J. Chem. Eng. Data 1998, 43, 205-214
205
Partial Molar Volumes and Isentropic Compressibilities of Amino Acids in Dilute Aqueous Solutions Yuri Yasuda,* Naoya Tochio, Masao Sakurai, and Katsutoshi Nitta Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo 060, Japan
The densities and speeds of sound in dilute aqueous solutions of the amino acids, L-asparagine, L-glutamine, L-histidine, L-aspartic acid monosodium salt, L-glutamic acid monosodium salt, L-lysine monohydrochloride, L-arginine monohydrochloride, and L-histidine monohydrochloride, were measured at (5, 15, 25, 35, and 45) °C. Partial molar volumes and partial molar isentropic compressibilities of these amino acids at infinite dilution were evaluated and discussed in connection with the electrostatic interactions between the charged groups of amino acid side chains and solvent water.
Introduction It is well-known that the stabilization of native conformations of biopolymers is caused by the various noncovalent interactions such as hydrogen bonding, electrostatic, and hydrophobic interactions. In aqueous protein solutions, the amino acid residues of a polypeptide chain interact with each other and with the surrounding water by these noncovalent forces. Therefore, the characterization of the thermodynamic properties of hydration can assist in the understanding of the conformational stability and the functional properties of proteins in solution. However, since proteins are particularly complex macromolecules, it is difficult to resolve the various interactions that participate in protein hydration. One of the useful approaches for interpreting the thermodynamic properties of protein solutions is the comparative study of various lowmolecular-weight model compounds. Amino acids have been often used as the most convenient substances to estimate the individual contribution of monomeric units in proteins. Volumetric properties of solution are believed to be sensitive to the nature of hydration. A number of experimental data of the partial molar volumes of amino acids have been reported (Hakin et al., 1995; Høiland, 1986). It is, however, recognized that amino acids in aqueous solution have oppositely charged carboxyl and amino groups that may influence the hydration of the adjacent amino acid side chains. For this reason the use of the other model compounds such as N-acetyl amino acid amides (Hedwig et al., 1991; Mishra and Ahluwalia, 1984) or oligopeptides (Hedwig, 1988) has been recommended. Kikuchi et al. (1995, 1996) have shown that the partial molar volumes, V2°(Ra), or isentropic compressibilities, K2°(Ra), of nonpolar side chains evaluated from amino acid series are, in most cases, smaller than the values of V2°(Rn) or K2°(Rn) calculated from the data for acetyl amino acid amide series. For valine or leucine, for example, the V2°(Ra) value is smaller by about 0.5 cm3‚mol-1 than that for V2°(Rn) and the K2°(Ra) value is smaller by about 2 cm3‚mol-1‚GPa-1 than that for K2°(Rn). On the other hand, it may be expected that such a charge effect is less significant in the case of amino acids with a polar side chain. In a recent communication from this laboratory, it was shown that the partial molar volumes
or compressibilities of seryl or threonyl groups are almost the same when calculated from the data of the two series, amino acids and acetyl amino acid amides (Mizuguchi et al. 1997). In the present work, we have studied other hydrophilic amino acids having a polar or an ionic side chain. Experimental Section The amino acids used in this study, L-asparagine (Asn), (Gln), L-histidine (His), L-aspartic acid monosodium salt (AspNa), L-glutamic acid monosodium salt (GluNa), L-lysine monohydrochloride (LysHCl), L-arginine monohydrochloride (ArgHCl), and L-histidine monohydrochloride (HisHCl), were guaranteed reagents obtained from several suppliers: Asn (>99%) and Gln (>98%) were obtained from Nacalai Tesque, Inc., Kyoto; AspNa (>98%) and GluNa (>99%) were obtained from Junsei Chemical Co., Ltd., Tokyo; the other amino acids (>99%) were obtained from Wako Pure Chemical Industries, Ltd., Osaka. These samples were used without further purification. All solutions were made by mass with deionized and distilled water. The solution densities were measured by an oscillatingtube densimeter (DMA 60/601, Anton Paar, Austria) with an accuracy of (2 × 10-6 g‚cm-3. The speeds of sound in the solutions were measured at a frequency of about 5 MHz using a sing-around velocimeter constructed in our laboratory. The accuracy of the measurements of the speed of sound was estimated to be better than 1 cm‚s-1 for the dilute solution range studied. The temperature of the fluid surrounding the measuring cell of the densimeter or velocimeter was maintained within (0.002 °C by using a quartz temperature controller. Details of the apparatus, calibrations, and experimental procedures have been described previously (Sakurai et al., 1994). L-glutamine
Results and Discussion The apparent molar volumes, Vφ, of the solutes were calculated by using the following equation
Vφ ) M2/F - (F - F1)/mFF1
(1)
where M2 is the solute molar mass, m is molality, and F1 and F are the densities of water and the solution, respec-
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206 Journal of Chemical and Engineering Data, Vol. 43, No. 2, 1998 Table 1. Density Differences and Apparent Molar Volumes for Aqueous Solutions of Amino Acids at 5, 15, 25, 35, and 45 °C m/ mol‚kg-1
103(F - F1)/ g‚cm-3
Vφ/ cm3‚mol-1
m/ mol‚kg-1
103(F - F1)/ g‚cm-3
Vφ/ cm3‚mol-1
m/ mol‚kg-1
103(F - F1)/ g‚cm-3
Vφ/ cm3‚mol-1
L-Asparagine
0.010 33 0.021 78 0.032 36 0.040 92
0.599 1.261 1.872 2.364
74.10 74.13 74.11 74.16
0.049 60 0.059 97 0.070 34
t ) 5 °C 2.864 3.457 4.051
74.17 74.21 74.23
0.079 97 0.088 10 0.096 22
4.600 5.061 5.522
74.25 74.29 74.32
0.011 14 0.021 49 0.031 40 0.041 04
0.625 1.205 1.760 2.298
76.00 75.98 75.96 75.98
0.049 43 0.058 75 0.068 71
t ) 15 °C 2.765 3.280 3.832
75.99 76.05 76.07
0.079 34 0.088 43 0.096 45
4.418 4.920 5.359
76.11 76.13 76.16
0.011 69 0.022 15 0.031 11 0.041 63
0.640 1.212 1.702 2.274
77.42 77.39 77.36 77.38
0.051 32 0.060 30 0.068 84
t ) 25 °C 2.799 3.286 3.748
77.43 77.43 77.45
0.077 47 0.085 59 0.094 96
4.213 4.649 5.152
77.47 77.50 77.52
0.008 83 0.019 45 0.032 12 0.042 15
0.476 1.046 1.724 2.257
78.29 78.41 78.44 78.53
0.050 78 0.058 79 0.066 68
t ) 35 °C 2.719 3.145 3.563
78.50 78.53 78.55
0.074 49 0.082 33 0.091 28
3.978 4.393 4.865
78.56 78.55 78.58
0.005 14 0.010 98 0.020 04 0.028 63
0.271 0.582 1.061 1.516
79.62 79.34 79.36 79.31
0.038 96 0.049 39 0.058 86
t ) 45 °C 2.060 2.610 3.106
79.34 79.33 79.35
0.066 73 0.073 72 0.079 69
3.519 3.886 4.197
79.35 79.36 79.36
L-Glutamine
0.008 62 0.017 39 0.028 28 0.035 92
0.483 0.973 1.578 2.001
90.02 90.12 90.20 90.26
0.046 66 0.056 82 0.066 75
t ) 5 °C 2.597 3.156 3.703
90.26 90.32 90.34
0.076 21 0.086 91 0.097 85
4.221 4.806 5.404
90.37 90.41 90.43
0.009 23 0.020 19 0.031 10 0.041 93
0.500 1.090 1.676 2.256
91.98 92.08 92.13 92.16
0.051 82 0.059 85 0.068 27
t ) 15 °C 2.785 3.212 3.660
92.18 92.21 92.22
0.078 30 0.088 66 0.099 64
4.194 4.741 5.319
92.23 92.27 92.30
0.006 81 0.014 10 0.022 86 0.030 40
0.358 0.742 1.203 1.596
93.70 93.58 93.52 93.61
0.038 88 0.047 49 0.057 28
t ) 25 °C 2.038 2.488 3.000
93.64 93.64 93.61
0.066 75 0.077 81 0.088 45
3.490 4.063 4.613
93.65 93.66 93.68
0.006 81 0.013 04 0.021 86 0.031 78
0.351 0.673 1.128 1.637
94.76 94.71 94.69 94.73
0.042 64 0.052 00 0.063 92
t ) 35 °C 2.197 2.677 3.288
94.67 94.67 94.64
0.074 03 0.082 80 0.092 44
3.812 4.257 4.744
94.54 94.58 94.63
0.008 13 0.018 06 0.026 65 0.035 91
0.416 0.920 1.357 1.826
95.35 95.57 95.52 95.55
0.044 01 0.052 97 0.062 80
t ) 45 °C 2.240 2.690 3.188
95.46 95.53 95.50
0.073 34 0.082 66 0.091 71
3.721 4.192 4.645
95.49 95.46 95.49
L-Histidine
0.018 02 0.030 80 0.043 52
1.073 1.832 2.582
95.50 95.51 95.57
0.056 68 0.064 16 0.075 30
t ) 5 °C 3.358 3.796 4.451
95.58 95.63 95.62
0.083 16 0.092 06
4.911 5.429
95.63 95.66
0.022 97 0.040 32 0.056 19
1.332 2.338 3.246
97.07 96.97 97.11
0.067 69 0.076 95 0.085 11
t ) 15 °C 3.902 4.431 4.897
97.17 97.17 97.18
0.089 57 0.095 51
5.152 5.487
97.17 97.21
0.022 79 0.039 10 0.058 79
1.281 2.195 3.296
98.97 98.92 98.90
0.071 41 0.081 09 0.088 92
t ) 25 °C 3.998 4.531 4.966
98.90 98.96 98.95
0.096 72 0.103 31
5.399 5.762
98.93 98.94
Journal of Chemical and Engineering Data, Vol. 43, No. 2, 1998 207 Table 1 (Continued) m/ mol‚kg-1
103(F - F1)/ g‚cm-3
Vφ/ cm3‚mol-1
m/ mol‚kg-1
103(F - F1)/ g‚cm-3
Vφ/ cm3‚mol-1
m/ mol‚kg-1
103(F - F1)/ g‚cm-3
Vφ/ cm3‚mol-1
L-Histidine
0.031 68 0.050 88 0.064 63
1.748 2.809 3.562
100.08 99.94 99.96
0.077 97 0.087 37 0.095 71
t ) 35 °C 4.293 4.807 5.264
99.94 99.92 99.90
0.102 10 0.107 18
5.612 5.890
99.89 99.88
0.023 72 0.038 51 0.051 28
1.292 2.097 2.790
101.02 100.96 100.92
0.062 21 0.074 21 0.081 01
t ) 45 °C 3.382 4.032 4.399
100.91 100.87 100.85
0.089 18 0.095 96
4.841 5.207
100.84 100.82
L-Aspartic
Acid Monosodium
0.011 62 0.016 23 0.022 43 0.028 77
1.110 1.550 2.140 2.742
59.52 59.50 59.53 59.61
0.038 12 0.050 36 0.061 61 0.072 06
t ) 5 °C 3.626 4.782 5.842 6.822
0.007 43 0.011 63 0.020 67 0.028 46
0.687 1.075 1.908 2.621
62.51 62.60 62.61 62.82
0.041 63 0.054 33 0.068 83 0.086 51
t ) 15 °C 3.831 4.995 6.314 7.919
62.79 62.81 62.91 63.02
0.104 41 0.124 72 0.145 18
9.535 11.358 13.189
63.13 63.26 63.38
0.005 71 0.010 60 0.018 36 0.026 63
0.523 0.964 1.666 2.411
63.34 63.93 64.13 64.31
0.037 70 0.049 28 0.061 90 0.075 90
t ) 25 °C 3.407 4.444 5.569 6.816
64.43 64.53 64.67 64.76
0.092 42 0.111 31 0.130 75 0.142 48
8.280 9.947 11.656 12.677
64.88 65.00 65.10 65.21
0.017 48 0.034 39 0.046 27 0.057 66
1.559 3.058 4.107 5.107
65.63 65.81 65.92 66.05
0.064 40 0.072 31 0.077 12 0.080 45
t ) 35 °C 5.697 6.394 6.816 7.110
66.09 66.10 66.11 66.10
0.086 00 0.094 42 0.110 29
7.594 8.326 9.712
66.14 66.20 66.24
0.008 76 0.017 34 0.024 97 0.036 07
0.773 1.531 2.201 3.177
66.57 66.46 66.56 66.57
0.049 06 0.069 87 0.085 48
t ) 45 °C 4.314 6.124 7.477
66.65 66.81 66.89
0.103 61 0.019 07 0.139 01
9.043 10.372 12.082
66.99 67.07 67.15
L-Glutamic
59.74 59.85 59.92 60.00
0.092 91 0.113 29 0.135 04
8.765 10.657 12.653
60.20 60.37 60.61
Acid Monosodium
0.019 46 0.040 59 0.061 37 0.080 17
1.829 3.807 5.740 7.477
74.94 75.02 75.15 75.29
0.100 13 0.119 75 0.140 25
t ) 5 °C 9.308 11.099 12.961
0.025 53 0.041 58 0.055 92 0.070 83
2.331 3.787 5.082 6.420
77.60 77.73 77.82 77.96
0.086 50 0.103 12 0.118 67
t ) 15 °C 7.825 9.305 10.683
78.02 78.14 78.24
0.141 81 0.158 74 0.175 85
12.723 14.208 15.700
78.38 78.48 78.58
0.023 93 0.038 91 0.057 57 0.080 17
2.132 3.464 5.111 7.096
79.80 79.80 79.89 80.00
0.100 89 0.124 22 0.142 40
t ) 25 °C 8.905 10.926 12.517
80.10 80.24 80.18
0.156 21 0.174 29 0.187 41
13.687 15.235 16.354
80.36 80.45 80.51
0.015 65 0.026 73 0.046 02 0.059 46
1.375 2.340 4.025 5.192
81.12 81.31 81.27 81.33
0.077 42 0.091 35 0.104 96
t ) 35 °C 6.746 7.946 9.113
81.39 81.45 81.51
0.121 59 0.136 60 0.149 84
10.535 11.811 12.934
81.58 81.65 81.71
0.024 17 0.041 59 0.058 74
2.102 3.610 5.087
81.88 81.95 82.03
0.073 37 0.097 96 0.118 21
t ) 45 °C 6.344 8.442 10.165
82.07 82.18 82.24
0.170 13 0.219 80
14.535 18.665
82.44 82.62
0.156 89 0.170 15 0.195 38 0.207 64
9.227 9.979 11.404 12.088
122.70 122.77 122.88 122.95
L-Lysine
0.011 66 0.027 64 0.062 91 0.078 41
0.717 1.679 3.780 4.692
121.06 121.69 122.10 122.23
0.094 54 0.105 84 0.123 62 0.139 41
75.44 75.58 75.72
0.158 99 0.180 39 0.205 94
14.648 16.571 18.845
75.86 75.99 76.16
Monohydrochloride t ) 5 °C 5.636 6.294 7.324 8.230
122.34 122.41 122.50 122.61
208 Journal of Chemical and Engineering Data, Vol. 43, No. 2, 1998 Table 1 (Continued) m/ mol‚kg-1
103(F - F1)/ g‚cm-3
Vφ/ cm3‚mol-1
m/ mol‚kg-1
103(F - F1)/ g‚cm-3
L-Lysine
Vφ/ cm3‚mol-1
m/ mol‚kg-1
103(F - F1)/ g‚cm-3
Vφ/ cm3‚mol-1
Monohydrochloride
0.010 58 0.020 37 0.030 48 0.040 15 0.051 29
0.627 1.202 1.795 2.358 3.003
123.32 123.52 123.59 123.67 123.79
0.063 33 0.075 64 0.091 88 0.107 25 0.124 52
t ) 15 °C 3.697 4.404 5.328 6.200 7.171
123.86 123.94 124.05 124.13 124.22
0.140 31 0.159 92 0.177 54 0.197 94
8.054 9.144 10.115 11.231
124.30 124.39 124.47 124.57
0.014 32 0.023 58 0.035 41 0.040 38
0.828 1.364 2.038 2.313
124.94 124.82 125.02 125.28
0.049 14 0.069 84 0.088 89 0.113 33
t ) 25 °C 2.809 3.973 5.039 6.392
125.33 125.46 125.52 125.64
0.139 53 0.163 80 0.190 03 0.205 48
7.830 9.147 10.557 11.383
125.75 125.86 125.97 126.02
0.007 51 0.012 99 0.023 20 0.033 08 0.048 07
0.426 0.738 1.317 1.874 2.713
126.28 126.12 126.13 126.18 126.27
0.063 91 0.078 90 0.095 85 0.112 41
t ) 35 °C 3.596 4.427 5.359 6.266
126.33 126.40 126.48 126.53
0.132 04 0.154 91 0.177 64 0.201 13
7.335 8.564 9.779 11.023
126.59 126.70 126.78 126.87
0.009 82 0.023 75 0.031 56 0.041 31 0.047 46
0.558 1.341 1.779 2.323 2.667
126.44 126.68 126.75 126.81 126.81
0.055 81 0.064 12 0.080 25 0.091 82 0.109 94
t ) 45 °C 3.127 3.588 4.470 5.112 6.102
126.91 126.93 127.06 127.02 127.07
0.125 01 0.144 74 0.157 52 0.180 46 0.201 82
6.922 7.984 8.668 9.891 11.021
127.09 127.17 127.22 127.28 127.34
L-Arginine
Monohydrochloride
0.015 05 0.026 56 0.042 93 0.058 79
1.122 1.975 3.176 4.333
135.96 136.03 136.25 136.37
0.073 23 0.088 71 0.105 42 0.129 45
t ) 5 °C 5.379 6.493 7.687 9.384
136.47 136.59 136.69 136.89
0.150 60 0.185 17 0.200 54
10.867 13.261 14.313
137.01 137.23 137.32
0.011 77 0.023 47 0.032 40 0.049 48
0.850 1.693 2.333 3.544
138.39 138.36 138.36 138.62
0.069 78 0.090 16 0.115 67 0.141 07
t ) 15 °C 4.973 6.394 8.157 9.893
138.76 138.91 139.06 139.22
0.165 22 0.196 79 0.205 37
11.528 13.641 14.212
139.34 139.50 139.54
0.011 62 0.022 86 0.032 17 0.048 86
0.824 1.611 2.263 3.422
139.84 140.15 140.20 140.35
0.068 95 0.097 51 0.123 56
t ) 25 °C 4.807 6.758 8.513
140.47 140.62 140.77
0.152 70 0.181 63 0.203 15
10.459 12.366 13.771
140.90 141.05 141.15
0.009 07 0.014 74 0.029 97 0.045 23
0.640 1.035 2.087 3.134
140.43 140.69 141.14 141.35
0.063 19 0.080 24 0.113 66
t ) 35 °C 4.358 5.511 7.749
141.51 141.63 141.83
0.148 86 0.177 83 0.202 73
10.071 11.962 13.567
142.02 142.13 142.25
0.006 37 0.013 45 0.020 00 0.027 63
0.440 0.927 1.377 1.899
142.32 142.35 142.34 142.38
0.038 36 0.049 07 0.068 90 0.091 16
t ) 45 °C 2.632 3.357 4.693 6.182
142.38 142.50 142.60 142.68
0.112 16 0.134 27 0.170 84 0.208 00
7.573 9.025 11.403 13.784
142.79 142.88 143.02 143.16
L-Histidine
Monohydrochloride
0.016 70 0.029 22 0.038 37
1.360 2.375 3.112
110.04 110.08 110.19
0.046 12 0.054 08 0.061 83
t ) 5 °C 3.731 4.373 4.984
0.012 91 0.024 99 0.036 76
1.021 1.973 2.893
112.39 112.47 112.62
0.047 22 0.056 19 0.063 45
t ) 15 °C 3.709 4.405 4.967
112.68 112.75 112.79
0.071 14 0.077 39
5.562 6.043
112.84 112.88
0.023 32 0.038 89 0.053 27
1.805 3.000 4.097
114.10 114.24 114.35
0.067 56 0.075 45 0.083 27
t ) 25 °C 5.117 5.773 6.364
114.51 114.55 114.58
0.090 20 0.099 15
6.885 7.556
114.60 114.66
0.032 09 0.054 90 0.067 58
2.446 4.164 5.115
115.32 115.52 115.57
0.080 60 0.090 00
t ) 35 °C 6.085 6.785
115.65 115.68
0.097 73 0.105 18
7.357 7.909
115.72 115.74
0.016 04 0.028 26 0.038 58
1.213 2.134 2.909
116.22 116.22 116.29
0.047 58 0.055 46 0.062 27
t ) 45 °C 3.580 4.164 4.669
116.36 116.44 116.50
0.068 08 0.074 49 0.078 56
5.098 5.570 5.870
116.54 116.59 116.61
110.30 110.28 110.45
0.067 96 0.079 05 0.090 66
5.474 6.358 7.274
110.46 110.48 110.58
Journal of Chemical and Engineering Data, Vol. 43, No. 2, 1998 209 Table 2. Speed of Sound Differences and Apparent Molar Isentropic Compressibilities for Aqueous Solutions of Amino Acids at 5, 15, 25, 35, and 45 °C m/ mol‚kg-1
(u - u1)/ m‚s-1
Kφ/ cm3‚mol-1‚GPa-1
m/ mol‚kg-1
(u - u1)/ m‚s-1
Kφ/ cm3‚mol-1‚GPa-1
m/ mol‚kg-1
(u - u1)/ m‚s-1
Kφ/ cm3‚mol-1‚GPa-1
L-Asparagine
0.008 49 0.022 34 0.035 39 0.044 98
0.733 1.891 3.023 3.836
-51.62 -50.31 -50.74 -50.56
0.052 85 0.059 85 0.068 13
t ) 5 °C 4.504 5.107 5.816
-50.45 -50.47 -50.42
0.077 83 0.086 42 0.095 15
6.638 7.366 8.110
-50.29 -50.17 -50.09
0.012 11 0.026 88 0.038 21 0.047 22
0.960 2.094 2.992 3.677
-41.25 -40.27 -40.44 -40.10
0.056 18 0.062 91 0.069 38
t ) 15 °C 4.384 4.908 5.415
-40.14 -40.08 -40.06
0.076 26 0.085 01 0.094 67
5.946 6.624 7.349
-39.95 -39.86 -39.61
0.012 89 0.026 11 0.037 17 0.044 76
0.943 1.883 2.691 3.218
-33.92 -33.23 -33.33 -32.99
0.053 37 0.061 28 0.068 24
t ) 25 °C 3.835 4.396 4.901
-32.92 -32.81 -32.82
0.075 59 0.083 52 0.092 92
5.428 6.013 6.658
-32.78 -32.84 -32.59
0.016 57 0.026 54 0.035 27 0.044 37
1.148 1.792 2.355 2.996
-29.42 -28.38 -27.91 -28.31
0.052 99 0.060 54 0.069 31
t ) 35 °C 3.568 4.062 4.663
-28.16 -27.99 -28.06
0.077 47 0.085 74 0.095 49
5.178 5.741 6.408
-27.77 -27.81 -27.84
0.010 15 0.019 81 0.029 72 0.039 52
0.662 1.283 1.923 2.545
-25.83 -25.56 -25.50 -25.31
0.049 93 0.061 02 0.070 37
t ) 45 °C 3.201 3.892 4.493
-25.11 -24.89 -24.90
0.078 88 0.086 83 0.094 11
5.019 5.519 5.981
-24.76 -24.69 -24.66
L-Glutamine
0.010 99 0.022 89 0.032 29 0.042 46
1.035 2.160 3.067 4.026
-48.06 -48.07 -48.43 -48.22
0.051 93 0.061 14 0.069 08
t ) 5 °C 4.908 5.779 6.521
-47.93 -47.86 -47.71
0.078 44 0.088 19 0.098 41
7.382 8.300 9.246
-47.43 -47.35 -47.15
0.014 02 0.024 56 0.035 29 0.043 93
1.225 2.115 3.038 3.785
-37.87 -36.98 -36.90 -36.90
0.052 36 0.060 32 0.068 44
t ) 15 °C 4.504 5.195 5.895
-36.75 -36.77 -36.73
0.078 30 0.089 53 0.100 26
6.728 7.684 8.598
-36.53 -36.40 -36.29
0.011 66 0.026 90 0.042 13 0.053 45
0.937 2.144 3.348 4.256
-30.01 -29.55 -29.34 -29.39
0.063 30 0.071 28 0.077 57
t ) 25 °C 5.010 5.632 6.138
-29.07 -28.95 -29.00
0.084 63 0.090 49 0.099 56
6.685 7.137 7.835
-28.89 -28.79 -28.65
0.013 41 0.025 63 0.034 29
0.996 1.896 2.533
-24.33 -24.13 -24.06
0.043 48 0.052 14 0.060 39
t ) 35 °C 3.220 3.856 4.493
-24.17 -24.09 -24.34
0.066 82 0.074 81 0.082 79
4.939 5.495 6.093
-24.06 -23.78 -23.85
0.010 45 0.022 93 0.031 22 0.040 54
0.714 1.595 2.168 2.801
-19.83 -20.46 -20.40 -20.18
0.049 76 0.057 99 0.066 30
t ) 45 °C 3.452 4.002 4.577
-20.33 -20.12 -20.12
0.075 99 0.084 88 0.093 74
5.237 5.855 6.483
-20.03 -20.06 -20.14
L-Histidine
0.019 97 0.038 34 0.052 22
2.135 4.085 5.545
-55.94 -55.51 -55.14
0.063 36 0.072 13 0.081 09
t ) 5 °C 6.738 7.709 8.671
-55.14 -55.42 -55.38
0.089 09 0.094 96
9.419 10.002
-54.49 -54.16
0.021 19 0.039 52 0.050 37
1.926 3.564 4.541
-39.66 -39.10 -38.98
0.063 47 0.072 00 0.081 31
t ) 15 °C 5.717 6.478 7.288
-38.85 -38.73 -38.45
0.090 09 0.098 04
8.057 8.783
-38.27 -38.31
0.017 04 0.029 03 0.040 55
1.369 2.358 3.296
-29.28 -29.74 -29.72
0.048 24 0.057 06 0.066 58
t ) 25 °C 3.955 4.701 5.483
-30.13 -30.33 -30.28
0.073 96 0.082 29
6.029 6.800
-29.75 -30.38
0.015 86 0.029 09 0.041 52
1.264 2.310 3.287
-26.73 -26.56 -26.39
0.052 00 0.062 33 0.071 08
t ) 35 °C 4.099 4.925 5.607
-26.19 -26.29 -26.19
0.077 25 0.084 26
6.054 6.621
-25.90 -26.01
210 Journal of Chemical and Engineering Data, Vol. 43, No. 2, 1998 Table 2 (Continued) m/ mol‚kg-1
(u - u1)/ m‚s-1
Kφ/ cm3‚mol-1‚GPa-1
m/ mol‚kg-1
(u - u1)/ m‚s-1
Kφ/ cm3‚mol-1‚GPa-1
m/ mol‚kg-1
(u - u1)/ m‚s-1
Kφ/ cm3‚mol-1‚GPa-1
0.085 43 0.092 19
5.903 6.424
-19.42 -19.74
L-Histidine
0.023 44 0.039 06 0.050 63
1.638 2.723 3.520
-19.87 -19.77 -19.66
0.060 64 0.069 92 0.076 70
t ) 45 °C 4.220 4.848 5.282
L-Aspartic
-19.69 -19.55 -19.29
Acid Monosodium
0.019 84 0.035 55 0.046 03 0.056 35
2.810 4.973 6.415 7.835
-114.99 -113.27 -112.57 -112.03
0.076 92 0.091 13 0.104 15
t ) 5 °C 10.597 12.558 14.304
0.006 18 0.014 41 0.033 74 0.044 98
0.743 1.842 4.287 5.764
-90.54 -95.10 -94.16 -94.56
0.067 25 0.089 45 0.105 93
t ) 15 °C 8.508 11.377 13.430
-93.03 -92.96 -92.36
0.121 20 0.130 54 0.139 48
15.321 16.502 17.620
-91.81 -91.62 -91.38
0.009 28 0.015 98 0.020 91 0.032 87
1.113 1.902 2.470 3.893
-84.00 -83.33 -82.67 -82.59
0.046 14 0.060 09 0.079 13
t ) 25 °C 5.502 7.139 9.402
-82.76 -82.23 -81.84
0.097 75 0.119 65 0.142 84
11.565 14.118 16.811
-81.19 -80.58 -79.96
0.006 44 0.013 04 0.023 10 0.034 28
0.727 1.457 2.579 3.806
-75.73 -74.97 -74.72 -74.17
0.052 33 0.071 73 0.093 38
t ) 35 °C 5.830 7.969 10.355
-74.07 -73.59 -73.13
0.119 37 0.133 45 0.141 66
13.171 14.680 15.612
-72.44 -72.05 -72.04
0.015 48 0.028 32 0.035 69 0.042 59
1.680 3.058 3.839 4.553
-71.03 -70.49 -70.12 -69.63
0.054 36 0.069 42 0.084 44
t ) 45 °C 5.812 7.351 8.894
-69.46 -68.65 -68.11
0.093 71 0.111 91 0.142 28
9.901 11.766 14.921
-68.14 -67.60 -67.03
L-Glutamic
-110.53 -110.15 -109.46
0.111 32 0.120 28 0.136 61
15.282 16.474 18.648
-109.21 -108.74 -107.98
Acid Monosodium
0.022 78 0.049 71 0.072 25 0.093 98
3.515 7.639 11.042 14.332
-115.43 -114.13 -112.88 -112.02
0.111 80 0.127 33 0.143 50
t ) 5 °C 17.036 19.365 21.804
0.020 56 0.042 16 0.065 28 0.079 90
2.908 5.947 9.132 11.187
-96.21 -95.39 -94.10 -93.83
0.098 78 0.118 70 0.140 90
t ) 15 °C 13.804 16.549 19.605
-93.22 -92.58 -91.92
0.154 62 0.177 06 0.202 05
21.479 24.552 27.968
-91.48 -90.85 -90.18
0.023 74 0.039 20 0.060 61 0.076 62
3.154 5.196 7.999 10.078
-83.99 -83.49 -82.72 -82.15
0.094 56 0.111 94 0.132 17
t ) 25 °C 12.399 14.658 17.289
-81.56 -81.14 -80.70
0.161 00 0.184 67 0.202 83
20.871 23.902 26.211
-79.52 -79.00 -78.58
0.015 01 0.028 45 0.042 57 0.055 15
1.936 3.590 5.326 6.872
-77.64 -75.81 -74.94 -74.41
0.076 05 0.110 11 0.122 68
t ) 35 °C 9.431 13.568 15.053
-73.74 -72.76 -72.28
0.141 85 0.161 65 0.194 14
17.353 19.730 23.613
-71.79 -71.34 -70.65
0.007 14 0.017 18 0.036 51 0.058 53
0.857 2.045 4.307 6.859
-70.63 -69.89 -68.94 -68.15
0.086 17 0.102 00 0.119 08
t ) 45 °C 10.053 11.867 13.812
-67.45 -67.05 -66.62
0.134 26 0.152 32 0.168 23
15.510 17.585 19.408
-66.16 -65.87 -65.62
L-Lysine
-111.42 -110.79 -110.24
0.163 86 0.177 71 0.197 70
24.835 26.890 29.872
-109.43 -108.89 -108.21
Monohydrochloride
0.020 72 0.038 20 0.052 40 0.065 12
3.265 5.987 8.185 10.152
-78.38 -77.45 -76.81 -76.37
0.078 77 0.097 03 0.111 46 0.128 44
t ) 5 °C 12.237 15.059 17.239 19.806
-75.74 -75.30 -74.69 -74.09
0.147 04 0.160 35 0.181 29 0.208 99
22.626 24.628 27.750 31.835
-73.56 -73.15 -72.46 -71.53
0.018 57 0.035 82 0.061 03 0.084 87
2.765 5.371 8.975 12.339
-64.57 -64.94 -62.72 -61.33
0.113 65 0.133 67 0.152 83
t ) 15 °C 16.424 19.261 21.945
-60.39 -59.86 -59.29
0.175 78 0.196 57 0.211 60
25.136 27.964 30.033
-58.64 -57.93 -57.55
Journal of Chemical and Engineering Data, Vol. 43, No. 2, 1998 211 Table 2 (Continued) m/ mol‚kg-1
(u - u1)/ m‚s-1
Kφ/ cm3‚mol-1‚GPa-1
m/ mol‚kg-1
(u - u1)/ m‚s-1
L-Lysine
Kφ/ cm3‚mol-1‚GPa-1
m/ mol‚kg-1
(u - u1)/ m‚s-1
Kφ/ cm3‚mol-1‚GPa-1
Monohydrochloride
0.010 17 0.025 28 0.035 42 0.044 83 0.055 50
1.376 3.460 4.837 6.109 7.560
-51.25 -51.93 -51.60 -51.30 -51.14
0.063 59 0.074 12 0.086 25 0.101 23
t ) 25 °C 8.632 10.043 11.632 13.615
-50.76 -50.50 -49.98 -49.60
0.124 49 0.148 14 0.176 46 0.203 09
16.696 19.782 23.460 26.891
-49.13 -48.54 -47.92 -47.35
0.006 67 0.014 29 0.027 62 0.041 39 0.052 92
0.845 1.826 3.544 5.394 6.886
-43.29 -43.87 -44.00 -45.01 -44.75
0.066 64 0.080 07 0.096 66 0.110 75 0.129 12
t ) 35 °C 8.648 10.345 12.455 14.282 16.555
-44.41 -43.96 -43.60 -43.54 -42.96
0.144 73 0.163 75 0.183 31 0.195 14 0.202 83
18.509 20.883 23.299 24.749 25.720
-42.64 -42.28 -41.89 -41.64 -41.57
0.007 54 0.015 42 0.023 63 0.036 42
0.961 1.927 2.980 4.569
-42.27 -40.80 -41.39 -40.92
0.055 06 0.075 50 0.088 90 0.113 73
t ) 45 °C 6.861 9.359 10.981 13.984
-40.33 -39.88 -39.57 -39.14
0.139 43 0.159 79 0.179 86 0.203 79
17.046 19.464 21.815 24.612
-38.65 -38.34 -37.98 -37.62
L-Arginine
Monohydrochloride
0.013 24 0.026 47 0.043 94 0.061 68
2.059 4.105 6.776 9.494
-76.95 -76.33 -75.35 -74.79
0.077 86 0.099 72 0.127 48
t ) 5 °C 11.925 15.188 19.271
0.017 10 0.036 76 0.053 74 0.067 51
2.460 5.248 7.632 9.534
-60.66 -59.61 -58.86 -58.15
0.083 20 0.105 04 0.129 95
t ) 15 °C 11.729 14.688 18.034
-57.75 -56.72 -55.73
0.151 23 0.154 81 0.160 28
20.841 21.386 22.046
-54.85 -55.01 -54.57
0.013 35 0.029 99 0.046 64
1.788 3.988 6.190
-49.48 -48.64 -48.26
0.075 99 0.097 73 0.125 10
t ) 25 °C 10.010 12.776 16.244
-47.30 -46.45 -45.63
0.149 81 0.175 70 0.206 84
19.330 22.508 26.297
-44.89 -44.08 -43.22
0.012 43 0.023 79 0.044 82 0.061 26
1.495 2.920 5.495 7.503
-38.66 -39.93 -39.59 -39.33
0.086 07 0.113 41 0.150 51
t ) 35 °C 10.444 13.686 18.007
-38.42 -37.76 -36.83
0.169 72 0.190 42 0.206 17
20.240 22.609 24.387
-36.44 -35.96 -35.58
0.011 16 0.021 22 0.040 06 0.051 81
1.320 2.547 4.712 6.077
-35.78 -36.69 -35.12 -34.81
0.071 74 0.087 54 0.104 41
t ) 45 °C 8.357 10.156 12.047
-34.17 -33.77 -33.28
0.128 17 0.158 47 0.202 45
14.655 18.012 22.793
-32.52 -31.92 -31.03
L-Histidine
-73.94 -72.95 -71.67
0.164 38 0.190 48 0.209 09
24.654 28.410 31.059
-70.22 -69.22 -68.51
Monohydrochloride
0.029 09 0.053 23 0.070 29
3.812 6.955 9.121
-75.90 -75.12 -74.19
0.080 38 0.087 10 0.093 43
t ) 5 °C 10.424 11.264 12.074
0.033 62 0.058 88 0.079 12
3.901 6.864 9.238
-57.94 -57.90 -57.71
0.094 71 0.105 60 0.114 36
t ) 15 °C 11.056 12.334 13.334
-57.46 -57.35 -57.10
0.122 00 0.129 08
14.212 15.058
-56.93 -56.94
0.021 90 0.041 21 0.055 99
2.440 4.585 6.197
-50.54 -50.15 -49.60
0.066 65 0.076 25 0.086 02
t ) 25 °C 7.351 8.396 9.425
-49.23 -49.00 -48.56
0.094 16 0.100 79
10.322 11.017
-48.49 -48.22
0.023 47 0.041 04 0.057 04
2.419 4.248 5.884
-42.79 -42.83 -42.45
0.068 88 0.080 35 0.089 07
t ) 35 °C 7.120 8.278 9.134
-42.45 -42.13 -41.77
0.097 87 0.105 45
10.005 10.782
-41.51 -41.44
0.017 76 0.038 94 0.053 42
1.760 3.832 5.258
-39.08 -38.39 -38.22
0.064 65 0.073 25 0.080 11
t ) 45 °C 6.352 7.169 7.839
-37.99 -37.68 -37.60
0.086 08 0.091 47
8.436 8.949
-37.61 -37.46
tively. The F1 values were taken from the table given by Kell (1975): (0.999 964, 0.999 100, 0.997 045, 0.994 032, and 0.990 213) g‚cm-3 at (5, 15, 25, 35, and 45) °C,
-73.94 -73.58 -73.40
0.099 35 0.105 04
12.806 13.532
-73.07 -72.92
respectively. The density differences, F - F1, between solutions and pure water and the calculated Vφ values are summarized in Table 1.
212 Journal of Chemical and Engineering Data, Vol. 43, No. 2, 1998 The apparent molar isentropic compressibilities, Kφ, of the solutes were calculated using the equation
Kφ ) M2κ/F - (κ1F - κF1)/mFF1
(2)
where κ and κ1 are the isentropic compressibilities of the solution and water, respectively, and the other symbols are as defined for eq 1. The isentropic compressibility was determined from the speed of sound and density using the relation 2
κ ) 1/u F
V2°/ cm3‚mol-1
5 15 25 35 40 45 55
74.03(0.01)b 75.87(0.01) 77.29(0.01) 78.42(0.02)
5 15 25 35 40 45 55
90.13(0.01) 92.05(0.01) 93.56(0.02) 94.72(0.06)
79.32(0.02)
95.54(0.03)
5 95.48(0.02) 15 96.95(0.05) 25 98.89(0.03) 40 35 100.07(0.03) 45 101.05(0.01) 55
(4)
where X2° is infinite dilution value, which is equal to the partial molar quantity at infinite dilution, and Bx is the experimental slope. In the case of dilute electrolyte solutions, the apparent molar quantities of electrolytes are usually fitted to a Redlich-type equation in terms of molarity, c
Xφ ) X2° + Sxc1/2 + Bxc
t/ °C
(3)
The speed of sound differences, u - u1, between solution and water and the Kφ values are given in Table 2. The u1 values were taken from the table reported by Del Grosso and Mader (1972): (1426.162, 1465.931, 1496.687, 1519.808, and 1536.409) m‚s-1 at (5, 15, 25, 35, and 45) °C, respectively. The density values of the solutions used for the speed of sound measurements were calculated from eqs 4 or 5 for the apparent molar volume, described below. For sufficiently dilute nonelectrolyte solutions, the variation of the apparent molar quantities, Xφ (X refers to V or K in the present study), with molality can be represented by the linear relation
Xφ ) X2° + Bxm
Table 3. Limiting Partial Molar Volumes of Amino Acids in Water at 5, 15, 25, 35, and 45 °C
(5)
where Sx is a theoretical slope given by the Debye-Hu¨ckel limiting law and Bx is an empirical constant. The Sv values for the apparent molar volume of 1:1 electrolyte in water are (1.528, 1.696, 1.868, 2.046, and 2.233) cm3‚mol-3/2‚L1/2 at (5, 15, 25, 35, and 45) °C, respectively (Millero, 1979). For isentropic compressibility, unfortunately, the theoretical limiting slope Sk is not known, so we substituted isothermal Sk for isentropic Sk value, that is, (0.776, 1.744, 2.55, 3.26, and 3.913) cm3‚mol-3/2‚L1/2‚GPa-1 for 1:1 electrolyte in water at (5, 15, 25, 35, and 45) °C, respectively (Millero, 1979). By using these theoretical limiting slopes one can adequately determine the limiting partial molar quantity, X2°, from a linear plot of (Xφ - Sxc1/2) against c. Equation 4 was fitted to our Vφ and Kφ data for the hydrophilic amino acids (Asn, Gln, and His) and eq 5 for the ionic amino acids (AspNa, GluNa, LysHCl, ArgHCl, and HisHCl). The parameters of eqs 4 or 5 were determined by using the method of weighted least squares taking into consideration the density precision of 2 ppm and speed of sound precision of 7 ppm and are summarized in Tables 3 and 4. The V2° and K2° values available in the literature are also included in these tables. The comparison shows that, in most cases, there is a reasonable agreement between our values and those from literature within experimental uncertainties. The partial molar volumes of all the amino acids studied increase with increasing temperature, and furthermore their curves obtained are always concave downward. This feature is typical of aqueous electrolyte or hydrophilic nonelectrolyte solutions (Hepler, 1969). It is well-known that ionic groups of solute attract strongly surrounding
Bv/ cm3‚kg‚mol-2 a
V2°(lit.)/ cm3‚mol-1
L-Asparagine 2.9(0.2) 2.9(0.2) 76.0,c 76.28d 2.3(0.2) 77.2,c 77.63,d 77.18,e 77.52,f 1.7(0.3) 78.8,c 79.07d 0.5(0.3) 79.5,c 80.48d L-Glutamine 3.1(0.2) 2.5(0.1) 92.3,c 92.29d 1.2(0.3) 93.8,c 93.90,d 94.36,e 93.61,g -1.4(0.8) 94.9,c 95.14,d -0.7(0.4) 96.3,c 96.25,d L-Histidine 1.9(0.3) 2.6(0.7) 97.3c 0.4(0.4) 98.8,c 99.14,d 98.3,g 98.79h 100.4c -1.7(0.3) 99.9c -2.4(0.1) 101.6c L-Aspartic
5 15 25 35 45
59.13(0.02) 62.27(0.02) 63.93(0.02) 65.47(0.03) 66.11(0.03)
5 15 25 35 45
74.48(0.01) 77.21(0.02) 79.29(0.02) 80.76(0.02) 81.41(0.01)
5 15 25 35 45
121.52(0.01) 123.26(0.01) 124.77(0.02) 125.74(0.01) 126.35(0.01)
5 15 25 35 45
135.72(0.01) 138.08(0.02) 139.79(0.02) 140.82(0.02) 141.93(0.01)
5 15 25 35 45
109.76(0.04) 112.15(0.06) 113.79(0.04) 114.96(0.05) 115.75(0.05)
Acid Monosodium 6.6(0.2) 3.1(0.2) 4.0(0.2) 1.0(0.4) 1.6(0.2)
L-Glutaminic
Acid Monosdium 4.8(0.1) 3.8(0.1) 2.1(0.1) 0.9(0.2) 0.7(0.1)
L-Lysine
Monohydrochloride 3.6(0.1) 2.9(0.1) 124.5c 2.1(0.1) 125.9,c 124.76g 1.1(0.1) 0.0(0.1)
L-Arginine
Monohydrochloride 4.7(0.1) 3.5(0.1) 2.6(0.1) 140.06g 2.7(0.1) 1.1(0.1)
L-Histidine
Monohydrochloride 4.0(0.6) 3.3(0.9) 2.9(0.6) 1.3(0.5) 3.0(0.8)
a Unit for B in eq 5 for electrolyte solutions is cm3‚L‚mol-2. v Standard deviations are in parentheses. c Kharakoz (1989). d Hakin et al. (1995). e Jolicoeur et al. (1986). f Hedwig (1991). g Mishra et al. (1984). h Millero et al. (1978). b
water molecules, so-called electrostriction, which causes a large decrease both in volume and in compressibility of the solution. Since the partial molar compressibility is a more sensitive measure of solute-solvent interactions than is the partial molar volume (Høiland, 1986), the effect of electrostriction is more obvious for the former thermodynamic property. Thus, as can be seen from Table 4, the partial molar compressibilities of the amino acids are large negative at all temperatures studied.
Journal of Chemical and Engineering Data, Vol. 43, No. 2, 1998 213 Table 4. Limiting Partial Molar Isentropic Compressibilities of Amino Acids in Water at 5, 15, 25, 35, and 45 °C t/ K2°/ Bk/ K2°(lit.)/ °C cm3‚mol-1‚GPa-1 cm3‚kg‚mol-2‚GPa-1 a cm3‚mol-1‚GPa-1 L-Asparagine
5 15 25 35 40 45 55
-50.9(0.1)b -40.8(0.1) -33.4(0.1) -28.5(0.2)
5 15 25 35 40 45 55
-48.8(0.1) -37.4(0.1) -29.9(0.1) -24.5(0.2)
5 15 25 35 45 55
-56.7(0.7) -39.8(0.1) -29.6(0.5) -26.8(0.1) -19.7(0.3)
5 15 25 35 45
-114.8(0.1) -95.5(0.1) -84.2(0.1) -75.6(0.1) -71.1(0.1)
5 15 25 35 45
L-Glutaminic Acid Monosdium -115.8(0.1) 37.6(0.5) -96.5(0.1) 28.7(0.5) -84.8(0.1) 26.2(0.4) -76.3(0.1) 22.8(0.5) -70.0(0.1) 18.0(0.7)
9.1(1.4) 12.2(1.7) 9.0(1.9) 8.2(2.7)
-25.7(0.1)
11.5(1.1)
-40.4c -33.5,c -32.97,d -26.5c -22.5c
L-Glutamine
17.3(1.4) 11.0(1.1) 12.8(1.1) 8.0(3.6)
-20.3(0.1)
2.9(1.6)
-21.9c -16.6c
L-Histidine
24.1(9.4) 16.2(1.9) -7.8(7.4) 10.2(2.7) 2.1(4.7)
L-Aspartic
L-Lysine
-36.7c -28.9
-41.3c -32.5,c -31.84e -25.9c -18.0c
Acid Monosodium 49.0(1.2) 25.7(1.1) 23.3(0.9) 17.9(0.9) 19.8(0.9)
Monohydrochloride 33.2(0.4) 30.9(0.4) -61.6c 21.2(0.4) -51.8c 16.3(0.3) 13.0(0.4)
5 15 25 35 45
-78.6(0.1) -64.6(0.1) -53.6(0.1) -46.2(0.1) -41.9(0.1)
5 15 25 35 45
L-Arginine Monohydrochloride -77.3(0.1) 41.7(0.5) -61.2(0.1) 38.0(0.7) -50.0(0.1) 28.3(0.5) -41.1(0.1) 20.5(0.4) -36.5(0.1) 19.5(0.4)
5 15 25 35 45
L-Histidine Monohydrochloride -77.2(0.2) 39.9(2.4) -59.0(0.1) 11.8(1.4) -51.6(0.1) 26.3(2.1) -44.2(0.1) 17.0(1.9) -39.6(0.2) 11.5(2.6)
a Unit for Bk in eq 5 for electrolyte solutions is cm3‚L‚mol-2‚GPa-1. b Standard deviations are in parentheses. c Kharakoz (1991). d Hedwig (1991). e Millero et al. (1978).
In Figure 1 are shown the temperature dependences of the limiting partial molar isentropic compressibilities of the amino acids studied. The compressibility decreases steeply with decrease of temperature; that is characteristic for dilute aqueous mixtures, regardless of whether they are hydrophilic or hydrophobic solutes (Kikuchi et al., 1995; Sakurai et al., 1995). A particularly intriguing feature is that the negative partial molar isentropic compressibilities of two sodium salts (AspNa and GluNa) are significant in magnitude compared with the hydrochloride salts (LysHCl, ArgHCl, or HisHCl): the difference between the two salt
Figure 1. Temperature dependence of the limiting partial molar isentropic compressibilities of hydrophilic amino acids in dilute aqueous solutions: (0) L-glutamine; (b) L-asparagine; (4) Lhistidine; (2) L-arginine monohydrochloride; ([) L-lysine monohydrochloride; (O) L-histidine monohydrochloride; (]) L-glutamic acid monosodium; (9) L-aspartic acid monosodium.
series is more than 30 cm3‚mol-1‚GPa-1. This may be consistent with the difference between the partial molar isentropic compressibilities of sodium alkanecarboxylates and alkylammonium chlorides having the same alkyl groups: 40-50 cm3‚mol-1‚GPa-1 (Høiland, 1986). Therefore, if we assume the ionic partial molar compressibilities of Na+ and Cl- ions to be -33.5 and -17.0 cm3‚mol-1‚GPa-1, respectively (Conway, 1978), partial molar compressibility for the COO- ion is more negative than that for the NH3+ ion by 23-33 cm3‚mol-1‚GPa-1. Since the contribution of a methylene group to the partial compressibility is negative at ambient temperature (Sakurai, 1995), the ions having larger hydrophobic groups may be expected to have more negative values of the partial molar compressibility. Therefore, taking into account that the alkyl chain in AspNa or GluNa is shorter than that in the hydrochloride salts, such a large difference suggests a stronger electrostriction effect by COO- rather than by NH3+ ion. Literature Cited Conway, B. E. The evaluation and use of properties of individual ions in solution. J. Solution Chem. 1978, 7, 721-770. Del Grosso, V. A.; Mader, C. W. Speed of sound in pure water. J. Acoust. Soc. Am. 1972, 52, 1442-1446. Hakin, A. W.; Duke, M. M.; Groft, L. L.; Marty, J. L.; Rushfeldt, M. L. Calorimetric investigations of aqueous amino acid and dipeptide systems from 228.15 to 328.15 K. Can. J. Chem. 1995, 73, 725734. Hedwig, G. R. Thermodynamic properties of peptide solutions. 3. Partial molar volumes and partial molar heat capacities of some tripeptides in aqueous solution. J. Solution Chem. 1988, 17, 383397. Hedwig, G. R. The partial molar volumes and partial molar isentropic pressure coefficients of the amino acid L-asparagine in aqueous solution at the temperature 298.15 K. J. Chem. Thermodyn. 1991, 23, 123-127. Hedwig, G. R.; Reading, J. F.; Lilley, T. H. Aqueous solutions containing amino acids and peptides. 27. Partial molar heat capacities and partial molar volumes of some N-acetyl amino acid amides, some N-acetyl peptide amides and two peptides at 25 °C. J. Chem. Soc., Faraday Trans. 1991, 87, 1751-1758. Hepler, L. G. Thermal expansion and structure in water and aqueous solutions. Can. J. Chem. 1969, 47, 4613-4617. Høiland, H. In Thermodynamic Data for Biochemistry and Biotechnology; Hinz, H.-J., Ed.; Springer-Verlag: Berlin; 1986; Chapters 2 and 4. Jolicoeur, C.; Riedl, B.; Desrochers, D.; Lemelin, L. L.; Zamojska, R.; Enea, O. Solvation of amino acid residues in water and urea-water
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Millero, F. J.; Surdo, A. L.; Shin, C. The apparent molal volumes and adiabatic compressibilities of aqueous amino acids at 25 °C. J. Phys. Chem. 1978, 82, 784-792. Mishra, A. K.; Ahluwalia, J. C. Apparent molal volumes of amino acids, N-acetylamino acids, and peptides in aqueous solutions. J. Phys. Chem. 1984, 88, 86-92. Mizuguchi, M.; Sakurai, M.; Nitta, K. Partial molar volumes and adiabatic compressibilities of N-Acetyl-DL-serinamide and N-AcetylL-threoninaminde in dilute aqueous solutions. J. Solution Chem. 1997, 36, 579-594. Sakurai, M.; Nakamura, K.; Takenaka, N. Apparent molar volumes and apparent molar adiabatic compressions of water in some alcohols. Bull. Chem. Soc. Jpn. 1994, 67, 352-359. Sakurai, M.; Nakamura, K.; Nitta, K. Sound velocities and apparent molar adiabatic compressions of alcohols in dilute aqueous solutions. J. Chem. Eng. Data 1995, 40, 301-310.
Received for review July 23, 1997. Accepted November 14, 1997.
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