Hydration of amino acids from ultrasonic measurements

Article abstract of DOI:10.1021/jp105255b

In this paper the results of compressibility of aqueous solutions of amino acids in water and in aqueous HCl and NaOH solutions at 25 °C are presented. The effect of the charged protonated amino groups and deprotonated carboxylic groups on the hydration number was tested. The idea of additivity of the hydration number with the constituents of the solute molecule was successfully applied and discussed.

Full text of DOI:10.1021/jp105255b

J. Phys. Chem. B 2010, 114, 12157–12161
12157
Hydration of Amino Acids from Ultrasonic Measurements
Andrzej Burakowski* and Jacek Glin´ski
Faculty of Chemistry, UniVersity of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland
ReceiVed: June 8, 2010; ReVised Manuscript ReceiVed: August 18, 2010
In this paper the results of compressibility of aqueous solutions of amino acids in water and in aqueous HCl
and NaOH solutions at 25 °C are presented. The effect of the charged protonated amino groups and deprotonated
carboxylic groups on the hydration number was tested. The idea of additivity of the hydration number with
the constituents of the solute molecule was successfully applied and discussed.
1. Introduction
the total hydration number observed. Thus, the idea of determin-
ing such contributions for amino acids seems possible and
Compressibility is known as an independent and effective
tool for investigating biologically active compounds and finds
increasing use as a means for characterizing protein systems.1,2
attractive, because of the possibility of estimation of the impact
of water solvent on the geometrical aspects of the amino acids’
interactions and reactivity, as was shown by Pratt and Pohorille
in their review devoted to the models and structure simulations
of organic molecules, in particular amphiphilic, at an interface.3
We also applied two different solventss0.2 M HCl and 0.2
M NaOHsto check how the pH of the system affects the
calculated hydration numbers. It should be stressed that for
reacting systems, i.e., amino acid + HCl or NaOH, the Pasynski
method yields values of hydration numbers which result from
compressibility of substances engaged in occurring reactions.
Performing studies in aqueous HCl and NaOH solutions
essentially means that the pH values will change. Thus, the
conditions which may exist in biological systems, including the
human body, can be practically simulated in the laboratory.
Remembering that biological macromolecules are physiologi-
cally active in aqueous solutions, the knowledge of interaction
of proteins and their groups with water is necessary to
understand the role of this solvent when solvated to soluble
organics in living cells. Some difficulty arises from the fact that
amino acids exist mainly in zwitterionic forms in aqueous
solutions. This suggests that their volume and compressibility
properties are intermediate between those of electrolytes and
organic nonelectrolytes. The importance of hydrophobic hydra-
tion of these solutes is another problem, as this mechanism could
be competitive with the electrostatic one.3 Such competition was
also proved by Ide et al., who found that the structure of water
in solutions of amino acids with nonpolar side chains is
enhanced, while around amino acids with polar or charged side
chains it is destroyed.4
2. Experimental Section
Glycine (POCH, Poland, p.a.), L-alanine (ROTH, Germany,
for biochemistry g99%), L-valine (AppliChem, Germany,
>99%), L-leucine (Reanal, Hungary, 98%), L-isoleucine (Reanal,
Hungary, 98%), L-phenylalanine (AppliChem, Germany, pure
Ph. Eur., USP grade, >98.5%), L-serine (MP Biomedicals, LLC,
Germany, USP grade), and L-lysine monohydrate (Loba-
Feinchemie, Austria, pure) were used without additional
purification.
Glycine, L-alanine, L-valine, L-leucine, L-isoleucine, L-phe-
nylalanine and L-serine hydrochlorides, L-lysine dihydrochloride,
and glycine, L-alanine, L-valine, L-leucine, L-isoleucine, L-
phenylalanine, L-serine, and L-lysine sodium salts were prepared
by mixing equimolar amounts of the respective amino acid and
hydrochloric acid or sodium hydroxide. The latter were prepared
from Fixanal chemicals.
The volume properties of proteins and amino acids are
characterized by a decrease in volume when these substances
are dissolved in water, similar to what occurs for electrolytes.5,6
This phenomenon is usually attributed to electrostriction due
to water-solute interactions, suggesting that amino acids or
proteins behave as electrolytes. However, it is also possible that
the so-called “bound water” behaves more like water solvated
to organic solutes than bulk water.7,8 This interpretation is
evidently too simplified if one keeps in mind that amino acids
are traditionally considered as hydrophobic substances and that
their hydrophobicity is well correlated with the length of the
chain. Hecht et al. compared different rankings of the amino
acids’ hydrophobicity and found that the size of the water
clathrate around the hydrophobic side chain increases in the
order Gly < Ala < Val, Ile, Leu.9 Note that some hydrophobic
character can be attributed even to the shortest amino acid,
glycine.
Solutions were prepared before measurements by weighing
using doubly distilled, freshly prepared water.
The speed of sound was determined using a computer-steered
OPKUD 01/100 apparatus (Optel, Wrocław, Poland), with an
absolute accuracy better then (0.2 m·s-1 and a precision of
similar order. Measurements are based on the determination
of the time an acoustic signal needs to pass through a sample
of known length. All samples were degassed before measure-
ments in an ultrasonic cleaner.
In this work we apply a different approach. It was recently
found that the hydration numbers determined ultrasonically, i.e.,
from the density and speed of sound data, in two-component
aqueous solutions of simple organic nonelectrolytes are additive
with both the length and the character of the constituents of the
solute molecule.10–12 It was also proved that for different solute
molecules the functional groups bring the same increment to
The density was measured using a vibrating tube Ecolab
MG-2 (Krako´w, Poland) apparatus with an accuracy of ca. (0.1
* To whom correspondence should be addressed. Phone: +48 71
3757235. Fax: +48 71 3282348. E-mail: andrzej.burakowski@gmail.com.
kg·m-3
.
10.1021/jp105255b  2010 American Chemical Society
Published on Web 08/30/2010

12158 J. Phys. Chem. B, Vol. 114, No. 37, 2010
Burakowski and Glin´ski
The temperature of the measurements was 25 ( 0.01 °C,
stabilized by a precision Julabo F25-ME (Germany) thermostat.
Its stability was controlled by a digital thermometer built into
the density apparatus and the absolute value by a precision
mercury thermometer.
From the speed of sound and density data the adiabatic
compressibility coefficients, κS, were calculated, using the
Laplace equation:
1 ∂V
V ∂P S
1
du2
κS ) -
)
(1)
(
)
where V is the volume, P is the pressure, d is the density, u is
the speed of sound, and the index S denotes the adiabatic
condition.
The relative change of compressibility was assumed to be
caused by some fraction of water molecules being engaged in
hydration spheres, where they become incompressible. This
leads to the well-known formula for the hydration number, nh,
known as the Pasynski equation13,14 (see also a review by Stuehr
and Yeager15):
Figure 1. Example of experimental results: κS vs solute mole fraction
for amino acids in water at 25 °C. Key: b, glycine; O, L-alanine; 2,
L-serine; 4, L-valine; [, L-leucine; ], L-isoleucine; 9, L-lysine; 0,
L-phenylalanine.
al.,37 and others.38,39 For a series of poly(vinyl alcohol)s, Inzelt
and Grof found the hydration number additive with the number
of monomers (i.e., with the chain length) and equal to 3.7-4.0
per -OH group.40 This value is in perfect agreement with the
results of Sasahara and Uedaira, who estimated the contribution
of a monomer to the total hydration number of poly(ethylene
glycol) as 3.9.41
Assuming additivity of hydration numbers with the geometry
of the solute molecules and collecting respective experimental
data for a series of compounds, a number of linear equations
with the number of unknowns equal to the number of constitu-
ents were obtained which were easy to solve. A similar
procedure was applied in the study reported in ref 35 and is
described in detail there.
With this assumption, we determined the hydration numbers
of amino acids and respective amino acid hydrochlorides and
amino acid sodium salts in water at 25 °C. The obtained data
are collected and presented in Table 1 together with the data
available in the literature.
In the fitting procedure for amino acid hydrochlorides and
sodium salts we also used the ionic nh values of the Na+ cation
(4.0) and Cl- anion (3.1) given in ref 42, which were also
determined from the acoustic Pasynski method at 25 °C on the
basis of data collected by Allam and Lee.43,44
When taking the above into consideration, the set of data
obtained in this way allows the hydration numbers of different
groups (fragments) of a solute molecule to be fitted in the
manner presented in ref 35. These contributions are listed in
Table 2, and the nh values calculated from them for all the
solutes applied for the fitting are included in Table 1. The
differences between experimental and calculated hydration
numbers are surprisingly low, and thus, the assumption ex-
pressed in the Introduction seems to be very well confirmed. It
is worth noting that the calculated sum of squares of deviations
(SQ), the parameter minimalized in fitting and being the probe
of the goodness of the fitting, was as low as 2.66.
It could seem strange to some that constituents such as -CH3
or -CH2- also have contributed their values to the overall
hydration number. This was discussed in the Introduction (the
effect of the size of the molecule), but one should also remember
that dielectrically water + amino acid systems rather resemble
emulsions, containing two kinds of spherical solutes.31 There-
fore, this picture suggests the importance of hydrophobic
interactions in the overall hydration.
n1
n2
κS
κS0
nh )
1 -
(2)
(
)
where n1 and n2 are the numbers of moles of water and solute
in solution, respectively, κS is the adiabatic compressibility
coefficient of the solution, and κ0S is that of pure solvent. The
applicability of the Pasynski equation in nonelectrolytic aqueous
solutions was recently examined by us.10
There were three series of experiments performed, differing
by the type of solvent: pure water, 0.2 M HCl, and 0.2 M NaOH.
The two latter were prepared using Fixanal chemicals.
3. Results and Discussion
In this work only very dilute systems were investigated. The
mole fraction of the solutes never exceeded 0.01; i.e., there were
always more than 100 water molecules per solute molecule. For
such dilutions one can neglect interactions between the solute
molecules, as well as assume that individual solvation shells
are effectively separated by bulk water molecules.
It is both surprising and meaningful that, in the investigated
range of concentrations, all the solutes are characterized by a
perfectly linear dependence of the adiabatic compressibility
coefficients κS on the concentration, as can be seen in Figure 1
(the R2 coefficient was always 0.999 or better). This means that
the hydration numbers of nonelectrolytes obtained by the
Pasynski formula are undoubtedly constant for such low
concentrations.
3.1. Calculations of the Contributions of Functional Groups
to the Observed Total Hydration Numbers of Amino Acids.
The values of the hydration numbers are now assumed by us to
be a sum of the contributions from individual constituents of
the solute molecule, as was proposed recently.11 This idea arises
from the well-known fact that in a homological series of simple
organic solutes the changes of many physical parameters are
regular. The linear or almost linear variability with extension
of the solute hydrocarbon chain is very well-known for partial
molar volumes and adiabatic compressibilities.33–35 However,
the linearity of compressibility changes with elongation of the
solute molecule suggests a similar linearity of hydration numbers
also. Such ideas were presented by Hedwig et al.,36 Cheng et

Amino Acid Hydration from Ultrasonic Measurements
J. Phys. Chem. B, Vol. 114, No. 37, 2010 12159
TABLE 1: Hydration Numbers (Experimental and Calculated) of Amino Acids and Their Respective Hydrochlorides and
Sodium Salts in Water at 25 °C Together with Data Available in the Literaturea
hydration number (nh)
this work
solute
nh,exptl
nh,calcd
∆nh
from the literatureb
glycine
5.5
5.4
0.1
2.63, 3.34, 3.67, 3.97,8 5.5,14 4.4,16 5.7,17–19 3.40,20 12.8,21 3.26,22
3.56, 3.23, 2.63, 3.52,23 2.63, 3.52,24 8.2,25 2.72,26 2.9,27 7.0,28
2.2,29 11.430
L-alanine
6.2
6.4
-0.2
3.16 (D), 3.41 (D), 3.57 (D), 3.94 (D), 3.16 (L), 3.41 (L), 3.57 (L),
3.94 (L), 3.09 (DL), 3.57 (DL), 3.94 (DL),8 6.5,14 7.8,17–19 3.35,20
19.6,21 2.89 (DL),22 3.06 (L), 3.43 (L), 3.48 (L), 4.66 (L),23 3.41
(DL), 4.65 (DL),24 2.6 (DL), 3.3 (L),25 3.49 (DL),26 3.8,27 1.75 (L)31
3.43 (L), 3.94 (L), 3.78 (L), 4.16 (L),8 13.6 (DL),17–19 4.22,20 30.6
(L),21 3.40 (L), 3.92 (L), 3.54 (L), 5.15 (L),23 3.43 (L), 5.18 (L),24 4.9
(DL),25 3.21 (DL),26 3.9,27 7.37 (L)31
L-valine
8.5
9.7
8.5
9.6
0.0
0.1
L-leucine
3.93 (L), 3.94 (L), 4.30 (L), 4.96 (L),8 4.53,20 41.1 (L),21 4.96 (L),
7.09 (L),24 13.9 (DL), 8.3 (L),25 5.5,27 8.53 (L)31
38.5 (L),21 24.0 (L),25 8.60 (l)31
L-isoleucine
L-phenylalanine
9.8
10.6
9.6
10.7
0.2
-0.1
4.15, 4.28, 4.64, 5.22,8 14.1 (DL),19 4.48,20 46.9 (L),214.94 (DL),26
7.51 (L)31
L-serine
L-lysine
6.9
10.5
6.7
7.0
8.7
10.1
9.5
11.4
7.9
13.1
9.4
10.6
13.7
14.2
14.5
15.2
10.9
13.6
6.9
10.5
6.0
7.0
9.1
0.0
0.0
0.7
6.9 (DL),19 4.04,20 4.2 (DL),25 3.91 (L),26 3.1c,32
12.8 (DL),17–19 20.1 (L),25 2.8c,32
glycine hydrochloride
L-alanine hydrochloride
L-valine hydrochloride
L-leucine hydrochloride
L-isoleucine hydrochloride
L-phenylalanine hydrochloride
L-serine hydrochloride
L-lysine dihydrochloride
glycine sodium salt
L-alanine sodium salt
L-valine sodium salt
L-leucine sodium salt
L-isoleucine sodium salt
L-phenylalanine sodium salt
L-serine sodium salt
L-lysine sodium salt
0.0
-0.4
-0.1
-0.7
0.1
10.2
10.2
11.3
7.5
13.7
9.9
10.9
13.0
14.1
14.1
15.2
11.4
15.0
17.2 (L)19
15.4 (L)19
15.5 (DL)19
0.4
-0.6
-0.5
-0.3
0.7
0.1
0.4
0.0
-0.5
-1.4
a The theoretical (calculated) hydration numbers, nh,calcd, were obtained using values from Table 2. Note that the literature hydration numbers
b
were obtained using different methods, including nonacoustic ones. D stands for the D-enantiomer, L for the L-enantiomer, and DL for a
mixture of D- and L-stereoisomers. c Hydration number of the amino acid side chain.
TABLE 2: Calculated Hydration Numbers of Functional
Groups, That Is, Their Contributions to the Observed Total
Hydration Numbers, Together with Their Values Available
in the Literature
TABLE 3: Comparison of Experimental and Calculated
Values of Hydration Numbers of Amino Acids in Water, 0.2
M HCl, and 0.2 M NaOHa
in water
in 0.2 M HCl
in 0.2 M NaOH
contribution to nh
from the literature
solute
glycine
L-alanine
L-valine
L-leucine
L-isoleucine
L-phenylalanine
L-serine
nh,exptl
nh,exptl
nh,calcd
nh,exptl
nh,calcd
group
this work
5.5
6.2
8.5
9.7
9.8
10.6
6.9
10.5
4.9
5.3
6.7
7.9
7.7
9.4
6.0
9.5
3.9
4.9
7.0
8.1
8.1
9.2
5.4
9.5
0.2
1.6
3.9
4.7
5.0
5.3
1.2
4.4
0.0
1.0
3.1
4.2
4.2
5.3
1.5
5.1
-NH2
0.92
1.32
0.45
3.83
0.80
1.47
1.17
0.63
4.64
-1 to 017
2-317
-COOH
+
NH3
-
+
8
17
-NH3
for the -CH<
group: 4.1 ( 0.4, 4
COO
-COO-
-OH
2.6-4.030
1-217
-CH3
>CH2
>CH-
-C6H5
1-3,17 1.0,30 3,31 2.145
L-lysine
a The calculated values of nh in 0.2 M HCl and in 0.2 M NaOH
were obtained using eqs 4 and 6, respectively.
The contribution for a nonprotonated amino group is a little
higher than for its protonated form, a rather strange behavior
when electrostriction is supposed to cause a more pronounced
effect. By analogy, the relatively high value for -COO-
compared to the much lower one for -COOH is caused by
additional electrostriction. This effect is undoubtedly worth
further investigation.
3.2. Hydration Numbers of Amino Acids in Acidic and
Alkaline Solutions. We also determined the hydration numbers
of amino acids in aqueous 0.2 M HCl and 0.2 M NaOH at 25
°C to check how they are influenced by the pH of the system.
Table 3 collects these results together with respective values
obtained in pure water.
Amino acids exist in aqueous solution in zwitterionic form,
and their oppositely charged groups undergo protolytic reactions
with acids or bases. As can be seen from Table 3 the values of
the hydration numbers of amino acids in 0.2 M HCl are on
average lower by about 1.5 than those in pure water. The
hydration numbers of amino acids in 0.2 M NaOH are much

12160 J. Phys. Chem. B, Vol. 114, No. 37, 2010
Burakowski and Glin´ski
lower than those in aqueous and acidic solutions. The difference
between those in pure water and alkaline solutions is on average
about 5.0.
When making a balance of water molecules engaged in the
process of formation of ions and their solvates, the hydration
numbers of amino acids in sodium hydroxide solution can be
calculated as follows:
The answer comes from the analysis of hydration numbers
of simple electrolytic solutes, presented by us recently42 and
calculated on the basis of data collected by Allam and Lee.43,44
It was found that nh of the OH- anion is equal to 5.9, while
that of the H+ cation is equal to -1.0. Independent of the
negative nh of the latter, which is unique and unexpected and
undoubtedly needs deeper investigations and explanation, these
values suggest that applying alkaline or acid solvents influences
the obtained hydration numbers if the solute reacts with OH-
or H+ ions. For amino acids in HCl one should consider the
following reaction (R denotes a hydrocarbon chain):
where
0.2 M NaOH+
NH3
n
h,RCH<
COO-
is the hydration number of the amino acid in 0.2 M NaOH
NH2
nh,RCH<
-
This means that when an amino acid is added to a HCl solution,
a reaction of the -COO- group with H+ ions takes place. As
a result of this reaction, the respective cations of the amino acid
appear in solution, and simultaneously an equivalent number
of H+ ions disappear from the system, while the number of Cl-
anions remains unchanged. In the case of L-lysine protonation
of bare -NH2 group also occurs.
Therefore, taking the above into consideration, i.e., making
a balance of water molecules engaged in the process of
formation of ions and their solvates, the hydration numbers of
the amino acid in hydrochloride acid solution can be calculated
from the following equation:
COO
is the hydration number of the respective amino acid anion in
pure water, and nh,OH is the hydration number of the OH anion.
Therefore, the values of the hydration numbers of cations
-
-
+
NH3
nh,RCH<
COOH
and anions
NH2
nh,RCH<
-
COO
can be easily calculated from the contributions of the functional
groups presented in Table 2. The results of these theoretical
calculations of the hydration numbers of amino acids in acidic
and alkaline solutions are shown in Table 3.
where
Such a simple arithmetic analysis explains very well the
observed values of the hydration numbers and leads to calculated
nh values very close to the experimental ones. This was also
recently proved by us for reacting systems such as amines in
acidic and carboxylic acids in alkaline solutions.46
0.2 M HCl
+
NH3
n
h,RCH<
COO-
is the hydration number of the amino acid in 0.2 M HCl
The hydration numbers presented in this work cannot be
understood as absolute values. The acoustic method observes
an average number of water molecules which are assumed to
become incompressible as an effect of electrostriction, formation
of hydrogen bonds, increased rigidity of the water network close
to hydrophobic groups of molecules, etc. The latter phenomenon
meaningfully affects neither intermolecular water-water dis-
tances nor angles between them. For example, it was found from
neutron diffraction that there are ca. 3.0 water molecules
coordinated to the amino group of glycine. The intermolecular
distances suggest that a hydrogen bond is formed between the
amino group and the nearest neighbor water molecules in
aqueous solution.47 The value listed in Table 1 (nh ) 5.5) is
higher, most probably because of the hydrophobic hydration
effect mentioned above, hardly detectable by neutron scattering.
In general, inspection of Table 1 shows that the reproduction
of hydration numbers when applying the assumed additivity of
them with solute molecule constituents and applying the
contributions determined and shown in Table 2 is surprisingly
good. This is a good test of the correctness of our initial
assumptions and the applied procedure. However, the reason
the protonation of the amino group has little effect on its
+
NH3
nh,RCH<
COOH
is the hydration number of the respective amino acid cation in
pure water, and nh,H is the hydration number of the H cation
+
+
+
(nh,H ) -1.0; see ref 42).
In a similar way we can explain why the values of the
hydration numbers of amino acids in 0.2 M NaOH solutions
are much more lower than their respective values in pure water.
Let us consider the following reaction:
As a result of the above reaction 1 H2O molecule appears in
the system as a product of deprotonation of the -NH3+ group,
and also 5.9 molecules of water appear which formed before
the hydration shell of the OH- anion (nh,OH ) 5.9; see ref 42).
-
The number of Na+ cations remains unchanged.

Amino Acid Hydration from Ultrasonic Measurements
J. Phys. Chem. B, Vol. 114, No. 37, 2010 12161
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4. Conclusions
In this study, the speeds of sound and densities of diluted
aqueous solutions of chosen simple amino acids and their
hydrochlorides and sodium salts were measured at 25 °C. On
the basis of these data, the hydration numbers of the solutes
were calculated using the concept of Pasynski. These values
are very close to the literature nh values obtained by the same
technique and on the same order as those obtained by other
methods.
It was shown for diluted aqueous solutions of amino acids
that the effect of solutes on the hydration numbers is strongly
and directly dependent on the length of the solute molecules
and their constituents. It seems possible now to predict the
hydration numbers of amino acids and their salts only from their
structural formulas.
It was also tested how the pH of the system affects the
calculated hydration numbers. It should be stressed that for
reacting systems, i.e., amino acid + HCl or NaOH, the Pasynski
method yields values of hydration numbers which are combina-
tions of the compressibilities of substances engaged in the
occurring reactions.
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