414
`
`J. Chem. Eng. Data 1996, 41, 414-417
`
`Solubilities of L-Cystine, L-Tyrosine, L-Leucine, and Glycine in
`Aqueous Solutions at Various pHs and NaCl Concentrations
`
`Renzo Carta*,† and Giuseppe Tola
`
`Dipartimento di Ingegneria Chimica e Materiali, Universita` di Cagliari, 09123 Cagliari, Italy
`
`Solubilities at 25 °C of four amino acids (L-cystine, L-tyrosine, L-leucine, and glycine) in aqueous solutions
`at various pHs and NaCl concentrations (0, 1, 3 mol dm-3) are reported. The pH was varied from 0 to 13
`using HCl or NaOH. The NaCl concentration strongly affects the solubilities of L-cystine, L-tyrosine,
`and L-leucine. In particular, the addition of NaCl increases the L-cystine solubility but reduces the
`solubility of L-tyrosine and L-leucine. The NaCl concentration has only a slight effect on the solubility of
`glycine. Experimental solubilities are correlated with the use of a chemical equilibrium model, good
`agreement in the pH range 1-12 was found. The analysis confirms the dependence reported in the
`literature between the solubility of amino acids and the salt concentration. For the amino acids examined,
`the parameters to calculate their solubilities in solutions with NaCl are also given.
`
`Introduction
`The recovery of amino acids by hydrolysis of keratinic
`material is of increasing importance due to progress in
`separation technologies. Typically, the hydrolysis is carried
`out in either strong acid or basic solutions. Moreover, some
`amino acids are recovered as precipitates by neutralization
`of the solution, and because the neutralization process
`leads to solutions with high salt contents, it is of interest
`to investigate the salt effect on the solubility of amino acids.
`In recent years, the study of the equilibrium of amino
`acids has received renewed attention because of industrial
`interest in these products. These studies have also been
`motivated by the lack of data and by the fact that the only
`complete experimental solubilities date back to many years
`ago (Greenstein and Winitz, 1961; Dalton and Schmidt,
`1933, 1935). Additional solubility data have been reported
`by various researchers, i.e. Needham et al. (1971), Orella
`and Kirwan (1989), Zumstein and Rousseau (1989), and
`Gatewood and Rousseau (1994). All the references deal
`with single amino acids, while some data on mixtures were
`published by Chen et al. (1989) and Jin and Chao (1992).
`Our objective is to measure the solubility of amino acids
`in water at various pHs and NaCl concentrations. These
`results will be useful in the development of solution models
`to describe the thermodynamic properties of amino acid
`solutions. In particular, this work focuses on the solubility
`of L-cystine, L-tyrosine, L-leucine, and glycine.
`
`Experimental Section
`Solutions without salt are prepared by adding com-
`mercial HCl (Carlo Erba RPE 1 mol dm-3) or NaOH (Carlo
`Erba RPE 0.1 mol dm-3) to bidistilled water so as to attain
`a fixed pH. An excess of amino acid (L-cystine and
`L-tyrosine, Aldrich Chemie 99%; L-leucine and glycine,
`Jansen >99%) was then added to the solutions and the
`flask was maintained in a thermostatic bath at (25 ( 0.1)
`°C. The amino acid solutions were continuously shaken
`with a magnetic drive agitator to establish equilibrium.
`Mixtures formed by the acid or basic solutions and the
`added amino acid were put in flasks and stirred for
`sufficient time (48 h) to reach equilibrium between the solid
`amino acid and the liquid solution.
`
`The stirring time of 48 h was determined so as to achieve
`equilibrium conditions. The possibility of reaching the
`equilibrium condition at 25 °C by starting from different
`temperatures was checked by approaching the target of 25
`°C with two different solutions of amino acids: the first
`was equilibrated at 30 °C and the second at 20 °C; then
`the two solutions were brought to 25 °C and left to
`equilibrate at this temperature. No significant differences
`were revealed.
`The hydration of the solid phase was checked in the
`following way: a weighed quantity of each of the amino
`acids examined was kept in contact with HPLC water for
`48 h; the mixture thus obtained was filtered, and the solid,
`after being left to dry at room temperature in vacuo (<0.5
`Torr), was weighed. The mass of the amino acids and the
`amount dissolved coincide with the original quantities that
`Aldrich Chemie and Jansen certify as anhydrous.
`After equilibrium was established, a syringe was used
`to take a sample of about 10 cm3. This sample was filtered
`in a thermostated filter, and the sample for the analysis
`was taken from the clear liquid.
`L-Cystine and L-tyrosine exhibited very long dissolution
`times (about 24 h), especially when using a low amount of
`solid material. Due to these difficulties, we operated with
`amounts of solids at least 2 times greater than those
`required to achieve equilibrium. The experimental solu-
`bilities as a function of pH, obtained in the solutions
`without salt, are reported in Table 1. The pH values,
`measured with a pH meter (Metrohm 691 precision (0.01
`units), are not the equilibrium values, but those of the
`initial solutions. This is very significant, especially in pH
`regions close to neutral. In fact, in these zones pHs are
`very different from those of the equilibrium solutions, but
`they represent the acid or base concentrations.
`The solubility of the amino acids in salt solutions was
`studied using the same procedure. The only difference was
`that the original solutions were prepared by adding the
`amount of anhydrous NaCl (Carlo Erba RPE 99.5 mol %)
`required to reach the selected molar concentrations. The
`measured solubilities, at different salt concentrations, are
`shown in Tables 2 and 3.
`Each solubility was calculated by averaging the values
`obtained from three series of samples taken from three
`different flasks. The maximum deviations in the nine
`samples were (1 (cid:2) 10-5 mol dm-3 for L-cystine and
`0021-9568/96/1741-0414$12.00/0 © 1996 American Chemical Society
`
`† E-mail: carta@ndchem3.unica.it.
`
`Downloaded via 50.239.31.27 on May 13, 2024 at 23:10:13 (UTC).
`
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`1
`
`

`

`the major problem was represented by the interference of
`the absorption spectra of HCl and NaOH with that of
`L-cystine. Indeed, hydrochloric acid and sodium hydroxide
`exhibit high-intensity absorbance in the region where a
`maximum ((cid:25)210 nm) for L-cystine is attained. In contrast,
`L-tyrosine has a maximum at about 270 nm where both
`the HCl and NaOH absorbance is negligible. For this
`reason it was very important, particularly for the L-cystine
`analyses, to have, as a blank, solutions with acid or base
`concentrations the same as those of the samples because
`small differences lead to faulty measurements.
`Calibration curves were made for L-cystine at 215 nm
`and for L-tyrosine at 270 nm. Absorption data at various
`L-cystine and L-tyrosine concentrations were correlated by
`a linear regression; the extinction coefficients are 6.6778
`(dm3 g-1) for L-cystine (regression coefficient 0.998) and
`6.2641 (dm3 g-1) for L-tyrosine (regression coefficient 0.998).
`The solutions containing L-leucine and glycine were
`analyzed by determining the mass of the residue obtained
`by evaporating known volumes of samples. Due to their
`solubility, the small volumes (1-5 cm3) considered contain
`at equilibrium detectable masses of solid amino acids. This
`method, which was extensively used by Jin and Cao (1992)
`and Needham et al. (1971), was first tested by determining
`the mass of the dissolved L-leucine and glycine from dried
`samples obtained from solutions with known amounts of
`dissolved amino acids. Quantitative detection was carried
`out from the mass of the solid obtained after the liquid of
`the sample was evaporated at (50 to 60) °C at atmospheric
`pressure. This maximum temperature was fixed to avoid
`a rapid evaporation, with possible losses of amino acids,
`and any thermal decomposition. The solid residue masses
`were determined, and when present, the mass of added
`NaCl was taken into account. A series of preliminary
`measurements was made to verify the reliability of the
`method.
`
`Journal of Chemical and Engineering Data, Vol. 41, No. 3, 1996 415
`Table 1. Experimental Solubilities s at 25 (cid:176)C (Solutions
`without NaCl)
`L-tyrosine
`L-cystine
`s/
`s/
`mol(cid:226)dm-3
`mol(cid:226)dm-3
`pH
`pH
`pH
`0.00
`1.00 0.035 75
`1.00 0.017 01
`0.27
`1.43 0.013 48
`1.41 0.004 09
`0.53
`1.74 0.008 01
`1.82 0.001 80
`0.73
`1.95 0.006 13
`2.29 0.000 90
`1.11
`2.69 0.003 48
`3.04 0.000 70
`2.00
`3.48 0.002 83
`3.42 0.000 70
`3.02
`4.45 0.002 86
`4.02 0.000 70
`3.50
`5.39 0.002 70
`5.08 0.000 71
`4.50
`7.00 0.002 80
`7.00 0.000 69
`7.00
`9.12 0.002 71
`8.61 0.000 70
`9.53 0.002 77 10.70
`10.71 0.001 13
`11.29 0.002 10 11.05 0.003 60 11.76
`11.75 0.004 25 11.48 0.005 78 12.20
`12.13 0.008 79 12.00 0.007 80 12.54
`12.65 0.026 21 12.57 0.028 56 12.71
`12.96 0.027 83 12.92 0.053 48 13.00
`13.00 0.028 25 13.00 0.060 02
`Table 2. Experimental Solubilities s at 25 (cid:176)C (Solutions
`with 1 mol(cid:226)dm-3 of NaCl)
`L-cystine
`L-tyrosine
`s/
`s/
`mol(cid:226)dm-3
`mol(cid:226)dm-3
`pH
`pH
`pH
`0.00
`1.00 0.023 65
`0.88 0.036 35
`0.30
`1.41 0.006 17
`1.37 0.013 41
`1.03
`1.93 0.002 12
`1.88 0.006 09
`1.97
`2.47 0.001 16
`2.44 0.003 67
`3.11
`2.96 0.000 95
`2.96 0.002 97
`4.05
`4.16 0.000 85
`3.06 0.002 90
`5.07
`5.50 0.000 84
`3.19 0.002 83
`5.40
`5.78 0.000 84
`4.65 0.002 71
`7.00
`7.00 0.000 85
`7.00 0.002 76
`8.04
`8.79 0.000 83
`9.53 0.002 76
`8.98
`9.28 0.000 87 10.42 0.003 03
`11.11 0.001 34 10.92 0.003 35 10.07
`11.89 0.003 30 11.68 0.004 76 11.03
`12.52 0.007 83 12.31 0.009 52 12.02
`12.91 0.019 67 12.80 0.022 46 13.00
`13.00 0.033 92 13.00 0.059 12
`Table 3. Experimental Solubilities s at 25 (cid:176)C (Solutions
`with 3 mol(cid:226)dm-3 of NaCl
`glycine
`L-leucine
`L-cystine
`L-tyrosine
`s/
`s/
`s/
`s/
`mol(cid:226)dm-3
`mol(cid:226)dm-3
`mol(cid:226)dm-3
`mol(cid:226)dm-3
`pH
`pH
`pH
`pH
`3.313
`0.00
`0.4514
`0.00
`0.67 0.039 67
`0.95 0.025 89
`2.916
`0.31
`0.1029
`0.31
`0.77 0.025 00
`1.49 0.006 22
`2.679
`1.03
`0.1189
`1.03
`1.18 0.007 83
`2.05 0.003 84
`2.636
`2.07
`0.0914
`2.07
`1.68 0.003 08
`2.56 0.002 17
`2.619
`3.12
`0.0700
`3.12
`2.62 0.001 28
`2.73 0.002 08
`2.617
`4.18
`0.0804
`4.18
`2.91 0.001 13
`2.89 0.002 00
`2.599
`4.88
`0.0891
`4.88
`3.97 0.001 10
`3.17 0.001 91
`2.595
`6.11
`0.0750
`6.11
`5.00 0.001 10
`4.39 0.001 82
`2.651
`7.00
`0.0741
`7.00
`7.00 0.001 10
`7.00 0.001 97
`2.604
`7.87
`0.0834
`7.87
`8.61 0.001 09
`9.45 0.001 97
`2.619
`9.12
`0.0763
`9.12
`10.61 0.001 61 10.52 0.002 21
`2.656
`10.07
`0.0921
`11.50 0.002 77 10.82 0.002 34 10.07
`2.649
`11.04
`0.0914
`12.02 0.003 96 11.02 0.002 64 11.04
`2.720
`11.98
`0.0837
`12.69 0.008 08 11.65 0.003 81 11.98
`12.91 0.022 00 12.33 0.008 46 13.00
`0.1832
`13.00
`3.083
`13.00 0.042 12 13.00 0.046 74
`L-tyrosine, (2.5 (cid:2) 10-4 mol dm-3 for L-leucine, and (5 (cid:2)
`10-4 mol dm-3 for glycine.
`Chemical analyses were carried out using two different
`procedures. The solutions containing L-cystine and L-
`tyrosine were analyzed spectrophotometrically (Shimatzu
`UV 160A). Due to their low solubilities, it was sufficient
`to dilute the sample, taken from the solution at equilibri-
`um,
`in a 1:2 ratio so as to avoid saturation of the
`absorbance signal.
`Preliminary spectrophotometric analyses were carried
`out to assess the reliability of the method. In particular,
`
`
`
`A(aq)( ) A(aq)- + H(aq)
`
`
`
`+
`
`0
`are the amino acids in the solid and
`where A(s) and A(aq)
`aqeous phases, respectively, A( represents the zwitterion,
`and A+, A2+, A-, and A2- are the charged forms of the amino
`acids.
`In the case of L-tyrosine (trivalent) and L-cystine (tet-
`ravalent), the equilibrium equations are similar but it is
`necessary to consider one (eq 6) or two (eqs 6 and 7) more
`equations.
`
`
`A(aq)- ) A(aq)2- + H(aq)
`
`
`A(aq)2+ ) A(aq)+ + H(aq)
`
`+
`
`+
`
`
`
`L-leucine
`s/
`mol(cid:226)dm-3
`0.9237
`0.7427
`0.4938
`0.3619
`0.3145
`0.2154
`0.1745
`0.1721
`0.1689
`0.1772
`0.1741
`0.2234
`0.1916
`0.2032
`0.2136
`0.3357
`
`glycine
`s/
`mol(cid:226)dm-3
`3.504
`3.089
`2.759
`2.755
`2.747
`2.721
`2.713
`2.728
`2.667
`2.741
`2.712
`2.757
`2.656
`2.667
`2.805
`3.052
`
`pH
`0.00
`0.27
`0.53
`0.73
`1.11
`2.00
`3.02
`3.50
`4.50
`7.00
`8.98
`9.99
`11.51
`12.11
`12.49
`13.00
`
`L-leucine
`s/
`mol(cid:226)dm-3
`0.6053
`0.5824
`0.3739
`0.1595
`0.1372
`0.1308
`0.1305
`0.122
`0.1289
`0.1308
`0.1173
`0.1008
`0.1181
`0.1838
`0.2412
`
`glycine
`s/
`mol(cid:226)dm-3
`3.580
`3.203
`2.896
`2.780
`2.732
`2.724
`2.729
`2.711
`2.720
`2.741
`2.723
`2.693
`2.708
`2.745
`3.045
`
`pH
`0.00
`0.33
`1.06
`1.48
`2.51
`3.50
`4.10
`5.15
`7.00
`8.04
`8.98
`10.06
`11.03
`12.02
`13.00
`
`Discussion of the Results
`The solubilities were compared with those predicted by
`a simple model which takes into account only the chemical
`equilibria with the activities considered equal to the
`concentrations. For a divalent amino acid (A) in aqueous
`solution the following equilibria have to be considered:
`
`A(s) ) A(aq)
`
`0
`
`(
`A(aq)0 ) A(aq)
`
`
`
`K1
`
`K2
`
`
`
`H2O(l) ) OH(aq)- + H(aq)
`
`+
`
`
`
`A(aq)+ ) A(aq)( + H(aq)
`
`
`
`+
`
`(1)
`
`(2)
`
`(3)
`
`(4)
`
`(5)
`
`K3
`
`K4
`
`K5
`
`K6
`
`K7
`
`(6)
`
`(7)
`
`2
`
`

`

`416 Journal of Chemical and Engineering Data, Vol. 41, No. 3, 1996
`
`Table 4. Solubility Constants K8 (mol(cid:226)dm-3) for
`Zwitterions
`L-leucine
`L-tyrosine
`L-cystine
`1.72 (cid:2) 10-1
`2.74 (cid:2) 10-3
`6.92 (cid:2) 10-4
`Table 5. Chemical Equilibrium Constants
`components K4/mol(cid:226)dm-3 K5/mol(cid:226)dm-3 K6/mol(cid:226)dm-3 K7/mol(cid:226)dm-3
`1.62 (cid:2) 10-1
`1.55 (cid:2) 10-2
`5.75 (cid:2) 10-9
`3.24 (cid:2) 10-9
`L-cystine
`6.31 (cid:2) 10-3
`7.76 (cid:2) 10-10 8.51 (cid:2) 10-11
`L-tyrosine
`2.29 (cid:2) 10-3
`5.02 (cid:2) 10-10
`L-leucine
`2.18 (cid:2) 10-3
`5.02 (cid:2) 10-10
`glycine
`
`glycine
`2.44
`
`Greenstein and Winitz (1961) reported values of KD (ratio
`of zwitterion concentrations to concentrations uncharged
`forms of amino acids) for amino acids in the range 105 to
`106. Thus, reactions 1 (characterized by the thermody-
`namic solubility constant K1) and 2 can be combined as
`follows (Pinho et al., 1994):
`(
`A(s) ) A(aq)
`
`K8
`
`(8)
`
`Thus, for pratical purposes
`K1 ) K8 ) a(
`The equilibrium constants (Ki) for the reactions 3-7 are
`given as follows:
`
`(9)
`
`K3 ) aH+aOH-
`K4 ) aA(aH+/aA+
`K5 ) aA-aH+/aA(
`K6 ) aA2-aH+/aA-
`K7 ) aA+aH+/aA2+
`
`ai ) (cid:231)imi
`
`(10)
`
`(11)
`
`(12)
`
`(13)
`
`(14)
`
`(15)
`
`where
`
`(cid:231)i is the molal activity coefficient, mi and ai are respectively
`the molality and activity coefficient of species i.
`If the activity coefficient of the zwitterion ((cid:231)z) can be
`assumed equal to unity, K1 corresponds to the solubility of
`the zwitterion species. Thus, in the isoelectric range, where
`amino acid concentrations are low, we can assume that (cid:231)z
`) 1 and evaluate Ks by measuring the solubility in this
`
`Figure 2. Comparison between experimental and calculated
`solubilities at 25 °C for L-tyrosine in aqueous solutions at various
`pHs (experimental data: (O) this work, (4) Dalton and Schmidt,
`1933; (s) calculated).
`
`Figure 3. Comparison between experimental and calculated
`solubilities at 25 °C for L-leucine in aqueous solutions at various
`pHs (experimental data: (O) this work; (4) Dalton and Schmidt,
`1933; (]) Gatewood and Rousseau, 1994; (s) calculated).
`
`Figure 4. Comparison between experimental and calculated
`solubilities at 25 °C for glycine in aqueous solutions at various
`pHs (experimental data: (O) this work; (4) Dalton and Schmidt,
`1933; (]) Orella and Kirwan, 1989; (s) calculated).
`
`range. The assumption that (cid:231)z ) 1 seems reasonable for
`L-cystine, L-tyrosine, and L-leucine since their solubilities
`are low (mole fractions <10-3). This assumption becomes
`less accurate for glycine because of its relatively high
`solubility (mole fraction about 5 (cid:2) 10-2).
`The values of the solubility constants used in the model
`are given in Table 4. For L-cystine and L-tyrosine these
`parameters were evaluated from the solubility data in the
`
`Figure 1. Comparison between experimental and calculated
`solubilities at 25 °C for L-cystine in aqueous solutions at various
`pHs (experimental data: (O) this work; (4) Dalton and Schmidt,
`1933; (s) calculated).
`
`3
`
`

`

`Journal of Chemical and Engineering Data, Vol. 41, No. 3, 1996 417
`
`Figure 5. Amino acid solubilities as a function of NaCl concen-
`tration: L-cystine (calc (s), exp (b )), L-tyrosine (calc (- -), exp
`(O)), L-leucine (calc (- - -), exp (4), glycine (calc (- - -), exp (])).
`Table 6. Salting Constants Kso (mol(cid:226)dm-3)
`L-cystine
`L-tyrosine
`L-leucine
`-0.044
`-0.112
`0.068
`
`glycine
`-0.002
`
`L-tyrosine and L-leucine decreases with increased NaCl
`concentration.
`
`Conclusions
`We have presented experimental results on the solubili-
`ties of L-cystine, L-tyrosine, L-leucine, and glycine which
`are in good agreement with results previously reported in
`the literature. As the pH of the solution is lowered, the
`chemical equilibria, as shown in eqs 3-7, move toward the
`left and amino acid solubility increases because of the
`stabilization of the cation species. For the less soluble
`amino acids (L-cystine and L-tyrosine) and for L-leucine, the
`use of a simple model that only accounts for the chemical
`equilibria and which assumes the activities equal to the
`concentrations appears to be adequate to represent the
`solubilities in the pH range from 1 to 12. For more soluble
`amino acids, or for higher acid or base concentrations, the
`model is not adequate.
`The addition of sodium chloride has different effects on
`the solubility of the amino acids studied. The solubility of
`L-cystine increases by about 50% as the NaCl concentration
`is changed from (0 to 3) mol(cid:226)dm-3; with the same increase
`in NaCl concentration, the solubility of L-tyrosine decreases
`by about 40% and that of L-leucine by more than 50%. The
`solubility of glycine, in the pH region 0-13, is only slightly
`influenced by changes in the salt concentration.
`The linear dependence of the logarithm of the ratio of
`the solubilities of the amino acids with and without salt
`confirms the relationship (16) and allows us to calculate
`the effect of the salt on the amino acid concentration at
`saturation conditions.
`Literature Cited
`Chen, C.-C.; Zhu, Y.; Evans, L. B. Phase Partitioning of Biomolecules:
`Solubilities of Amino Acids. Biotechnol. Prog. 1989, 5, 111-118.
`Dalton, J. D.; Schmidt, C. L. A. The Solubility of Certain Amino Acids
`in Water, The Densities of Their Solutions at Twenty-Five Degrees,
`and the Calculated Heats of Solution and Partial Molar Volumes.
`J. Biol. Chem. 1933, 103, 539-545.
`Dalton, J. D.; Schmidt, C. L. A. The Solubility of Certain Amino Acids
`in Water, The Densities of Their Solutions at Twenty-Five Degrees,
`and the Calculated Heats of Solution and Partial Molar Volumes
`II. J. Biol. Chem. 1935, 109, 241-248.
`Fasman G. D. Handbook of Biochemistry and Molecular Biology, 3rd
`ed.; CRC Press: Boca Raton, FL, 1976.
`Gatewood, M. D.; Rousseau, R. W. Effect of Sodium Hydroxide on the
`Solubilities of L-Isoleucine, L-Leucine, and L-Valine. Biotechnol. Prog.
`1994, 10, 253-257.
`Greenstein, J. P.; Winitz, M. Chemistry of the Amino Acids; John Wiley
`& Sons: New York, 1961; Vol. I.
`Jin, X. Z.; Chao, K.-C. Solubility of Four Amino Acids in Water and
`Four Pairs of Amino Acids in Their Water Solutions. J. Chem. Eng.
`Data 1992, 37, 199-203.
`Needman, T. E.; Paruta, A. N.; Gerraughty, R. J. Solubility of Amino
`Acids in Pure Solvent Systems. J. Pharm. Sci. 1971, 60, 565-567.
`Orella, C. J.; Kirwan, D. J. The Solubility of Amino Acids in Mixtures
`of Water and Aliphatic Alcohols. Biotechnol. Prog. 1989, 5, 89-91.
`Pinho, S. P.; Silva, C. M.; Macedo, E. A. Solubility of Amino Acids: A
`Group Contribution Model Involving Phase and Chemical Equilib-
`ria. Ind. Eng. Chem. Res. 1994, 33, 1341-1347.
`Zumstein, R. C.; Rousseau, R. W. Solubility of L-isoleucine and
`Recovery from Neutral and Acidic Solutions. Ind. Eng. Chem. Res.
`1989, 28, 1226-1231.
`
`(16)
`
`Received for review July 28, 1995. Revised November 17, 1995.
`Accepted January 16, 1996.X
`JE9501853
`
`isoelectric range obtained in this work, while the values
`for L-leucine and glycine were calculated using a relation
`introduced by Pinho et al. (1994)
`The equilibrium constants for reactions 4-7 of the amino
`acids were taken from the literature (Greenstein and
`Winitz, 1961) and are shown in Table 5.
`In Figures 1-4 the solubilities obtained from the model
`are compared with our experimental values, and literature
`data are also shown. The agreement is quite good for three
`of the amino acids studied, while calculated solubilities for
`glycine differ from the experimental values, in the isoelec-
`tric region, by about 20%. This high difference may be due
`to the value of the solubility constant which was obtained
`by using an equation with great sensitivity to the param-
`eters (Pinho et al., 1994). Also the assumption that (cid:231)z ) 1
`may not be sufficiently accurate and could explain these
`high differences. In fact the activity coefficient reported
`in the literature (Fasman, 1976) for solutions of glycine
`containing 3.0 mol dm-3 of amino acid is 0.742.
`The effect of NaCl is shown in Figure 5, which is a plot
`of solubility in neutral solutions as a function of the NaCl
`concentration (C). It appears to be a linear dependence
`between the logarithm of the ratio of the solubility with
`(s) and without (s0) NaCl and the salt concentration that
`allows us to calculate the salting out or salting in constants,
`Kso, by a linear regression. The values so obtained are
`reported in Table 6. The straight line confirms the
`dependence usually given in the literature (Greenstein and
`Winitz, 1961):
`
`log( s
`
`) ) KsoC
`
`s0
`
`The solubility of L-cystine increases with salt concentra-
`tion (C), glycine is little affected, while the solubility of
`
`X Abstract published in Advance ACS Abstracts, March 1, 1996.
`
`4
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`