Table 1.
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Table 1 Chemical name, CAS No., molecular mass, purity, source and analysis method of chemicals used.
Molecular mass
Mass fraction
purity
Water content
(mass fraction)
Chemical name
CAS no.
Source
Analysis method
−
1
(
g·mol )
132.12
174.20
188.22
189.17
101.19
60.05
a
Glycyl-glycine
Glycyl-ւ-valine
Glycyl-ւ-leucine
Triglycine
556-50-3
1963-21-9
869-19-2
556-33-2
121-44-8
64-19-7
>0.99
Sigma
Hi-Media
a
0.98
a
0.98
Hi-Media
a
0.99
Sigma
a
Triethylamine
>0.99
Merck
a
Acetic acid
>0.99
Merck
a
Phosphorous pentaoxide
Sodium chloride
Potassium chloride
1314-56-3
7647-14-5
7447-40-7
5204-74-0
51009-80-4
178461-51-3
−
141.95
58.44
>0.98
Merck
a
>0.999
BDH
a
74.55
>0.998
BDH
b
b
d
TEAA
161.24
175.27
177.24
189.25
≥ 0.98
0.0179
Synthesized in lab
Synthesized in lab
Synthesized in lab
Synthesized in lab
NMR
NMR
NMR
NMR
c
c
d
TEAP
≥ 0.98
0.0021
c
c
d
TEAG
≥ 0.98
0.0109
c
c
d
TEAPy
≥ 0.99
0.0059
a
As stated by supplier.
b
1
The PIL, Triethylammonium acetate [TEAA] is synthesized in laboratory using procedure reported earlier [51]. The purity was checked by H
1
NMR spectroscopic techniques. No traces of impurities were detected in H NMR spectra (See Supplementary Material).
c
PILs namely Triethylammonium propionate [TEAP], Triethylammonium glycolate [TEAG] and Triethylammonium pyruvate [TEAPy] used
here are synthesized and characterized earlier [51].
5

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d
Water content in the PILs estimated using TKF-55 Karl-Fischer titrator and values reported are average of at least three measurements for each
PIL. This water content was treated as an impurity while reporting the PIL purity and considered as a part of solvent while preparing aqueous
solutions of PILs to correct the concentrations accordingly.
6

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2
.2. Synthesis of Protic Ionic Liquids
All the studied protic ionic liquids (PILs) namely triethylammonium acetate [TEAA],
triethylammonium glycolate [TEAG], triethylammonium pyruvate [TEAPy] and
triethylammonium propionate [TEAP] were synthesized in our laboratory. The details on
synthesis, purification and characterization of these PILs are reported already in our recent
article [51] except triethylammonium acetate [TEAA] for which also a similar procedure of
synthesis and purification were adopted. Accordingly, for synthesis of TEAA PIL, equimolar
quantities of triethylamine and acetic acid were used. The triethylamine were taken into a
round bottom flask equipped with a reflux condenser and a dropping funnel with the tip under
stopcock. An equimolar quantity of glacial acetic acid was added dropwise with constant
stirring into triethylamine with the help of dropping funnel. An equimolar mixture in the
round bottom flask with constant stirring was kept in a circulating heated water-bath initially
at 333.15 K for 1 hour and then at 343.15 K for 2 hour for completion of reaction to form
TEAA. The traces of water molecules in the TEAA PIL were removed by drying at 343.15 K
under vacuum for 48 hours. Finally, the resulting PIL was stored under dry conditions i.e.
over phosphorous pentaoxide (P O ) in a desiccator. The structure of the TEAA was
2
5
1
confirmed by H NMR spectra in CDCl solvent obtained using Bruker 300 MHz NMR
3
instrument (see Supplementary Material, Fig. S1). The details of purity estimation and water
1
content analysis of studied PILs using the H NMR spectral data were reported already in our
recent work [51]. The chemical structures of studied peptides and PILs are schematically
represented in scheme 1. Water contents in the synthesized PILs were estimated using TKF-
5
5 Karl-Fischer titrator and found to be 0.0179 mass fraction in TEAA, 0.0109 mass fraction
in TEAG, 0.0059 mass fraction in TEAPy and 0.0021 mass fraction in TEAP which were
taken into consideration during the preparation of binary aqueous solutions of PILs. The water
content given here is the average over at least three measurements for each PIL.
7

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Peptides
glycyl-glycine
glycine-ւ-valine
glycine-ւ-leucine
Triglycine
Protic Ionic Liquids
Scheme 1. Chemical structures of peptides and cations and anions of protic ionic liquids.
2
.3. Preparation of Sample Solutions
Fresh doubly quartz distilled water was used for the entire work. All aqueous binary
(PIL + water) and ternary (peptide + PIL + water) solutions were prepared on the molality
basis at room temperature in the airtight glass vessels using a Mettler Toledo ML204/A01
weighing balance having readability of ± 0.0001 g. The standard uncertainties in the molality
−
4
−1
of solutions were found to be ± 1·10 mol·kg .
2
.4.
Density Measurements
The densities ( ) of the ternary solutions were measured using vibrating–tube digital

densitometer (Anton Paar, DMA 60/602 M) at 298.15 K and at atmospheric pressure. The
temperature of the measuring cell was maintained constant at 298.15 K using Julabo F-34HE
cryostat, having temperature stability of ± 0.01 K. The densitometer was calibrated using
water and air, the densities of which are known at 298.15 K and at atmospheric pressure.
8

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Density of air used for calibration was corrected for laboratory pressure and humidity.
Oscillation period of the vibrating tube (either filled with water or with air) was measured at
2
98.15 K and used to determine instrumental calibration constants A and B of the equation
2

 A B . After knowing constants, A and B, the densities of peptides in aqueous PIL
solutions at different concentrations were obtained from measured  values of the
corresponding sample solutions at 298.15 K. The combined uncertainty in the density
−
2
−3
measurements was found to be ± 1·10 kg·m . The authenticity of density measurements
was ascertained by comparing the density data of known system i.e. of aqueous solutions of
NaCl and KCl, with those available in literature [59] (see Supplementary Material, Fig. S2).
The information regarding the calibration, testing and error analysis for density measurements
are also reported in earlier work [56].
3
.
Result and discussion
3
.1. Apparent and Partial Molar Volumes
The experimental density ( ) data for peptides in the aqueous PIL solutions
Peptides + PIL + water ternary systems) at 298.15 K are reported in Tables 2 to 5 which
were used to calculate apparent molar volume (V ), using the following standard Eq. (1)
60].

(
[


  

  M 
0
2
V  

  
 


(1)
2
m 0
 



.
−1
where M2 is the molar mass of a peptide in kg mol , m2 is the molality of peptide in aqueous
.
−1
.
−3
PIL expressed in mol kg and, 0 and

represents the density in kg m of solvent (~0.2
−
1
mol·kg PIL + water) and of solution respectively. The data of apparent molar volume (V
)
of peptides thus obtained are collected in Tables 2 to 5 along with estimated uncertainties at
9

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each concentration. Fig. 1 represents comparison of concentration dependent V data for
−
1
studied glycine-based peptides in aqueous ~0.2 mol∙kg PIL solutions at 298.15 K. It has
been observed that the apparent molar volume for different peptides in aqueous PIL solutions
increases initially with increase in concentration of peptide and remains more or less constant
at studied higher concentrations. Increase in V at low concentration is due to peptide
solvation through hydrogen bonding between PIL and peptide which gets dominated by
peptide association at higher peptide concentration in aqueous PIL solutions. The apparent
0
molar volume at infinite dilution (V ) was obtained from the extrapolation of the plot of
apparent molar volume (V ) versus the molal concentration of the peptide (m2 ) (see Fig. 1)
to infinite dilution using the Eq. (2).
0
V    B m
(2)
V
V
2
1
0

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Table 2 Experimental densities (

), apparent molar volume (V ), partial molal volume of solute (
V
2
) and solvent (
V
1
) at different molality (
.
−1
a
m2 ) of peptides in aqueous solution containing 0.2000 mol kg of TEAA (m1 ) at 298.15 K and 0.1 MPa.
6
6
6
6
6
6
m2

10 ·V
m2

10 ·V
10 ·
V
2
10 ·
V
1
10 ·
V
2
10 ·
V
1
.
−1
.
−3
3.
−1
3.
−1
3.
−1
.
−1
.
−3
3.
−1
3.
−1
3.
−1
/
mol kg
/kg m
/m mol
/m mol
/m mol
/mol kg
/kg m
/m mol
/m mol
/m mol
.
−1
.
−1
Glycyl-glycine + 0.2000 mol kg of TEAA + Water
Glycyl-ւ-valine + 0.2000 mol kg of TEAA + Water
0
0
0
0
0
0
0
0
0
0
0
.0000
.0214
.0415
.0603
.0798
.0972
.1468
.1937
.2447
.2962
.3438
999.02
18.547
0.0000
0.0203
0.0399
0.0605
0.0794
0.0987
0.1449
0.1689
0.2006
999.00
18.547
1000.22
1001.33
1002.36
1003.43
1004.38
1007.05
1009.57
1012.27
1014.98
1017.46
76.01 ± 0.47
76.31 ± 0.24
76.49 ± 0.17
76.56 ± 0.13
76.61 ± 0.10
76.79 ± 0.07
76.85 ± 0.05
76.97 ± 0.04
77.05 ± 0.03
77.08 ± 0.03
76.07 ± 0.47
76.41 ± 0.24
76.64 ± 0.17
76.76 ± 0.13
76.85 ± 0.10
77.14 ±0.07
77.33 ± 0.05
77.57 ± 0.04
77.77 ± 0.03
77.92 ± 0.03
18.547
18.547
18.547
18.547
18.547
18.546
18.545
18.544
18.543
18.542
1000.06
1001.07
1002.12
1003.08
1004.04
1006.33
1007.50
1009.03
121.86 ± 0.49 121.98 ± 0.49
122.10 ± 0.25 122.34 ± 0.25
122.36 ± 0.17 122.72 ± 0.17
122.47 ± 0.13 122.94 ± 0.13
122.56 ± 0.10 123.15 ± 0.10
122.73 ± 0.07 123.60 ± 0.07
122.89 ± 0.06 123.89 ± 0.06
123.03 ± 0.05 124.22 ± 0.05
18.547
18.547
18.547
18.547
18.546
18.545
18.544
18.543
1
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0
.3974
1020.25
77.08 ± 0.03
78.05 ± 0.03
18.540
.
−1
.
−1
Glycyl-ւ-leucine + 0.2000 mol kg of TEAA + Water
Triglycine + 0.2000 mol kg of TEAA + Water
0
0
0
0
0
0
0
0
0
.0000
999.02
1000.01
1000.47
1000.92
1001.40
1001.89
1002.38
1003.09
1003.82
18.547
0.0000
0.0199
0.0385
0.0584
0.0789
0.1178
0.1607
0.2009
999.02
18.547
18.547
18.547
18.547
18.547
18.546
18.545
18.544
.0203
.0298
.0391
.0490
.0592
.0695
.0844
.1000
139.31 ± 0.49 139.41 ± 0.49
139.32 ± 0.34 139.46 ± 0.34
139.34 ± 0.26 139.52 ± 0.26
139.39 ± 0.20 139.62 ± 0.20
139.41 ± 0.17 139.69 ± 0.17
139.46 ± 0.14 139.79 ± 0.14
139.52 ± 0.12 139.92 ± 0.12
139.69 ± 0.10 140.15 ± 0.10
−4
18.547
18.547
18.547
18.547
18.547
18.547
18.546
18.546
−1
1000.55
1001.98
1003.48
1005.03
1007.94
1011.11
1014.05
111.93 ± 0.50 112.01 ± 0.50
112.12 ± 0.26 112.28 ± 0.26
112.23 ± 0.17 112.46 ± 0.17
112.34 ± 0.13 112.66 ± 0.13
112.51 ± 0.08 112.98 ± 0.08
112.58 ± 0.06 113.22 ± 0.06
112.70 ± 0.05 113.50 ± 0.05
a
−2
−3
−9
Standard uncertainties u are u(T) = 0.02 K, u (m2 ) = 1·10 mol·kg . The combined standard uncertainties in u (

) = 1·10 kg·m and u (
V
1
) = 2·10
c
c
3
.
−1
m mol .
1
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Table 3 Experimental densities (

), apparent molar volume (V ), partial molal volume of solute (V2 ) and solvent (V1 ) at different molality ( m2
.
−1
a
)
of peptides in aqueous solution containing 0.2009 mol kg of TEAG (m1 ) at 298.15 K and 0.1 MPa.
6
6
6
6
6
6
m2

10 ·

10 ·V2
10 ·V1
m2

10 ·V
10 ·V2
10 ·V1
V
.
−1
.
−3
3.
−1
3.
−1
3.
−1
.
−1
.
−3
3.
−1
3.
−1
3.
−1
/
mol kg
kg m
/m mol
/m mol
/m mol
/mol kg
/kg m
/m mol
/m mol
/m mol
.
−1
.
−1
Glycyl-glycine + 0.2009 mol kg of TEAG + Water
Glycyl-ւ-valine + 0.2009 mol kg of TEAG + Water
1
003.00
1003.00
b
b
0
0
0
0
0
0
0
0
0
0
.0000
.0400
.0600
.0811
.0976
.1449
.1953
.2443
.2950
.3442
1002.75
18.533
0.0000
0.0206
0.0383
0.0595
0.0783
0.0984
0.1285
0.1683
0.2001
1002.75
1004.07
1004.98
1006.05
1006.96
1007.98
1009.44
1011.35
1012.87
18.533
1005.23
1006.34
1007.48
1008.39
1010.94
1013.65
1016.24
1018.90
1021.45
76.06 ± 0.25
76.13 ± 0.17
76.34 ± 0.12
76.40 ± 0.10
76.60 ± 0.07
76.71 ± 0.05
76.84 ± 0.04
76.91 ± 0.03
77.01 ± 0.03
76.17 ± 0.25
76.31 ± 0.17
76.57 ± 0.12
76.68 ± 0.10
77.02 ± 0.07
77.27 ± 0.05
77.54 ± 0.04
77.75 ± 0.03
78.00 ± 0.03
18.533
18.533
18.533
18.533
18.532
18.531
18.530
18.528
18.527
121.75 ± 0.48 121.94 ± 0.48
122.03 ± 0.26 122.38 ± 0.26
122.31 ± 0.17 122.85 ± 0.17
122.83 ± 0.13 123.54 ± 0.13
122.78 ± 0.10 123.68 ± 0.10
123.07 ± 0.08 124.23 ± 0.08
123.31 ± 0.06 124.83 ± 0.06
123.40 ± 0.05 125.21 ± 0.05
18.533
18.533
18.533
18.532
18.531
18.530
18.528
18.526
1
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0
.3995
1024.27
77.14 ± 0.03
78.29 ± 0.03
18.525
.
−1
.
−1
Glycyl-ւ-leucine + 0.2009 mol kg of TEAG + Water
Triglycine + 0.2009 mol kg of TEAG + Water
1
003.00
1003.00
b
b
0
0
0
0
0
0
0
0
0
.0000
.0195
.0298
.0398
.0497
.0591
.0689
.0844
.0998
1002.75
18.533
18.533
18.533
18.533
18.533
18.532
18.532
18.532
18.531
−1
0.0000
0.0206
0.0390
0.0589
0.0798
0.1174
0.1600
0.1999
1002.75
18.533
18.533
18.533
18.533
18.533
18.533
18.532
18.532
1003.95
1004.45
1004.92
1005.39
1005.84
1006.30
1007.02
1007.72
138.98 ± 0.51 139.20 ± 0.51
139.11 ± 0.33 139.45 ± 0.33
139.25 ± 0.25 139.71 ± 0.25
139.41 ± 0.20 139.99 ± 0.20
139.42 ± 0.17 140.11 ± 0.17
139.52 ± 0.14 140.31 ± 0.14
139.69 ± 0.12 140.66 ± 0.12
139.98 ± 0.10 141.13 ± 0.10
−4
1004.58
1005.99
1007.51
1009.09
1011.92
1015.08
1018.02
111.82 ± 0.48 111.86 ± 0.48
111.93 ± 0.25 112.01 ± 0.25
111.99 ± 0.17 112.11 ± 0.17
112.03 ± 0.12 112.19 ± 0.12
112.06 ± 0.08 112.30 ± 0.08
112.19 ± 0.06 112.52 ± 0.06
112.21 ± 0.05 112.61 ± 0.05
a
−2
−3
−9
Standard uncertainties u are u(T) = 0.02 K, u(m ) = 1·10 mol·kg . The combined standard uncertainties in u (ρ) = 1·10 kg·m and u (V1 ) = 2·10
2
c
c
3
.
−1
m mol .
b
Density data for TEAG + water solutions taken from Reference [51].
1
4

Journal Pre-proof
Table 4 Experimental densities (

), apparent molar volume (V ), partial molal volume of solute (V2 ) and solvent (V1 ) at different molality ( m2
.
−1
a
)
of peptides in aqueous solution containing 0.1994 mol kg of TEAPy ( m1 ) at 298.15 K and 0.1 MPa.
6
6
6
6
6
6
m2

10 ·

10 ·V2
10 ·V1
m2

10 ·V
10 ·V2
10 ·V1
V
.
−1
.
−3
3.
−1
3.
−1
3.
−1
.
−1
.
−3
3.
−1
3.
−1
3.
−1
/
mol kg
/kg m
/m mol
/m mol
/m mol
/mol kg
/kg m
/m mol
/m mol
/m mol
.
−1
.
−1
Glycyl-glycine + 0.1994 mol kg of TEAPy + Water
Glycyl-ւ-valine + 0.1994 mol kg of TEAPy + Water
1
000.91
1000.91
b
b
0
0
0
0
0
0
0
0
0
0
.0000
.0212
.0412
.0598
.0793
.0983
.1478
.1954
.2432
.2979
1001.13
18.610
0.0000
1001.13
1002.12
1003.05
1004.00
1004.94
1006.00
1007.45
1009.40
1010.98
18.610
18.610
1002.10
1003.22
1004.26
1005.33
1006.37
1009.07
1011.64
1014.17
1017.01
75.86 ± 0.47
75.95 ± 0.24
75.96 ± 0.17
76.06 ± 0.13
76.14 ± 0.10
76.31 ± 0.07
76.41 ± 0.05
76.58 ± 0.04
76.82 ± 0.03
75.94 ± 0.47
76.10 ± 0.24
76.18 ± 0.17
76.35 ± 0.13
76.50 ± 0.10
76.84 ± 0.07
77.12 ± 0.05
77.46 ± 0.04
77.90 ± 0.03
18.610
18.610
18.610
18.610
18.610
18.609
18.608
18.606
18.604
0.0229
0.0409
0.0597
0.0784
0.0995
0.1283
0.1681
0.2006
121.10 ± 0.44 121.27 ± 0.44
121.66 ± 0.24 121.96 ± 0.24
122.01 ± 0.17 122.46 ± 0.17
122.28 ± 0.13 122.87 ± 0.13
122.41 ± 0.10 123.16 ± 0.10
122.38 ± 0.08 123.34 ± 0.08
122.61 ± 0.06 123.88 ± 0.06
122.69 ± 0.05 124.20 ± 0.05
18.610
18.610
18.610
18.609
18.608
18.606
18.605
1
5

Journal Pre-proof
0
0
.3469
.3959
1019.51
1021.99
77.07 ± 0.03
78.33 ± 0.03
78.67 ± 0.03
18.602
18.600
77.24 ± 0.03
.
−1
.
−1
Glycyl-ւ-leucine + 0.1994 mol kg of TEAPy + Water
Triglycine + 0.1994 mol kg of TEAPy + Water
1
000.91
1000.91
b
b
0
0
0
0
0
0
0
0
0
.0000
.0198
.0298
.0399
.0492
.0594
.0695
.0846
.1001
1001.13
18.610
18.610
18.610
18.610
18.610
18.610
18.610
18.610
18.609
−1
0.0000
0.0204
0.0393
0.0591
0.0787
0.1181
0.1581
0.2029
1001.13
18.610
18.610
18.610
18.610
18.610
18.609
18.608
18.606
1001.89
1002.37
1002.86
1003.31
1003.80
1004.28
1004.99
1005.72
138.91 ± 0.50 139.04 ± 0.50
138.98 ± 0.33 139.18 ± 0.33
139.08 ± 0.25 139.35 ± 0.25
139.17 ± 0.20 139.50 ± 0.20
139.22 ± 0.17 139.62 ± 0.17
139.26 ± 0.14 139.72 ± 0.14
139.35 ± 0.12 139.91 ± 0.12
139.46 ± 0.10 140.13 ± 0.10
−4
1002.49
1003.93
1005.43
1006.90
1009.84
1012.79
1016.07
111.63 ± 0.49 111.74 ± 0.49
111.86 ± 0.25 112.08 ± 0.25
112.19 ± 0.17 112.52 ± 0.17
112.39 ± 0.13 112.84 ± 0.13
112.55 ± 0.08 113.22 ± 0.08
112.69 ± 0.06 113.59 ± 0.06
112.70 ± 0.05 113.85 ± 0.05
a
−2
−3
−9
Standard uncertainties u are u(T) = 0.02 K, u(m ) = 1·10 mol·kg . The combined standard uncertainties in u (ρ) = 1·10 kg·m and u (V1 ) = 2·10
2
c
c
3
.
−1
m mol .
b
Density data for TEAPy + water solutions taken from Reference [51].
1
6

Journal Pre-proof
Table 5 Experimental densities (

), apparent molar volume (V ), partial molal volume of solute (V2 ) and solvent (V1 ) at different molality ( m2
.
−1
a
)
of peptides in aqueous solution containing ~0.2116 mol kg of TEAP (m1 ) at 298.15 K and 0.1 MPa.
6
6
6
6
6
6
m2

10 ·

10 ·V2
10 ·V1
m2

10 ·V
10 ·V2
10 ·V1
V
.
−1
.
−3
3.
−1
3.
−1
3.
−1
.
−1
.
−3
3.
−1
3.
−1
3.
−1
/
mol kg
/kg m
/m mol
/m mol
/m mol
/mol kg
/kg m
/m mol
/m mol
/m mol
.
−1
.
−1
Glycyl-glycine + 0.2116 mol kg of TEAP + Water
Glycyl-ւ-valine + 0.2116 mol kg of TEAP + Water
9
98.57
998.57
b
b
0
0
0
0
0
0
0
0
0
.0000
.0398
.0590
.0801
.0981
.1453
.1973
.2467
.3481
998.43
18.638
0.0000
0.0191
0.0402
0.0588
0.0797
0.0990
0.1291
0.1684
0.1994
998.43
18.638
1000.80
1001.87
1003.04
1004.04
1006.57
1009.32
1011.95
1017.26
75.83 ± 0.25
75.96 ± 0.17
76.03 ± 0.12
75.99 ± 0.10
76.45 ± 0.07
76.82 ± 0.05
76.91 ± 0.04
77.02 ± 0.03
75.99 ± 0.25
76.19 ± 0.17
76.34 ± 0.12
76.38 ± 0.10
77.02 ± 0.07
77.60 ± 0.05
77.89 ± 0.04
78.40 ± 0.03
18.638
18.638
18.638
18.638
18.637
18.636
18.634
18.629
999.59
1000.67
1001.62
1002.67
1003.63
1005.15
1007.09
1008.59
120.96 ± 0.53 121.12 ± 0.53
121.64 ± 0.25 121.99 ± 0.25
122.15 ± 0.17 122.65 ± 0.17
122.40 ± 0.13 123.08 ± 0.13
122.54 ± 0.10 123.39 ± 0.10
122.56 ± 0.08 123.66 ± 0.08
122.70 ± 0.06 124.14 ± 0.06
122.80 ± 0.05 124.51 ± 0.05
18.638
18.638
18.638
18.637
18.637
18.636
18.634
18.632
1
7

Journal Pre-proof
0
.4166
1020.71
77.30 ± 0.03
78.96 ± 0.03
18.626
.
−1
.
−1
Glycyl-ւ-leucine + 0.2115 mol kg of TEAP + Water
Triglycine + 0.2115 mol kg of TEAP + Water
9
98.55
998.55
b
b
0
0
0
0
0
0
0
0
0
.0000
.0198
.0349
.0392
.0494
.0595
.0695
.0894
.1003
998.43
18.639
18.639
18.638
18.638
18.638
18.638
18.638
18.637
18.637
−1
0.0000
0.0211
0.0394
0.0593
0.0783
0.1184
0.1588
0.2003
998.43
18.639
18.639
18.638
18.638
18.638
18.637
18.635
18.633
999.53
1000.27
1000.48
1000.96
1001.45
1001.93
1002.87
1003.39
138.74 ± 0.51 138.90 ± 0.51
139.01 ± 0.29 139.30 ± 0.29
139.10 ± 0.26 139.43 ± 0.26
139.20 ± 0.20 139.62 ± 0.20
139.29 ± 0.17 139.80 ± 0.17
139.34 ± 0.14 139.93 ± 0.14
139.43 ± 0.11 140.19 ± 0.11
139.49 ± 0.10 140.34 ± 0.10
−4
1000.20
1001.59
1003.09
1004.53
1007.53
1010.52
1013.54
111.16 ± 0.47 111.31 ± 0.47
111.72 ± 0.25 112.00 ± 0.25
112.16 ± 0.17 112.57 ± 0.17
112.27 ± 0.13 112.82 ± 0.13
112.37 ± 0.08 113.21 ± 0.08
112.51 ± 0.06 113.63 ± 0.06
112.71 ± 0.05 114.12 ± 0.05
a
−2
−3
−9
Standard uncertainties u are u(T) = 0.02 K, u(m ) = 1·10 mol·kg . The combined standard uncertainties in u (ρ) = 1·10 kg·m and u (V1 ) = 2·10
2
c
c
3
.
−1
m mol .
b
Density data for TEAP + water solutions taken from Reference [51].
1
8

Journal Pre-proof
1
1
40.0
39.5
140.0
(A)
(B)
1
1
39.3
38.6
1
1
1
1
39.0
23.5
22.2
20.9
1
1
1
23.2
22.1
21.0
1
1
1
13.5
12.6
11.8
1
1
1
12.8
11.6
10.4
7
7
7
7.6
6.8
6.0
7
7
7
8.0
6.5
5.0
0
.0
0.1
0.2
0.3
0.4
0.0
0.1
0.2
0.3
0.4
−
1
−1
m /(mol·kg )
m /(mol·kg )
2
2
1
1
1
40.0
39.2
38.4
1
1
1
40.0
(D)
(C)
39.2
38.4
1
1
1
23.6
22.4
21.2
1
1
22.4
21.5
120.6
113.0
1
1
1
12.7
12.0
11.3
1
1
12.2
11.3
78.0
7
7
7
8.0
6.7
5.4
7
7
6.5
5.0
0
.0
0.1
0.2
0.3
0.4
0.0
0.1
0.2
0.3
−1
0.4
−
1
m /(mol·kg )
m /(mol·kg )
2
2
Fig. 1. Variation of apparent molar volume (V ) for different peptide: digly, ♦; trigly, ●; gly-
−
1
val, ■; gly-leu, ▲ in ~0.2 mol·kg aqueous solutions of different PILs: A) TEAA, B) TEAP,
C) TEAG and D) TEAPy at 298.15 K.
1
9

Journal Pre-proof
0
wherein

is the limiting apparent molar volume of a peptide in aqueous PIL solution and
V
0
BV is the experimental slope. The data of V and BV thus obtained for studied glycine based
0
peptides in aqueous PIL solutions at 298.15 K are compiled in Table 6. The parameter V
excludes ion–ion interactions and hence is helpful to understand precisely the ion–solvent
interaction. It is noted that the limiting apparent molar volume of peptides in aqueous PIL
solutions is lower than that in water which indicates the hydrophobic solvation of peptides in
aqueous PIL solutions more important in presence PIL in water that its absence. Furthermore,
the effect of hydrophobic solvation of peptides in aqueous PIL solutions becomes more
0
apparent with increase in hydrophobicity of PILs as revealed from more decrease in V
values with PIL hydrophobicity (see the Table 6). A similar trends were also observed for
amino acids and peptides in the aqueous of tetra-n-alkylammonium bromide and 1-n-pentyl-3-
methylimidazolium chloride ([C mim] Cl) solutions [18−20,29] resulting an increased
5
electrostriction in the vicinity of charged centers of the peptide. The reorganization and
strengthening of hydrogen bonded network of water molecules in the vicinity of peptide
functionality arises due to electric field exerted by charged centers in solution and hence
0
diminution of the void spaces leads to decrease in V values for peptides implying
hydrophobic solvation. Since, the effect is more prominent if ternary mixture contains
relatively more hydrophobic species i.e. either gly-leu or TEAP.
2
0

Journal Pre-proof
0
−1
Table 6 The limiting apparent molar volume (

) and experimental slope ( BV ) for the different peptides in the ~0.20 mol·kg of different PILs
V
a
aqueous solutions at 298.15 K and 0.1MPa.
Glycyl-glycine
Triglycine
Glycine-ւ-valine
Glycyl-ւ-leucine
Solvents
0
0
0
0

BV

BV

BV

BV
0.9 ± 1
V
V
V
V
b
b
c
c
c
c
c
c
Water
76.30 ± 0.07 2.55 ± 0.35 112.11 ± 0.03 5.4 ± 0.4 121.99 ± 0.02 1.7 ± 0.4 139.70 ± 0.07
TEAA
TEAG
TEAPy
76.58 ± 0.07 1.41 ± 0.22 112.10 ± 0.05 3.01 ± 0.30 122.04 ± 0.05 5.00 ± 0.28 139.10 ± 0.05 5.60 ± 0.69
76.23 ± 0.04 2.30 ± 0.14 111.89 ± 0.04 1.64 ± 0.24 122.16 ± 0.14 6.46 ± 0.87 138.72 ± 0.08 12.14 ± 0.97
75.67 ± 0.04 3.93 ± 0.13 112.11 ± 0.13 3.11 ± 0.82 121.83 ± 0.13 4.40 ± 0.83 138.83 ± 0.03 6.24 ± 0.31
76.04 ± 0.13 3.03 ± 0.40 111.82 ± 0.11 4.44 ± 0.70 121.97 ± 0.15 4.27 ± 0.94 138.85 ± 0.06 6.51 ± 0.72
TEAP
a
6
0
3
−1
6
3
−2
Units: 10 ·

/(m .mol ); 10 · BV /(m ·kg·mol ).
V
b,c
0
V values for peptides + water solutions taken from Reference [10,12].
2
1

Journal Pre-proof
The partial molar volumes of non-electrolytes (peptide) (
V
) and partial molar
2
−
1
volume of solvent (
V
) (i.e. water + ~0.2 mol·kg of PIL) were obtained from the Eqs. (3) and
1
(4), respectively [60]

 
V
V    m 

(3)
(4)
2
V
2



m2



M    
0
S
2
V
V  V  
m 


1
1
2


m2




0
where, V1 is the molar volume of solvent obtained from density of mixed solvent system, ʋ is
the number of ions produced by peptides (ʋ =1). M S is the molecular weight of mixed solvent
.
−1
(
water + ~0.2 mol kg of PIL) which was estimated using Eq. (5) [60].
1
M 
(5)
S

wwater
wPIL






Mwater
MPIL


where, wwater and wPIL are mass fraction of water and PIL respectively for the binary mixtures
.
−1
used here as a mixed solvent, whereas Mwater and MPILs are molar masses in kg mol of water
and PILs respectively. The data estimated for partial molar volume of solute (peptides) and
−
1
solvent (~0.2 mol·kg of aqueous solutions of PILs) are given in Tables 2 to 5 and graphically
shown in Figs. 2 to 5. From Figs. 2 to 5, it is observed that, the partial molar volumes of
peptides (
V
2
) increases with increasing concentration. Further in case of increasing
hydrophobicity of PIL in aqueous solutions, the decrease in partial molar volume of peptides
is observed. The partial molar volume of solute increases at lower concentration however at
higher concentration the values becomes more or less constant for all studied peptides and the
trends observed are in line with the V data so Supplementary the above interpretation offered
2
2

Journal Pre-proof
to concentration dependent behavior of V . The partial molar volumes of solvent (water +
−
1
~
0.2 mol.kg of PIL) (V1 ) decreases with increasing the concentration of peptides and
increases slightly with increasing the hydrophobicity of PILs indicating hydrophobic
solvation of peptides in aqueous PIL solutions with PIL mediated water structure making
effect [58]. This kosmotropic effect in peptide solutions in presence of PILs is further
supported by sign and magnitude of limiting transfer molar volumes obtained for transfer of
peptides from water to aqueous PIL solutions which accounts the interactions between solute
peptide and cosolute PIL, the details of which are given below.
140.1
139.8
139.5
18.547
(D)
18.546
8.546
1
1
1
1
24.0
23.0
22.0
18.548
18.545
18.543
(
C)
B)
113.5
113.0
112.5
112.0
18.547
18.546
18.545
(
18.544
78.0
77.0
76.0
18.546
(A) 18.543
8.540
1
0.0
0.1
0.2
0.3
0.4
−
1
/
(mol·kg )
Fig. 2. The variation of the partial molar volume of peptide (
V
2
) (rigid triangle ▲) and
)) (open square □) as function of the molality of
peptides (m ) at 298.15 K: A) digly, B) trigly, C) gly-val and D) gly-leu.
−
1
solvent (water + ~0.2 mol·kg of TEAA (
V
1
2
2
3

Journal Pre-proof
141.0
140.0
139.0
18.533
(D)
18.532
8.531
1
124.8
123.6
122.4
18.532
C)
(
18.529
18.526
112.8
112.4
112.0
(
B) 18.533
18.532
8.532
1
1
8.535
78.4
77.7
77.0
76.3
(A)
18.530
18.526
0.0
0.1
0.2
0.3
0.4
−
1
/
(mol·kg )
Fig. 3. The variation of the partial molar volume of peptide (
V
2
) (rigid triangle ▲) and
)) (open square □) as function of the molality of
peptides (m ) at 298.15 K: A) digly, B) trigly, C) gly-val and D) gly-leu.
−
1
solvent (water + ~0.2 mol·kg of TEAG (
V
1
2
1
8.610
8.610
140.0
139.5
139.0
(D)
1
18.610
18.609
124.0
123.0
122.0
121.0
18.610
(C)
18.608
8.605
1
1
8.612
114.0
113.0
112.0
(B)
18.608
18.605
18.610
18.605
18.600
78.4
77.0
75.6
(
A)
0.0
0.1
0.2
0.3
0.4
−
1
/
(mol·kg )
Fig. 4. The variation of the partial molar volume of peptide (
V
2
) (rigid triangle ▲) and
)) (open square □) as function of the molality of
peptides (m ) at 298.15 K: A) digly, B) trigly, C) gly-val and D) gly-leu.
−
1
solvent (water + ~0.2 mol·kg of TEAPy (
V
1
2
2
4

Journal Pre-proof
140.4
139.8
139.2
18.638
(D)
18.638
18.637
18.637
1
1
1
8.638
8.635
8.633
1
1
1
24.0
22.5
20.9
(C)
(B)
18.639
114.0
112.8
111.6
18.636
18.633
7
7
7
9.3
8.0
6.7
18.636
18.630
18.624
(A)
0
.0
0.1
0.2
0.3
0.4
−
1
/
(mol·kg )
Fig. 5. The variation of the partial molar volume of peptide (
V
2
) (rigid triangle ▲) and
)) (open square □) as function of the molality of
peptides (m ) at 298.15 K: A) digly, B) trigly, C) gly-val and D) gly-leu.
−
1
solvent (water + ~0.2 mol·kg of TEAP (
V
1
2
3
.2. Apparent Transfer Molar Volume
0
Limiting apparent transfer molar volumes ( trV ) for the peptides due to its transfer
from water to aqueous PILs solutions were calculated using Eq. (6) and the data obtained for
0
trV of studied peptides are collected in Table 7.
0
0
V
1
0
V
trV  (~ 0.2mol kg aqueousPILSolution)  (water)
(6)
0
where, V (water) is the limiting apparent molar volume of peptide in pure water at 298.15 K
0
−1
data of which were taken from the literature [10,12] and V (~0.2 mol·kg aqueous PIL
−
1
solution) is the limiting apparent molar volume of peptides in ~0.2 mol·kg aqueous PIL
0
solution. The obtained   values are negative (Table 7 and Fig. 6) for all studied peptides
tr
V
in aqueous PIL solutions which means that the limiting apparent molar volumes for studied
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peptides in aqueous PILs solutions are smaller than those in pure water. From Table 7, it is
0
−1
observed that the trV for digly in ~0.2 mol·kg of aqueous TEAA solutions at 298.15 K


are close to zero indicating that the zwitterionic  NH3 and  COO groups interacts water
0
molecules with same strength as the ions of TEAA with water. However, trV values
become more negative by increasing the hydrophobicity of PILs (see Fig. 6) due to
hydrophobic–hydrophobic interaction between non–polar group of PILs and non−polar group
0
of peptides dominating over ion−ion interactions. Furthermore, the trV value shift towards
more negative with increasing the hydrophobicity of peptides and the effect is more
pronounced for peptides in aqueous solutions of TEAPy and TEAP (shown in Scheme 2). A
0
comparison in trV values for studied peptides in different APILs, traditional salts, tetra-n-
alkyl ammonium halide salt, carbohydrates and surfactants within studied concentration range
of PILs are given in the Table 7. Banipal [15], Ali [20] and Yan [21] have also observed
similar trends for amino acids and peptides in sodium salts of carboxylic acids, tetra-n-
alkylammonium bromide and sodium butyrate in aqueous solutions, respectively. Fang et al.
0
V
[
27] reported the negative magnitudes of   for amino acids such as glycine, ւ-
tr
phenylalanine and ւ-alanine for their transfer from water to aqueous solution of APIL namely
1
‑ethyl-3-methylimidazolium bromide ([Emim][Br]). Similarly, few other researchers
0
V
[
37,38,49] also reported negative magnitude of   for amino acids on transfer them from
tr
water to aqueous 1-n-alkyl-3-methylimidazolium based ionic liquids. Furthermore, Singh et
0
al. [49] demonstrated that that DL-α-alanine and glycine have negative   for their
tr
V
transfer from water to aqueous PIL namely 3-hydroxypropylammonium formate (3-HPAF).
Recently, we observed that ion−hydrophobic interaction between the for amino acids ionic
liquids (AAILs) and polyethylene glycols (PEGs) assisted with solute–cosolute H−bonding
along with electrostriction effect exists in ternary aqueous solutions containing AAILs and
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0
PEGs as revealed from observed negative magnitude of trV [58]. A similar behavior has
also been seen for the amino acids/peptides with aqueous tetraalkylammonium salts solutions
0
[
[
18−20]. The positive trV are reported for peptides in aqueous NaCl [7], sodium butyrate
21], K SO [23], KNO [23], imidazolium based ionic liquids [29−46], glucose [8,61],
2
4
3
sucrose [8], D-galactose [62], xylitol [63], CPC and CPB [64] and tartrazine [65] solution at
0
2
98.15 K (See the Table 7). The positive values of trV represent stronger hydrophilic
interactions between solute and co-solute in aqueous medium causing expansion of volume
with release of some hydrated water molecules on transfer of solute from to aqueous co-solute
solutions. Seen in this light, H−bond assisted hydrophobic solvation of peptides occurs in
aqueous protic ionic liquid solutions. These results can be justified based on co-sphere
overlap model [12]. Accordingly, when interactions between ionic (or dipolar) groups
between two different molecules occurs through electrostatic forces then they are called as
ion-ion interactions or dipolar-dipolar interactions or ion-dipolar interaction which results in
increase in volume on transferring solute from binary to ternary aqueous solutions. On
contrary if interactions between non-polar moieties of two molecules occurs in aqueous
medium then they are called as hydrophobic-hydrophobic interactions which causes large
decrease in volumes on transferring solute from aqueous binary to aqueous ternary solutions.
Similar decrease is in volume of transfer is observed but with less negative magnitudes when
the ionic (or dipolar) groups of one molecule interact with non-polar moieties of other
molecules in aqueous solutions. Thus, according to co-sphere overlap model, the positive
0
0
trV values signify ion (or dipolar)−ion (or dipolar) interactions whereas negative trV
denotes ion (or dipolar)−hydrophobic and/or hydrophobic−hydrophobic interactions. The
nature of solute species affects the properties of water molecules in the cosphere hydration
shell. In the perspective of co-sphere overlap model, for ternary systems peptides + PILs +
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0
water, three different types of interactions exist based on sign and magnitude of trV values.
These are: (i) ion−ion interactions, (ii) ion−hydrophobic interactions and (iii)
hydrophobic−hydrophobic interactions. Accordingly, ionic–hydrophobic interaction between
ionic group of PILs/peptides and non−polar group of peptides/PILs along with H-bond
assisted hydrophobic association of peptides dominates over ion–ion interactions causing
more electrostriction alongwith water structure making effect (Kosmotropic effect) (shown in
Scheme 2). Thus, aqueous peptide solutions containing PIL TEAA favors ion-ion or ion-
diploar interactions over hydrophobic-hydrophobic interactions when the peptides less
hydrophobic e.g. gly-gly, trigly and gly- -val. However, as the hydrophobicity increases
L
either for the PIL or for the peptide, the hydrophobic-hydrophobic interactions amongst the
PIL and peptide molecules dominates over ion-ion or ion-diploar interactions as noted from
0
the sign and magnitude of trV reported in Table 7. This observation indicates that fine
structural changes in PILs can help to tune the physicochemical behavior of aqueous PIL
solutions for their appropriation use in different fields such as protein purification,
stabilization, drug formulations etc.
TEAA
TEAG TEAPy TEAP
0
.4
.0
0
-
-
-
0.4
0.8
1.2
0
Fig. 6. Apparent transfer molar volume ( trV ) of peptides: digly,■; trigly,■; gly-val, ■; gly-
−
1
leu, ■ in ~0.2 mol·kg PILs aqueous solution at 298.15 K.
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TEAR
Digly
Trigly
Gly-Val
Gly-Leu
Glycine Based Peptides + Triethylammonium Carboxylate PILs Hydrophobic Interaction Increases
R= −CH , −CH OH, −COCH ,−CH CH
3
3
2
3
2
Scheme 2. Glycine based peptides and triethylammonium based carboxylate PILs
interactions.
2
9