Russian Journal of General Chemistry, Vol. 75, No. 11, 2005, pp. 1723!1728. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 11, 2005, pp. 1806!1811. Original Russian Text Copyright + 2005 by Afanas’ev, Tyunina, Ryabova.
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Hydration of Aliphatic N-Acetyl Amino Acid Amides in Solution V. N. Afanas’ev, E. Yu. Tyunina, and V. V. Ryabova Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, Russia Received March 31, 2004
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Abstract The temperature and concentration dependences of the ultrasound speed and density of aqueous solutions of aliphatic N-acetyl a-amino acid amides were studied using the adiabatic compression method. Parameters of the hydrate complexes formed therein were estimated on a quantitative level. The hydration number and molar compressibility of the complexes were shown to be almost independent of temperature in the range from 5 to 35oC. Water molecules interacting with the dissolved substance in aqueous solutions of peptides and proteins become different from those in the bulk solution. Therefore, estimation of thermodynamic parameters of hydration could favor understanding of the conformational stability and functional activity of proteins in solution. Proteins are complex macromolecules, so that low-molecular weight compounds, such as amino acids and amino acid amides, are used as model compounds [134]. Amino acids and amides derived therefrom give rise to hydrophobic and hydrophilic interactions with water. Both these effects involve reduction in the entropy [5]. The formation of a strained hydrogen bond network by water favors a three-dimensional protein structure in which hydrophobic centers reside are turned inside the helix, while hydrophilic centers appear outside, thus creating specific conditions for intermolecular solvation via hydrogen bonding. In this connection, hydration may be regarded as an important factor responsible for the state of biomolecules in aqueous solution. In the present work we made an attempt to generalize the available data on volume properties and compressibility, which are sensitive to variations in the solvent3solute interaction, and quantitatively estimate the state of hydrate water in the solvate shell of aliphatic N-acetyl a-amino acid amides. The latter were selected as subjects for study on the basis of the following considerations. First, aliphatic N-acetyl a-amino acid amides may be regarded as models of side chains in globular proteins. Second, unlike amino acids, N-acetyl-substituted amino acid amides lack charged terminal groups like COO! and NH+3 , which could appreciably affect the hydration of side chains and mask the interaction pattern of water with hydrophobic radicals [1]. Our analysis was based on the approach according to which a solution is considered
to be a mixture of hydrate complexes formed by the solute and water molecules not included in the hydrate shell. This approach utilizes variation of the molar adiabatic compressibility (bSVm) as a quantitative measure of hydration of the dissolved substance [639]. The molar volume of a solution is given by Eq. (1): Vm = (X1
3 hX2)V1 + X2Vh.
(1)
Here, h is the hydration number, V1 and Vh are the molar volume of the solvent and hydrated solute, respectively, and X1 and X2 are their mole fractions. Differentiation of Eq. (1) with respect to pressure [provided that the entropy remains constant and taking into account that the adiabatic compressibility bS = 3(1/V)(§V/§P)S and that variation of the molar compressibility of a solution may be caused by change in the molar compressibility of the hydrate complexes of N-acetyl amino acid amides (bS,hVh) and [free] solvent (bS,1V1), gives Eq. (2):
bSVm = (X1 3 hX2)bS,1V1 + X2bS,hVh.
(2)
Here, bS,1 and bS,h are the adiabatic compressibility coefficients of water and hydrate complex of the solute, respectively. It should be noted that the hydration number h is the number of water molecules in the hydrate complex per molecule of the solute whose density and compressibility differ from the corresponding parameters of water. The apparent molar adiabatic compressibility of a substance dissolved in water (jc,S) is expressed by Eq. (3):
jc,S = (bSVm 3 X1bS,1V1)/X2.
(3)
By substituting Eq. (2) into (3) we obtain the main expression (4) which describes isoconcentration dependences of jc,S.
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jc,S = bS,hVh 3 hbS,1V1.
(4)
The apparent molar volume of a substance dissolved in water (jV) is given by Eq. (5):
jV = (Vm 3 X1V1)/X2.
(5)
The molar volume of a hydrate complex is expressed by Eq. (6): Vh = V2h + hV1h.
(6)
Here, V1h is the molar volume of water in the hydrate shell, and V2h is the volume of a cavity containing 1 mol of a solute without hydrate shell [6, 7]. As the concentration of a solute rises, its hydrate shells overlap each other, leading to decrease in h, Vh, and bS,hVh. In terms of the above approach, from Eqs. (1), (5), and (6) we obtain Eq. (7) for the parameter jV:
jV = V2h 3 h(V1 3 V1h).
(7)
Here, V1 3 V1h is the volume compression of water in the hydrate shell relative to the bulk solution. In the present work we examined the volume3tension properties of aqueous solutions of N-acetylsubstituted glycine (Ac3Gly3NH2), alanine (Ac3Ala3 NH2), valine (Ac3Val3NH2), and leucine amides (Ac3 Leu3NH2). The densities of solutions and ultrasound speeds therein in the concentration range from 0.01 to 0.15 mol kg!1 and temperature range from 5 to 35oC were taken from [1]. From the densities (r) and ultrasound speeds (u) we calculated the adiabatic compressibilities and apparent molar parameters using Eqs. (8)3(10). 8.8
1
8.4
2
8.2 3 8.0 4 7.8 0
0.5
1.0
1.5
2.0
2.5
3.0
X2 0 103 Fig. 1. Concentration dependences of the molar compressibility (bSVm) of aqueous solutions of Ac3Ala3NH2 at (1) 5, (2) 15, (3) 25, and (4) 35oC.
bS = (ru2)!1, jc,S = 103(r1bS 3 rbS,1)/mrr1 + M2bS/r, jV = 103(r1 3 r)/mrr1 + M2/r.
(8) (9) (10)
Here, r1 and r are the densities of water and aqueous solution, respectively; bS,1 and bS are the adiabatic compressibilities of water and aqueous solution, respectively; M2 is the molecular weight of the solute; and m is the molality of the solution. The concentration dependences of bSVm of aqueous solutions of Ac3X3NH2 in the examined limited concentration range are almost linear (r > 0.999) for all the compounds under study. The bSVm values decrease with rise in the solute concentration, and increase in temperature leads to an appreciable variation of the slope of the bSVm = f(X2) plot. Figure 1 shows the dependences bSVm = f(X2) for aqueous solutions of Ac3Ala3NH2 as an example. The observed decrease in bSVm may be interpreted in terms of the two-structure model of water. This model implies the presence of a loose structure in equilibrium with a tighter packing. As the temperature rises (within the examined range, 5335oC), the overall compressibility of such medium decreases to a value typical of the tight structure due to the corresponding displacement of the above equilibrium. Therefore, the temperature dependence of bSVm for aqueous solutions of Ac3X3 NH2 may be explained primarily by the temperature dependence of the molar compressibility of water bS,1V1 within the examined range of variation of the state parameters [10]. Unlike a-amino acids (Gly, Ala, Val, Leu) for which decrease in bSVm with rise in the concentration results mainly from electrostriction of water molecules in the vicinity of charged centers (COO!, NH+3 ), an analogous effect observed for the corresponding amides can be produced by hydrophobic hydration of the hydrocarbon radical, which makes the structure of water near solute molecules tighter [11, 12]. Thus, despite different mechanisms of interaction with water of amino acid zwitterions and the corresponding amides having no charged groups, both these give a negative contribution to the compressibility of solution. In both cases, the environment of a dissolved molecule is less compressible than bulk water. It should be noted that the reduction in bSVm of Ac3X3NH2 solution becomes concentration-independent as the temperature rises, i.e., as water becomes less structuralized. Presumably, bulk water molecules (which are linked to the hydrate shells of nonelectrolyte amide molecules through weak hydrogen bonds) lose their structure first as the temperature rises.
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(a)
2
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(b)
32
1
33
0
1
31
34
32
1 2
33
2
35
3
3
36
34
4
4 8.0
8.4
8.0
8.8
bS,1V1 0
1010,
cm5
dyn!1 mol!1
8.4
8.8
Fig. 2. Plots of the apparent molar adiabatic compressibility (jc,S) of the solute versus molar adiabatic compressibility of water (bS,1V1) for (a) N-acetyl a-amino acid amides (c = 0.15 mol kg!1) and (b) amino acids (c = 0.1 mol kg!1): (1) Ac3Gly3 NH2, Gly; (2) Ac3Ala3NH2, Ala; (3) Ac3Val3NH2, Val; and (4) Ac3Leu3NH2, Leu.
The temperature dependences of the apparent molar adiabatic compressibilities jc,S of N-acetyl amino acid amides in water were analyzed at fixed concentrations. The temperature dependences are expressed through the molar adiabatic compressibility of pure water bS,1V1 according to Eq. (4). In terms of the selected approach [6, 7], the dependences jc,S = f(bS,1 V1) are linear and are characterized by high correlation coefficients (r > 0.995). This means that there is no appreciable dependence of bS,hVh and h for the examined nonelectrolytes on the temperature. Figure 2 shows the dependences jc,S = f(bS,1V1) for aqueous solutions of the amides under study and the corresponding a-amino acids. The values of bS,hVh and h given in Table 1 tend to decrease as the concentration increases. Relatively small decrease in h with rise in the concentration of amino acid amide in water is explained primarily by the limited concentration range. The decrease in h with rise in the concentration is described by Eq. (11), which is analogous to the relation found for aqueous solutions of electrolytes [639]: h = h0exp(3kX2).
600
4
3
500
400 2
300
(11)
Here, k is a constant, and h0 is the hydration number at infinite dilution. The h0 values for aliphatic N-acetyl amino acid amides range as follows: Ac3 Gly3NH2 (23.67) < Ac3Ala3NH2 (27.89) < Ac3Val3 NH2 (39.54) < Ac3Leu3NH2 (58.23). Increase in h0 is accompanied by increase in the molar heat capaciRUSSIAN JOURNAL OF GENERAL CHEMISTRY
ties at infinite dilution (C 0p,2) [13] (Fig. 3). First of all, this reflects different contributions of hydrophobic hydration of hydrocarbon groups to the overall hydration effect upon dissolution of different amino acid
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30
40
50
60 h0
Fig. 3. Plot of C 0p,2 for (1) Ac3Gly3NH2, (2) Ac3Ala3 NH2, (3) Ac3Val3NH2, and (4) Ac3Leu3NH2 versus their hydration numbers h0 at infinite dilution. No. 11
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Table 1. Molar adiabatic compressibilities (bS,hVh, cm5 din!1 mol!1) of hydrate complexes derived from N-acetyl amino acid amides and their hydration numbers (h) in aqueous solutions with various concentrations
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÒÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄ h ³ bS,hVh 0 108 ³ r º h ³ bS,hVh 0 108 ³ r Mole fraction, X2 0 103³ ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄ×ÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄ Ac3Ala3NH2 Ac3Gly3NH2 0.1801 ³ 23.5 ³ 1.895 ³ 0.9997 º 27.7 ³ 2.238 ³ 0.9954 0.3601 ³ 23.4 ³ 1.886 ³ 0.9996 º 27.6 ³ 2.228 ³ 0.9955 0.5401 ³ 23.3 ³ 1.876 ³ 0.9996 º 27.5 ³ 2.218 ³ 0.9957 0.7200 ³ 23.2 ³ 1.867 ³ 0.9996 º 27.3 ³ 2.208 ³ 0.9959 0.8999 ³ 23.0 ³ 1.857 ³ 0.9995 º 27.2 ³ 2.199 ³ 0.9962 1.0810 ³ 22.9 ³ 1.848 ³ 0.9995 º 27.1 ³ 2.189 ³ 0.9963 1.2590 ³ 22.8 ³ 1.838 ³ 0.9995 º 27.0 ³ 2.179 ³ 0.9965 1.4391 ³ 22.7 ³ 1.829 ³ 0.9994 º 26.8 ³ 2.169 ³ 0.9967 1.6190 ³ 22.6 ³ 1.820 ³ 0.9994 º 26.7 ³ 2.160 ³ 0.9969 1.7983 ³ 22.5 ³ 1.810 ³ 0.9994 º 26.6 ³ 2.150 ³ 0.9972 2.6950 ³ 21.9 ³ 1.764 ³ 0.9992 º 26.0 ³ 2.102 ³ 0.9979 Ac3Leu3NH2 Ac3Val3NH2 0.1801 ³ 39.4 ³ 3.177 ³ 0.9979 º 58.0 ³ 4.688 ³ 0.9941 0.3601 ³ 39.3 ³ 3.168 ³ 0.9976 º 57.7 ³ 4.670 ³ 0.9942 0.5401 ³ 39.1 ³ 3.159 ³ 0.9975 º 57.5 ³ 4.651 ³ 0.9942 0.7200 ³ 39.0 ³ 3.150 ³ 0.9975 º 57.2 ³ 4.633 ³ 0.9942 0.8999 ³ 38.9 ³ 3.141 ³ 0.9975 º 57.0 ³ 4.615 ³ 0.9943 1.0810 ³ 38.8 ³ 3.132 ³ 0.9975 º 56.7 ³ 4.597 ³ 0.9943 1.2590 ³ 38.6 ³ 3.123 ³ 0.9975 º 56.5 ³ 4.579 ³ 0.9943 1.4391 ³ 38.5 ³ 3.114 ³ 0.9975 º 56.3 ³ 4.561 ³ 0.9944 1.6190 ³ 38.4 ³ 3.106 ³ 0.9975 º 56.0 ³ 4.543 ³ 0.9944 1.7983 ³ 38.3 ³ 3.097 ³ 0.9975 º 55.8 ³ 4.525 ³ 0.9944 2.6950 ³ 37.7 ³ 3.053 ³ 0.9974 º 54.6 ³ 4.438 ³ 0.9946 ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÐÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄ amides in water. It should be noted that hydrophobic hydration of Ac3X3NH2 molecules and hydrogen bonding between the amide groups and water molecules leads to reduction in the translational and rotational mobilities of water molecules in the vicinity of amide molecule and subsequent decrease in the heat capacity [14]. The less compressible the environment of solute molecules as compared to bulk water, the lower the thermodynamic parameters C 0p,2. Presumably, aqueous solutions of Ac3Gly3NH2 are structuralized to a greater extent. Variations of the hydration numbers h of Ac3X3 NH2 with concentration result mainly from overlap of their hydrate shells and are accompanied by reduction of the molar compressibility according to Eq. (12):
bS,hVh = bS,2hV2h + hbS,1hV1h.
(12)
Here, bS,1hV1h and bS,2hV2h are the molar compressibilities of the hydrate water and the proper cavity containing 1 mol of a solute, respectively. The dependence bS,hVh = f(h) is linear (r > 0.999) for all the examined compounds (Fig. 4). The slopes of the
bS,hVh = f(h) plots are similar, indicating that the molar compressibility of the hydrate shell bS,1hV1h does not depend on the amino acid amide concentration. The bS,1hV1h value is equal to 8.01 0 10!10 + 9.1 0 10!13 cm5 din!1 mol!1 for all the examined substrates, and it is close to the corresponding value of water (8.083 0 10!10 cm5 din!1 mol!1 at 25oC). These data suggest relatively weak interactions in the hydrate environment of the dissolved nonelectrolytes. It is seen (Fig. 4, Table 1) that increase in the length of the hydrocarbon chain (X), which is accompanied by increase in the volume of the cavity occupied by the solute molecule, increases the hydrate number h and compressibility of the hydrate complex of Ac3X3NH2. This means possible variation of the contribution of hydrophobic hydration. The bS,1hV1h values for aliphatic N-acetyl amino acid amides are greater than bS,1hV1h = 6.84 0 10!10 cm5 din!1 mol!1 for the corresponding a-amino acids [11]. Our results indicate the predominant contribution of electrostriction to hydration of amino acids having similar charged centers COO! and NH+3 , in contrast to the corresponding N-acetyl amides which are nonelectrolytes.
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Table 2. Molar volumes (V1h, cm3 mol!1) and adiabatic compressibilities (b1h, cm2 din!1) of hydrate water in the hydrate shells of N-acetyl a-amino acid amides at various temperatures
ÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ ³ Ac3Ala3NH2 ³ Ac3Val3NH2 ³ Ac3Leu3NH2 ³ Ac3Gly3NH2 T, oC ÃÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄ V1h ³ 1011b1h ³ V1h ³ 1011b1h ³ V1h ³ 1011b1h ³ V1h ³ 1011b1h ³ ÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄ 5 ³ 17.95 ³ 4.52 ³ 18.00 ³ 4.50 ³ 17.88 ³ 4.53 ³ 17.93 ³ 4.52 15 ³ 17.93 ³ 4.52 ³ 18.13 ³ 4.47 ³ 17.88 ³ 4.53 ³ 17.96 ³ 4.52 25 ³ 17.96 ³ 4.51 ³ 18.13 ³ 4.47 ³ 17.94 ³ 4.52 ³ 18.02 ³ 4.50 35 ³ 17.98 ³ 4.53 ³ 18.15 ³ 4.47 ³ 18.06 ³ 4.49 ³ 18.09 ³ 4.48 ÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄ In the framework of the approach in use, the weak concentration dependence of the apparent molar volume jV is determined by small variation of the hydration number h at a constant temperature according to Eq. (7) (r > 0.996). As an example, Figure 5 shows the dependence jV = f(h) for aqueous solutions of Ac3Val3NH2. Taking into account insufficient accuracy of the experimental densities in the region of low concentrations, we can reveal only a tendency for the slope of jV = f(h) to decrease relative to aqueous solutions of the corresponding a-amino acids. We estimated the molar volume V1h and adiabatic
compressibility b1h hydrate water in the hydrate shells of the solutes (Table 2). The obtained values approach those typical of pure water (V1 and b1, respectively), while the corresponding values for aqueous solutions of aliphatic amino acids are considerably lower [11]. Some tendency for V1h to increase and a weak temperature dependence of b1h may be noted. These results are consistent with the determining contribution of charged groups in amino acids and appreciable contribution of hydrophobic hydration of side hydrocarbon chains in the amides under study to the properties of hydrate complexes formed therefrom.
5.0 a
Thus, variation of the volume properties of solutions with rise in the nonelectrolyte concentration is accompanied by change of the compressibility of
4 4.0
140
4
139
3
3 b
3.0 2
138
7
1
2.0
2 137
6 1
1.0 5 10
136 20
30
40
50
60 h
Fig. 4. Plots of the molar compressibilities (bS,hVh) of hydrate complexes derived from (a) N-acetyl a-amino acid amides and (b) a-amino acids versus their hydration numbers (h): (1) Ac3Gly3NH2, (2) Ac3Ala3NH2, (3) Ac3Val3NH2, (4) Ac3Leu3NH2, (5) Gly, (6) Ala, (7) Val, and (8) Leu. RUSSIAN JOURNAL OF GENERAL CHEMISTRY
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37.6
38.0
38.4
38.8
39.2
39.6 h
Fig. 5. Plots of the apparent molar volume (jV) of Ac3Val3NH2 in water versus hydration number (h) at (1) 5, (2) 15, (3) 25, and (4) 35oC. No. 11
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water in the hydrate shells of N-acetyl a-amino acid amides and reduction in their hydration numbers. Our results allow us to consider the hydration number h as the main component of the concentration dependence, and the temperature, as a factor affecting the hydration through variation of the solvent structure. We also revealed an appreciable effect of hydrophobic hydration in the interaction between N-acetyl a-amino acid amides and water and increase in its contribution for more complex biomolecules.
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6. Onori, J., J. Chem. Phys., 1988, vol. 89, no. 1, p. 510. 7. Afanas’ev, V.N., Tyunina, E.Yu., and Ryabova, V.V., Zh. Fiz. Khim., 2003, vol. 77, no. 7, p. 1192. 8. Afanas’ev, V.N. and Tyunina, E.Yu., Zh. Obshch. Khim., 2002, vol. 72, no. 3, p. 386. 9. Afanas’ev, V.N., Panenko, E.S., and Kutepov, A.M., Dokl. Ross. Akad. Nauk, 2001, vol. 311, no. 1, p. 1. 10. Bukin, V.A. and Sarvazyan, A.P., Issledovanie vody i vodnykh sistem fizicheskimi metodami (Studies on Water and Aqueous Systems by Physical Methods), Leningrad: Leningr. Gos. Univ., 1989, p. 153. 11. Afanas’ev, V.N., Tyunina, E.Yu., and Ryabova, V.V., Zh. Strukt. Khim., 2004, vol. 45, no. 5, p. 883. 12. Belousov, V.P. amd Panov, M.Yu., Termodinamika vodnykh rastvorov neelektrolitov (Thermodynamics of Aqueous Nonelectrolyte Solutions), Leningrad: Khimiya, 1983, p. 63. 13. Hakin, A.W. and Hedwig, G.R., Phys. Chem. Chem. Phys., 2000, vol. 2, no. 3, p. 1795. 14. Barrett, G.C., Chemistry and Biochemistry of the Aminoacids, London: Chapman and Hall, 1985, p. 600.
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