Saturated Solubility and Thermodynamic Evaluation of l-Tryptophan in

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Saturated Solubility and Thermodynamic Evaluation of L‑Tryptophan in Eight Pure Solvents and Three Groups of Binary Mixed Solvents by the Gravimetric Method at T = 278.15−333.15 K Wenjun Zhu, Yuwen Fan, Qiang Xu, Xinxin Liu, Bin Heng, Wenge Yang, and Yonghong Hu*

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College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, No. 30, South Puzhu Road, Nanjing 211816, People’s Republic of China ABSTRACT: In this research, solubility of L-tryptophan in water, methanol, ethanol, acetone, isopropanol, n-butanol, acetonitrile, ethyl acetate, organic pure solvent, and in three binary mixed solvents (water + acetonitrile), (water + methanol), and (water + isopropanol) at the temperature range of T = 278.15−333.15 K was determined with the gravimetric method. The experimental results show that the solubility of L-tryptophan in the above solvents increased as temperature rose, and it achieved the highest dissolution efficiency in water. The improved Apelblat model, the Buchowski−Ksiazaczak λh model, the Redlich− Kister (CNIBS/R−K) model, and the Jouyban−Acree model were used to nonlinearly fit the experimental data of solubility, and the consistency was good. The physicochemical information of L-tryptophan provided in this report may be helpful for its extraction/separation, recrystallization, purification, and formulation development.

1. INTRODUCTION L-Tryptophan has the chemical name L-2-amino-3-indolyl-1propionic acid, also known as L-trypsin amino acid, which is a white or almost white crystalline powder. Its molecular formula is C11H12N2O2, molecular weight is 204 g mol−1, and CAS is 7322-3.1 Its molecular structure is depicted in Figure 1.

widely used in the pharmaceutical, feed, and food industries. At present, the production method is mainly fermentation, and the fermentation liquid is an extremely complicated multiphase system. How to take effective measures to separate and extract tryptophan from the fermentation liquid, thereby increasing the total yield, is currently an urgent problem to be solved in the industrial production of L-tryptophan. The purpose of this study was to determine the solubility of Ltryptophan in water, methanol, ethanol, acetonitrile, acetone, isopropanol, n-butanol, ethyl acetate, (water + acetonitrile), (water + methanol), and (water + isopropanol) with gravimetric analysis at the temperature range of 273.15−333.15 K. To better understand the solvation ability of L-tryptophan, its relative stability in the solvent was explained; the correlation between thermodynamic data and solubility can guide the separation and characterization of L-tryptophan.

Figure 1. Structure of L-tryptophan.

2. EXPERIMENTAL SECTION 2.1. Materials and Apparatus. L-Tryptophan with a mass fraction purity >98% was purchased from Aladdin (China). Its purity was detected by high-performance liquid chromatography (HPLC type DIONEX P680 DIONEX Technologies). The water sample was purified by distillation (Milli-Q, Nanjing Kelinde Scientific Instrument Co., Ltd.). Details of solvents are enumerated in Table 1. In our laboratory, all solvents are analytical-grade reagents, and the mass fraction purities of its

L-Tryptophan

is one of the common amino acids that assemble proteins.2 It is one of the eight essential amino acids required by mammals and is also a raw sugar amino acid.3 LTryptophan is capable of producing serotonin, which is essentially a neurotransmitter that regulates mood disorders and relieves symptoms such as irritability, anxiety, sleep disorders, and depression.4−6 At the same time, it was found that L-tryptophan can increase satiety and stimulate intestinal hormone release.7 L-Tryptophan can regulate protein synthesis,8 regulate immune and digestive functions, increase serotonin metabolism, and enhance cognitive ability,9 so it plays an important role in human metabolism and growth. These nutritional and medicinal properties of L-tryptophan make it © XXXX American Chemical Society

Received: June 16, 2019 Accepted: August 20, 2019

A

DOI: 10.1021/acs.jced.9b00562 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Properties of Materials materials

CAS reg. no.

molar mass (g/mol)

provenance

mass fraction purity (%)

purification method

analysis method

L-tryptophan

73-22-3 7732-18-5 67-56-1 71-36-3 67-64-1 64-17-5 75-05-8 141-78-6 67-63-0

204.23 18.01 32.04 74.12 58.08 46.07 41.05 88.11 60.06

Aladdin self-preparation Shenbo chemicals Shenbo chemicals Shenbo chemicals Shenbo chemicals Shenbo chemicals Shenbo chemicals Shenbo chemicals

99.9 100 99.9 99.5 99.5 99.7 99.9 99.7 99.7

none none none none none none none none none

HPLC GC GC GC GC GC GC GC GC

water methanol 1-butanol acetone ethanol acetonitrile ethyl acetate isopropanol

where mi = 1−3 and Mi = 1−3 denote the mass and the molar mass of L-tryptophan, respectively, in which index i = 1 and 2 represents L-tryptophan and water, respectively, and i = 3 denotes acetonitrile, methanol, or isopropanol. All molar quantities are based on the IUPAC relative atomic mass table.

were determined by gas chromatography, which were higher than 99%. Chemicals were used without further purification during the experiment. The analytical balance (Sartorius, BS210s, Germany) was purchased from Suzhou Spirit Weighing Scientific Instrument Co., Ltd. with a resolution of ±0.1 mg. The smart water circulating thermostatic bath system (model: CH1015) was bought from Shanghai Pingxuan Scientific Instrument Co., Ltd, with accuracy of ±0.05 K. 2.2. Solubility Measurement. Throughout the experimental design, the analytical stirred-flask pattern method and the gravity method were used to determine the molar fraction solubility of L-tryptophan.10−14 Different solvents were separately added to eight 10 mL glass test tubes with stoppers (to avoid solvent evaporation during agitation); then, excess Ltryptophan was added to the tubes. The glass tubes were fixed in a glass container filled with water and maintained at a constant temperature by a smart water circulating thermostatic bath system at the temperature range of T = 273.15−333.15 K. The accuracy of system temperature for all of the experiments was under ±0.05 K. Meanwhile, the glass container was placed on a magnetic stirrer, the solvent and the solute were thoroughly stirred and mixed for 12 h to ensure a solid−liquid equilibrium, and finally a supersaturated solution was obtained. Subsequently, it was allowed to stand under constant temperature for 12 h so that the excess solute and the solution were completely separated. A 1 mL supernatant was quickly transferred to a previously weighed 5 mL beaker and weighed again using a pipette. The beaker was placed in the desiccator and weighed repeatedly until getting a constant weight. Each experiment was repeated 3 times, and the average was adopted to calculate the molar fraction solubility. The mole fraction solubility for pure solvents is calculated as follows x=

m1/M1 m1/M1 + m2 /M 2

3. RESULTS AND DISCUSSION 3.1. Solid−Liquid Equilibrium Data. Under 0.1 MPa, molecular fraction solubility of L-tryptophan in water, methanol, ethanol, acetone, isopropanol, n-butanol, acetonitrile, and ethyl acetate in the temperature range T = 278.15−333.15 K is presented in Table 2, and the image is shown in Figure 2 (Tables 3 and 4). Because the boiling point of acetone is 329.4 K, the solubility data of L-tryptophan above 328.15 K is not included in the single solvent acetone experiment. Meanwhile, the molar fraction solubility data in (water + acetonitrile), (water + methanol), and (water + isopropanol) are summarized in Tables 5−7, and the images are shown in Figures 3−5, respectively. 3.2. Simulation Models. To select suitable models to explain the dissolution behavior of L-tryptophan in the selected solvent, during our study, four models were utilized to correlate the solubility data: the modified Apelblat equation, the Buchowski−Ksiazaczak λh model, the CNIBS/R−K model, and the Jouyban−Acree model. 3.2.1. Modified Apelblat Equation. As is widely known, the modified Apelblat equation is a semiempirical equation with three parameters. This is a very authoritative mathematical description of the binary solid−liquid phase equilibrium and has been widely used to correlate the solubility of solutes in solvents. The modified Apelblat equation shows the trend of temperature solubility of L-tryptophan in a single solvent or in binary solvents.15,16 The concrete equation can be described as eq 4

(1)

ln x = A +

where m1 and M1 present the mass and molar mass of Ltryptophan, whereas m2 and M2 are that of the solvents. The mole fraction solubility of L-tryptophan (x) in (water + acetonitrile), (water + methanol), and (water + isopropanol) binary solvent mixtures is calculated using eq 2. The mole fraction of water (xA) in the binary solvent mixtures is calculated by eq 3 x=

xA =

m1/M1 m1/M1 + m2 /M 2 + m3 /M3

m2 /M 2 m2 /M 2 + m3 /M3

B + C ln(T /K) T /K

(4)

where x is the molar fraction solubility of L-tryptophan in an organic solvent, and T is the absolute temperature. A, B, or C is an empirical constant obtained by nonlinear multiple regression analysis, in which parameters A and B, respectively, indicate the effect of solution nonideality on the solubility of solute and the change of the solute activity coefficient, and parameter C reveals the effect of temperature on enthalpy, all of which are listed in Table 3. 3.2.2. Buchowski−Ksiazaczak λh Model. The Buchowski− Ksiazaczak λh model is another semiempirical equation that illustrates the dissolution behavior of L-tryptophan in various pure solvents with two parameters λ and h, presented in eq 517

(2)

(3) B



Article a

Table 2. Mole Fraction Solubility x of L-Tryptophan in Different Single Solvents from 278.15 to 333.15 K under 0.1 MPa 100 RD

100 RD T/K

103x

278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

0.62 0.66 0.72 0.80 0.90 1.02 1.15 1.31 1.49 1.66 1.89 2.17

eq 4

eq 5

T/K

3.23 0.03 −1.31 −2.59 −1.01 0.05 0.16 0.99 1.38 −0.54 −0.54 0.10

13.78 6.75 1.98 −2.06 −2.51 −2.69 −3.14 −2.19 −1.07 −1.74 0.06 2.90

288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

7.87 1.67 −2.58 −3.54 −3.62 −0.92 0.36 1.53 0.80 0.57 −0.21 −0.38

13.78 5.77 −0.22 −2.67 −3.87 −1.89 −0.97 0.15 −0.35 −0.07 −0.10 0.71

5.53 0.49 −1.91 −2.73 −2.09 −0.77 0.57 0.93 0.79 0.20 −0.03 −0.33

14.20 6.35 1.30 −1.71 −2.71 −2.45 −1.62 −1.28 −1.00 −0.77 0.22 1.48

5.58 0.01 −2.68 −4.68 −1.83 1.04 2.18 0.73 0.53 −1.00 −0.27 0.21

25.68 14.69 6.46 −0.42 −1.54 −1.33 −1.69 −3.67 −3.33 −3.38 −0.28 3.29

4.51 −1.60

28.11 16.10

Methanol

278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

Ethanol 278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

0.12 0.13 0.14 0.17 0.20 0.24 0.28 0.33 0.38 0.44 0.50 0.58

278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

Isopropanol 278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

0.11 0.12 0.14 0.15 0.18 0.21 0.24 0.28 0.33 0.37 0.43 0.50

278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

1-Butanol 278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

0.10 0.11 0.12 0.14 0.17 0.20 0.24 0.28 0.34 0.40 0.49 0.59

278.15 283.15

0.10 0.11

103x

eq 4

Acetone 0.13 −2.45 0.15 −1.79 0.18 −1.48 0.22 0.87 0.26 1.51 0.31 1.21 0.37 −0.36 0.45 −0.59 0.56 −0.28 0.69 0.22 Ethyl Acetate 0.04 4.24 0.05 1.06 0.07 2.41 0.09 −2.80 0.11 −1.99 0.14 −1.87 0.18 0.11 0.23 1.34 0.28 1.21 0.34 0.57 0.41 −2.04 0.50 0.69 Water 0.98 3.73 1.05 1.59 1.13 −0.75 1.24 −1.71 1.37 −2.50 1.54 −1.99 1.74 −1.40 1.99 0.00 2.29 1.64 2.61 1.98 2.94 0.86 3.28 −1.53 Acetonitrile 0.01 −9.10 0.02 −4.52 0.02 −3.79 0.03 2.86 0.04 0.39 0.05 1.79 0.07 1.03 0.08 0.27 0.10 −0.15 0.12 −0.89 0.14 −0.70 0.17 0.64

eq 5 8.75 3.74 −0.41 −1.23 −2.47 −3.48 −4.65 −3.32 −0.46 3.39 −5.24 −5.70 −1.82 −5.25 −2.91 −1.70 0.95 2.47 2.31 1.34 −1.91 −0.12 15.44 8.99 2.91 −1.14 −4.23 −5.18 −5.21 −3.63 −1.13 0.65 1.52 1.70 −43.63 −26.43 −17.09 −3.64 −1.82 2.63 3.73 3.80 3.22 1.40 −0.43 −1.99

Acetone

l l λ(1 − x) | λ(1 − x) | o o o o lnm 1+ = λhm } } o o1 + o o x x n ~ n ~

a

The relative standard uncertainty is ur(x) = 0.025, ur(xA) = 0.0001, ur(p) = 2 kPa, and u(T) = 0.05 K.

where x and Tm explain the molar fraction solubility of Ltryptophan and the melting point at standard pressure, respectively, and T is the absolute temperature. λ is considered

(5) C



Article

Table 4. Parameters of the λh Model for L-Tryptophan in Different Single Solvents at the Temperature Range from 278.15 to 333.15 Ka

a

solvent

λ*100

h

102RAD

methanol ethanol 1-butanol acetone acetonitrile ethyl acetate isopropanol water

0.03 0.0 0.04 0.06 0.0 0.08 0.0 0.04

77 422.25 157 276.40 96 685.96 63 902.30 194 359.00 54 425.75 222 852.50 52 150.31

3.41 2.55 5.48 6.34 9.15 2.64 2.92 4.31 ∑102RAD = 36.81

Standard uncertainty u is u(T) = 0.05 K.

(water + acetonitrile), (water + ethanol), and (water + isopropanol) is expressed by eq 6 N

ln x = xA ln XA + xB ln XB + xAxB ∑ Si(xA − xB)2 i=0

(6)

where x represents the molar fraction solubility of L-tryptophan in a multicomponent solvent. xA and xB represent the initial molar fraction composition of the binary solvent when no solute is added. XA and XB represent the saturated molar solubility of Ltryptophan in acetonitrile, methanol, and isopropanol, respectively. Si represents the model constant, N can be equal to 1, 2, and 3. When N = 2 and replaces xB with (1 − xA), it can be rearranged to eq 7 Figure 2. Mole fraction solubility (x) of L-tryptophan versus temperature (T) in different single solvents: (a) water, methanol, ethanol, isopropanol; and (b) acetone; 1-butanol; ethyl acetate; acetonitrile.

ln x − (1 − xA)ln XB − xA ln XA = (1 − xA)xA{S0 + S1(2xA − 1) + S2(2xA − 1)2 }

This is a transformation of the CNIBS/R−K model. The parameter Si can be obtained by nonlinear regression fitting

Table 3. Parameters of the Modified Apelblat Model for LTryptophan in Different Single Solvents at the Temperature Range from 278.15 to 333.15 Ka solvent

A

B

C

102RAD

methanol ethanol 1-butanol acetone acetonitrile ethyl acetate isopropanol water

−177.45 −87.12 −266.41 −297.66 264.70 90.52 −131.71 −198.31

5866.46 1131.59 9012.47 10257.86 −16186.50 −8199.90 3341.53 6875.07

26.46 13.13 39.93 44.69 −38.70 −12.65 19.64 29.61

0.99 2.00 1.73 1.41 2.18 1.69 1.36 1.64 ∑102RAD = 13.01

(7)

{ln x − (1 − xA)ln XB − xA ln XA} versus {(1 − xA) xA{S0 + S1(2xA − 1) + S2(2xA − 1)2 }}

The parameter values are listed in Tables 11−13. However, the CNIBS/R−K model can only be used to describe the solubility data for different proportions of mixed solvents at a fixed temperature. To describe the effect of changes in solvent composition and temperature on the solubility of L-tryptophan, we use another equation. 3.2.4. Jouyban−Acree Model. The mature model is a universal equation for solid−liquid equilibrium based on thermodynamic principles, providing a more rigorous approach to empirical models than thermodynamics. This is a relatively more general model to describe the change in the mole fraction solubility of a solute with temperature and the initial composition of a binary solvent mixture.19

a


to be the association constant between the solute−solute molecules in the binary system, h is related to the excess enthalpy of the solvent, and the two parameters λ and h are shown in Tables 4 and 8−10. 3.2.3. CNIBS/R−K Model. The solubility trend of Ltryptophan in different proportions of water under isothermal conditions was described by the combined nearly ideal binary solvent: Redlich−Kister, CNIBS: R−K, which correlated experiment dissolved data.18 The solubility of L-tryptophan in

N

ln x = xA ln XA + xB ln XB + xAxB ∑ i=0

Ji (xA − xB)i T

(8)

where Ji is the parameter and T is the absolute temperature in kelvin. The other symbols are the same as in eq 9. D



Article

Table 5. Mole Fraction Solubility (x) of L-Tryptophan in (Water + Acetonitrile) Binary Solution Mixtures from 278.15 to 333.15 Ka under 0.1 MPab xA

1000x

100|x − xcal|/ x (eq 4)

0.00 0.34 0.55 0.70 0.81 0.90 0.96 1.00

0.01 0.34 0.55 0.69 0.80 0.88 0.94 0.98

9.10 0.13 3.90 3.82 3.78 3.75 3.74 3.73

0.00 0.34 0.55 0.70 0.81 0.90 0.96 1.00

0.02 0.37 0.59 0.74 0.86 0.94 1.01 1.05

4.52 0.14 1.61 1.60 1.59 1.59 1.59 1.59

0.00 0.34 0.55 0.70 0.81 0.90 0.96 1.00

0.02 0.40 0.63 0.80 0.92 1.02 1.09 1.13

3.79 0.28 0.82 0.79 0.77 0.76 0.76 0.75

0.00 0.34 0.55 0.70 0.81 0.90 0.96 1.00

0.03 0.44 0.70 0.88 1.01 1.11 1.20 1.24

2.86 0.48 1.70 1.71 1.71 1.71 1.71 1.71

0.00 0.34 0.55 0.70 0.81 0.90 0.96 1.00

0.04 0.49 0.78 0.97 1.12 1.23 1.32 1.37

0.39 0.55 2.53 2.51 2.51 2.50 2.50 2.50

0.00 0.34 0.55 0.70 0.81 0.90 0.96 1.00

0.05 0.54 0.88 1.10 1.26 1.39 1.49 1.54

1.79 0.99 1.97 1.98 1.98 1.99 1.99 1.99

0.00 0.34 0.55 0.70 0.81

0.07 0.62 0.99 1.24 1.43

1.03 0.23 1.37 1.39 1.39

100|x − xcal|/ x (eq 7)

100|x − xcal|/ x (eq 10)

0.00 1.88 2.18 1.17 0.75 0.60 0.83 0.00

26.21 3.74 3.32 3.48 3.37 3.22 3.21 3.29

0.00 1.59 1.87 1.00 0.64 0.52 0.72 0.00

18.41 2.36 1.95 2.13 2.02 1.88 1.86 1.95

0.00 1.36 1.63 0.87 0.56 0.45 0.63 0.00

13.65 1.07 0.66 0.86 0.76 0.61 0.59 0.69

0.00 1.14 1.38 0.74 0.48 0.39 0.54 0.00

8.23 0.24 0.20 0.01 0.10 0.25 0.26 0.16

0.00 1.02 1.25 0.67 0.43 0.35 0.49 0.00

6.33 0.47 0.89 0.66 0.76 0.91 0.92 0.81

0.00 0.68 0.80 0.40 0.31 0.23 0.34 0.00

3.94 1.24 1.13 0.89 0.98 1.13 1.13 1.03

0.00 0.62 0.75 0.38 0.29

2.54 1.27 1.18 0.93 1.01

T = 278.15 K

T = 283.15 K

T = 288.15 K

T = 293.15 K

T = 298.15 K

T = 303.15 K

T = 308.15 K

E



Article

Table 5. continued xA

1000x

100|x − xcal|/ x (eq 4)

0.90 0.96 1.00

1.57 1.68 1.74

1.40 1.40 1.40

0.00 0.34 0.55 0.70 0.81 0.90 0.96 1.00

0.08 0.71 1.14 1.42 1.63 1.79 1.92 1.99

0.27 0.17 0.01 0.01 0.01 0.00 0.00 0.00

0.00 0.34 0.55 0.70 0.81 0.90 0.96 1.00

0.10 0.82 1.31 1.64 1.88 2.06 2.21 2.29

0.15 1.00 1.61 1.62 1.63 1.63 1.64 1.64

0.00 0.34 0.55 0.70 0.81 0.90 0.96 1.00

0.12 0.94 1.50 1.87 2.14 2.35 2.52 2.61

0.89 0.04 1.92 1.95 1.96 1.97 1.98 1.98

0.00 0.34 0.55 0.70 0.81 0.90 0.96 1.00

0.14 1.09 1.69 2.11 2.42 2.65 2.84 2.94

0.70 0.69 0.83 0.84 0.85 0.86 0.86 0.86

0.00 0.34 0.55 0.70 0.81 0.90 0.96 1.00

0.17 1.29 1.89 2.35 2.70 2.96 3.17 3.28

0.64 0.24 1.47 1.50 1.51 1.52 1.52 1.53

100|x − xcal|/ x (eq 7)

100|x − xcal|/ x (eq 10)

0.22 0.32 0.00

1.15 1.15 1.05

0.00 0.51 0.60 0.30 0.25 0.18 0.27 0.00

1.48 1.18 0.91 0.65 0.71 0.84 0.84 0.74

0.00 0.53 0.65 0.33 0.25 0.19 0.28 0.00

0.64 0.62 0.44 0.17 0.22 0.33 0.32 0.22

0.00 0.53 0.65 0.33 0.25 0.19 0.28 0.00

0.11 0.29 0.11 0.18 0.15 0.04 0.06 0.16

0.00 0.69 0.90 0.49 0.31 0.26 0.35 0.00

0.40 0.28 0.06 0.37 0.34 0.25 0.27 0.36

0.00 1.06 1.54 0.90 0.47 0.44 0.56 0.00

0.92 1.33 0.11 0.43 0.42 0.33 0.35 0.45

T = 308.15 K

T = 313.15 K

T = 318.15 K

T = 323.15 K

T = 328.15 K

T = 333.15 K

a

The relative standard uncertainty is ur(x) = 0.025, u(xA) = 0.0001, u(p) = 2 kPa, and u(T) = 0.05 K. bxA denotes the mole fraction of water in the binary solvent mixtures. x denotes the mole fraction solubility of L-tryptophan. xcal denotes the calculated solubility.

ln X3 = ln XB + (ln XA − ln XB)XA + + +

( −J0 + 3J1 − 5J2 )XA T ( −4J2 )XA T

2

+

(J0 − J1 + J2 )XA

T ln X3 = A 0 + A1T + A 2 TXA + A3XA + A4 XA 2 + A5XA 3

T ( −2J1 + 8J2 )XA 3

+ A 6XA 4

(10)

where A0, A1, A2, A3, A4, A5, and A6 were calculated by regressing T ln X3 against T, TXA, XA2, XA3, and XA4, by least-square analysis, which are listed in Tables 14−16. 3.3. Deviation. The relative deviation (RD) formula is as shown in eq 11

T

4

(9)

F



Article

Table 6. Mole Fraction Solubility (x) of L-Tryptophan in (Water + Methanol) Binary Solution Mixtures from 278.15 to 333.15 Ka under 0.1 MPab xA

1000x

100|x − xcal|/x (eq 4)

0.00 0.28 0.49 0.65 0.77 0.87 0.95 1.00

0.62 0.72 0.79 0.85 0.90 0.93 0.96 0.98

3.23 3.43 3.54 3.61 3.65 3.69 3.71 3.73

0.00 0.28 0.49 0.65 0.77 0.87 0.95 1.00

0.66 0.77 0.85 0.91 0.96 1.00 1.03 1.05

0.03 0.64 0.98 1.19 1.34 1.45 1.54 1.59

0.00 0.28 0.49 0.65 0.77 0.87 0.95 1.00

0.72 0.84 0.92 0.99 1.04 1.08 1.11 1.13

1.31 1.09 0.97 0.89 0.84 0.80 0.77 0.75

0.00 0.28 0.49 0.65 0.77 0.87 0.95 1.00

0.79 0.92 1.01 1.08 1.14 1.18 1.22 1.24

2.59 2.25 2.06 1.94 1.85 1.79 1.74 1.71

0.00 0.28 0.49 0.65 0.77 0.87 0.95 1.00

0.90 1.03 1.13 1.20 1.26 1.31 1.35 1.37

1.01 1.57 1.90 2.11 2.25 2.36 2.45 2.50

0.00 0.28 0.49 0.65 0.77 0.87 0.95 1.00

1.02 1.17 1.28 1.36 1.42 1.47 1.52 1.54

0.05 0.71 1.16 1.45 1.66 1.81 1.93 1.99

0.00 0.28 0.49 0.65 0.77

1.15 1.32 1.44 1.53 1.61

0.16 0.43 0.77 0.99 1.15

100|x − xcal|/x (eq 7)

100|x − xcal|/x (eq 10)

0.00 0.55 1.36 2.26 4.01 6.21 6.17 0.00

2.97 3.08 3.14 3.19 3.23 3.26 3.28 3.30

0.00 0.49 1.24 2.21 4.10 6.28 6.10 0.00

1.51 1.66 1.76 1.82 1.87 1.90 1.93 1.95

0.00 0.85 1.98 2.55 3.53 5.83 6.46 0.00

0.51 0.56 0.59 0.62 0.64 0.66 0.67 0.68

0.00 0.90 2.08 2.60 3.45 5.76 6.50 0.00

0.36 0.30 0.27 0.24 0.22 0.20 0.19 0.18

0.00 1.32 2.97 2.99 2.68 5.11 6.87 0.00

0.49 0.63 0.71 0.76 0.79 0.82 0.83 0.84

0.00 1.43 3.19 3.08 2.46 4.91 6.94 0.00

0.56 0.76 0.87 0.94 0.99 1.02 1.05 1.06

0.00 1.41 3.14 3.06 2.51

0.67 0.83 0.93 0.99 1.03

T = 278.15 K

T = 283.15 K

T = 288.15 K

T = 293.15 K

T = 298.15 K

T = 303.15 K

T = 308.15 K

G



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1000x

100|x − xcal|/x (eq 4)

0.87 0.95 1.00

1.66 1.71 1.74

1.26 1.35 1.40

0.00 0.28 0.49 0.65 0.77 0.87 0.95 1.00

1.31 1.50 1.64 1.75 1.83 1.90 1.96 1.99

0.99 0.62 0.40 0.26 0.16 0.09 0.03 0.00

0.00 0.28 0.49 0.65 0.77 0.87 0.95 1.00

1.49 1.72 1.88 2.01 2.11 2.19 2.25 2.29

1.38 1.48 1.53 1.57 1.60 1.61 1.63 1.64

0.00 0.28 0.49 0.65 0.77 0.87 0.95 1.00

1.66 1.93 2.13 2.28 2.39 2.49 2.57 2.61

0.54 0.43 0.98 1.33 1.58 1.76 1.90 1.98

0.00 0.28 0.49 0.65 0.77 0.87 0.95 1.00

1.89 2.19 2.41 2.57 2.70 2.80 2.89 2.94

0.54 0.00 0.30 0.50 0.64 0.74 0.82 0.86

0.00 0.28 0.49 0.65 0.77 0.87 0.95 1.00

2.17 2.49 2.72 2.89 3.03 3.14 3.23 3.28

0.10 0.51 0.87 1.10 1.26 1.38 1.48 1.53

100|x − xcal|/x (eq 7)

100|x − xcal|/x (eq 10)

4.96 6.93 0.00

1.06 1.08 1.09

0.00 1.36 3.04 3.02 2.61 5.04 6.89 0.00

0.49 0.60 0.67 0.71 0.74 0.76 0.77 0.78

0.00 1.21 2.73 2.88 2.90 5.30 6.77 0.00

0.26 0.27 0.27 0.27 0.27 0.27 0.27 0.27

0.00 0.83 1.94 2.53 3.56 5.86 6.44 0.00

0.38 0.19 0.09 0.02 0.03 0.06 0.09 0.10

0.00 1.03 2.34 2.71 3.24 5.59 6.61 0.00

0.01 0.13 0.19 0.23 0.26 0.28 0.30 0.30

0.00 1.42 3.17 3.07 2.49 4.93 6.93 0.00

0.62 0.54 0.49 0.45 0.43 0.41 0.39 0.38

T = 308.15 K

T = 313.15 K

T = 318.15 K

T = 323.15 K

T = 328.15 K

T = 333.15 K

a


RD =

xi − xci xi

RAD =

(11)

1 N

n

∑ i=1

xi − xci xi

(12)

where xi represents a label explicit value of the solubility of L-

where N denotes the number of experimental points, and xi and

tryptophan in the solvents, and xci represents the measured value

xci, respectively, represent the labeling value and the measured

of solubility. The relative average deviation (RAD) formula is

value. The root-mean-square deviation (RMSD) formula is H



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Table 7. Mole Fraction Solubility (x) of L-Tryptophan in (Water + Isopropanol) Binary Solution Mixtures from 278.15 to 333.15 K under 0.1 MPaa,b xA

1000x

100|x − xcal|/x (eq 4)

0.00 0.43 0.65 0.78 0.86 0.93 0.98 1.00

0.11 0.49 0.67 0.79 0.86 0.92 0.96 0.98

5.53 4.03 3.86 3.80 3.77 3.75 3.73 3.73

0.00 0.43 0.65 0.78 0.86 0.93 0.98 1.00

0.12 0.52 0.72 0.84 0.92 0.98 1.03 1.05

0.49 1.45 1.53 1.55 1.57 1.58 1.58 1.59

0.00 0.43 0.65 0.78 0.86 0.93 0.98 1.00

0.14 0.56 0.78 0.91 1.00 1.06 1.10 1.13

1.91 0.93 0.83 0.79 0.77 0.76 0.76 0.75

0.00 0.43 0.65 0.78 0.86 0.93 0.98 1.00

0.15 0.62 0.86 1.00 1.10 1.16 1.21 1.24

2.73 1.88 1.79 1.75 1.73 1.72 1.71 1.71

0.00 0.43 0.65 0.78 0.86 0.93 0.98 1.00

0.18 0.69 0.95 1.10 1.21 1.28 1.34 1.37

2.09 2.46 2.48 2.49 2.49 2.49 2.49 2.50

0.00 0.43 0.65 0.78 0.86 0.93 0.98 1.00

0.21 0.78 1.07 1.24 1.36 1.44 1.51 1.54

0.77 1.82 1.91 1.95 1.97 1.98 1.99 1.99

0.00 0.43 0.65 0.78 0.86

0.25 0.89 1.21 1.41 1.54

0.57 1.09 1.27 1.33 1.36

100|x − xcal|/x (eq 7)

100|x − xcal|/x (eq 10)

0.00 0.05 0.12 0.11 0.06 0.09 0.07 0.00

2.41 3.35 3.28 3.30 3.28 3.27 3.28 3.30

0.00 0.05 0.12 0.11 0.05 0.09 0.07 0.00

0.84 1.95 1.92 1.94 1.94 1.92 1.93 1.95

0.00 0.05 0.12 0.10 0.05 0.08 0.07 0.00

0.15 0.70 0.66 0.68 0.67 0.65 0.67 0.69

0.00 0.05 0.11 0.10 0.05 0.08 0.07 0.00

0.72 0.15 0.20 0.17 0.18 0.21 0.19 0.17

0.00 0.04 0.11 0.09 0.05 0.08 0.06 0.00

0.87 0.75 0.83 0.82 0.84 0.86 0.85 0.83

0.00 0.04 0.10 0.09 0.05 0.08 0.06 0.00

0.75 0.95 1.04 1.03 1.05 1.08 1.07 1.05

0.00 0.04 0.10 0.09 0.05

0.52 0.96 1.06 1.05 1.07

T = 278.15 K

T = 283.15 K

T = 288.15 K

T = 293.15 K

T = 298.15 K

T = 303.15 K

T = 308.15 K

I



Article


1000x

100|x − xcal|/x (eq 4)

0.93 0.98 1.00

1.63 1.70 1.74

1.38 1.40 1.40

0.00 0.43 0.65 0.78 0.86 0.93 0.98 1.00

0.28 1.02 1.39 1.61 1.76 1.87 1.95 1.99

0.93 0.16 0.07 0.04 0.02 0.01 0.01 0.00

0.00 0.43 0.65 0.78 0.86 0.93 0.98 1.00

0.33 1.17 1.59 1.85 2.03 2.15 2.24 2.29

0.79 1.51 1.58 1.61 1.62 1.63 1.63 1.64

0.00 0.43 0.65 0.78 0.86 0.93 0.98 1.00

0.38 1.34 1.82 2.11 2.31 2.45 2.55 2.61

0.20 1.70 1.86 1.91 1.94 1.96 1.97 1.98

0.00 0.43 0.65 0.78 0.86 0.93 0.98 1.00

0.43 1.52 2.05 2.38 2.60 2.76 2.88 2.94

0.03 0.72 0.80 0.83 0.84 0.85 0.86 0.86

0.00 0.43 0.65 0.78 0.86 0.93 0.98 1.00

0.50 1.70 2.29 2.66 2.90 3.08 3.21 3.28

0.33 1.32 1.44 1.48 1.50 1.51 1.52 1.53

100|x − xcal|/x (eq 7)

100|x − xcal|/x (eq 10)

0.08 0.06 0.00

1.10 1.10 1.08

0.00 0.04 0.10 0.09 0.05 0.07 0.06 0.00

0.38 0.70 0.77 0.75 0.77 0.79 0.79 0.77

0.00 0.04 0.10 0.09 0.05 0.07 0.06 0.00

0.25 0.27 0.29 0.26 0.27 0.29 0.28 0.26

0.00 0.04 0.10 0.09 0.05 0.07 0.06 0.00

0.12 0.05 0.06 0.11 0.10 0.08 0.09 0.11

0.00 0.04 0.10 0.08 0.05 0.07 0.06 0.00

0.17 0.25 0.27 0.31 0.31 0.29 0.30 0.31

0.00 0.04 0.09 0.08 0.05 0.07 0.06 0.00

0.52 0.36 0.36 0.40 0.40 0.37 0.38 0.39

T = 308.15 K

T = 313.15 K

T = 318.15 K

T = 323.15 K

T = 328.15 K

T = 333.15 K

a


ÄÅ ÅÅ 1 Å RMSD = ÅÅÅ ÅÅ N ÅÇ

ÉÑ1/2 ÑÑ ∑ (xci − xi) ÑÑÑ ÑÑÖ i=1 N

2Ñ Ñ

temperature rising. The sequence of solubility of the mole fraction from high to low is as follows: water > methanol > ethanol > isopropanol > acetone > 1-butanol > ethyl acetate > acetonitrile. Although the solubility of L-tryptophan increases as the polarity of the solvent increases, L-tryptophan is a nonpolar amino acid, so its solubility in a solvent was independent of polarity. On the one hand, from Figure 1, the molecular structure of L-tryptophan indicates that there is a pair of unshared lone pairs of electrons on the nitrogen atom of the

(13)

3.4. Analysis of Solubility and Thermodynamic Parameters in Solid−Liquid Equilibrium Experiments. 3.4.1. Analysis of Solubility in Pure Solvents. As shown in Figure 2, the solubility of L-tryptophan in eight different pure solvents is a function of temperature and increases with J



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Table 8. Parameters of the Modified Apelblat Equation for LTryptophan in (Water + Acetonitrile) Binary Solution Mixtures from 278.15 to 333.15 K under 0.1 MPaa

a

xA

A

B

C

MD

0.0000 0.3374 0.5529 0.7024 0.8123 0.8964 0.9629 1.0000

264.67 −310.47 −192.40 −195.26 −196.66 −197.49 −198.04 −198.31

−16186.47 11796.96 6535.00 6696.82 6777.72 6826.50 6858.94 6875.07

−38.70 46.21 28.67 29.12 29.34 29.47 29.561 29.605

2.18 0.41 1.65 1.64 1.64 1.64 1.64 1.64 (MD) = 1.55


Table 9. Parameters of the Modified Apelblat Equation for LTryptophan in (Water + Methanol) Binary Solution Mixtures from 278.15 to 333.15 K under 0.1 MPaa

Figure 3. Mole fraction solubility (x) of L-tryptophan versus temperature (T) in (water + acetonitrile) binary solvent mixtures.

a

xA

A

B

C

MD

0.0000 0.2839 0.4906 0.6477 0.7712 0.8708 0.9529 1.0000

−177.45 −185.37 −189.93 −192.88 −194.95 −196.48 −197.66 −198.31

5866.46 6248.58 6468.67 6611.71 6712.10 6786.42 6843.71 6875.07

26.46 27.65 28.34 28.79 29.10 29.33 29.51 29.61

0.99 1.10 1.29 1.41 1.50 1.56 1.61 1.64 (MD) = 1.39


Table 10. Parameters of the Modified Apelblat Equation for L-Tryptophan in (Water + isopropanol) Binary Solution Mixtures from 278.15 to 333.15 K under 0.1 MPaa

Figure 4. Mole fraction solubility (x) of L-tryptophan versus temperature (T) in (water + methanol) binary solvent mixtures.

a

xA

A

B

C

MD

0.0000 0.4891 0.6992 0.8161 0.8905 0.9421 0.9799 1.0000

−131.71 −190.01 −194.82 −196.45 −197.29 −197.79 −198.13 −198.31

3341.53 6403.05 6671.53 6765.66 6814.67 6844.55 6864.80 6875.07

19.64 28.31 29.05 29.31 29.44 29.52 29.58 29.61

1.36 1.59 1.62 1.63 1.63 1.64 1.64 1.64 (MD) = 1.59


a hydrogen bond with a proton solvent such as water. On the other hand, water has a smaller molecular structure than other solvents and has a higher dielectric constant (78.514 at 298.15 k),20 which makes it easier for water to overcome static electricity than other solvents, thus entering the L-tryptophan molecular gap. 3.4.2. Thermodynamic Study in Pure Solvents. According to Tables 3 and 4, within the actual measured temperature range (278.15−333.15 K), the solubility of the model used in this study for nonlinear fitting of L-tryptophan showed significant agreement. Taking the Apelblat model as an example, the relative mean deviations (100 RAD) of L-tryptophan in eight single pure solvents are 0.99, 2.00, 1.73, 1.41, 2.18, 1.69, 1.36, and 1.64. The absolute value of the relative deviation between them does not exceed 2.5, demonstrating that the improved

Figure 5. Mole fraction solubility (x) of L-tryptophan versus temperature (T) in (water + isopropanol) binary solvent mixtures.

amino group. As a result, the amino group has an affinity for a basic proton, and L-tryptophan can serve as a Lewis acid to form K



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Table 11. Parameters of the CNIBS/R−K Model for LTryptophan in (Water + Acetonitrile) Binary Solution Mixturesa

Table 14. Parameters of the Jouyban−Acree Model for LTryptophan in (Water + Acetonitrile) Binary Solution Mixturesa

T

S0

S1

S2

MD

T/K

MD

278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

6.37 5.74 5.24 4.72 4.44 4.12 3.92 3.78 3.68 3.60 3.53 3.48

−5.40 −4.76 −4.25 −3.73 −3.44 −2.99 −2.82 −2.64 −2.59 −2.52 −2.58 −2.84

2.88 2.51 2.22 1.92 1.75 1.41 1.32 1.20 1.19 1.17 1.28 1.62

0.93 0.79 0.69 0.58 0.53 0.34 0.32 0.26 0.28 0.28 0.37 0.62 (MD) = 0.5

278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

6.23 4.07 2.36 1.18 1.47 1.43 1.29 0.92 0.37 0.14 0.29 0.54 (MD) = 1.69

parameters A0 A1 A2 A3 A4 A5 A6

−3089.44 0.54 0.62 3258.37 −5949.87 5239.36 −1758.64

a

a

Table 12. Parameters of the CNIBS/R−K Model for LTryptophan in (Water + Methanol) Binary Solution Mixturesa

Table 15. Parameters of the Jouyban−Acree Model for LTryptophan in (Water + Methanol) Binary Solution Mixturesa



T

S0

S1

S2

MD

T

MD

278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

0.1077 0.1088 0.101 0.100 0.088 0.084 0.085 0.087 0.092 0.102 0.097 0.085

−0.0165 −0.0168 −0.0150 −0.0148 −0.0122 −0.0114 −0.0116 −0.0119 −0.0130 −0.0151 −0.0141 −0.0115

0.0024 0.0025 0.0021 0.0021 0.0016 0.0014 0.0015 0.0015 0.0017 0.0021 0.0019 0.0014

0.26 0.26 0.27 0.27 0.27 0.28 0.28 0.27 0.27 0.26 0.27 0.28 (MD) = 0.27

278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

3.18 1.80 0.62 0.24 0.73 0.91 0.96 0.69 0.27 0.12 0.21 0.46 (MD) = 0.85


−2342.62 0.87 0.32 70.38 −44.68 13.10 −2.39

a

a

Table 13. Parameters of the CNIBS/R−K Model for LTryptophan in (Water + Isopropanol) Binary Solution Mixturesa

Table 16. Parameters of the Jouyban−Acree Model for LTryptophan in (Water + Isopropanol) Binary Solution Mixturesa



T

S0

S1

S2

MD

T

MD

278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

2.03 2.01 1.94 1.88 1.81 1.75 1.70 1.68 1.68 1.67 1.632 1.59

−1.18 −1.17 −1.12 −1.07 −1.02 −0.98 −0.94 −0.93 −0.93 −0.92 −0.90 −0.87

0.48 0.47 0.45 0.43 0.41 0.39 0.38 0.38 0.37 0.37 0.36 0.35

0.62 0.61 0.59 0.57 0.55 0.53 0.52 0.51 0.51 0.51 0.50 0.49 (MD) = 0.54

278.15 283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15

3.18 1.80 0.61 0.25 0.83 1.00 0.99 0.71 0.27 0.09 0.28 0.40 (MD) = 0.87


−2699.21 0.47 0.71 1347.74 −2038.09 1574.70 −490.53

a

a

Apelblat equation is conducive to correlating the solubility data of L-tryptophan with a specific solvent. The same analytical method can be used to complete the solubility data and

parameter analysis of all models. However, we can realize that the modified Apelblat equation’s and the Buchowski− Ksiazaczak λh equation’s ∑102RAD values are 13.01 and


L




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the three models in another two binary solvents (water + methanol) (Tables 9, 12, and 15) and (water + isopropanol) (Tables 10, 13, and 16) 1.39, 0.27, 0.85 (water + methanol); 1.59, 0.54, 0.87 (water + isopropanol). The results are consistent, and the CNIBS/R−K model has the best nonlinear fitting results (Tables 15 and 16). 3.4.5. Comparison of Experimental Data. The experimental data in this work was compared to that in the literature,21,22 as shown in Table 17. In contrast, in water and ethanol solutions, the experimental data in the literature is close to the experimental data herein at the same temperature. The error is caused by the small amount of each extraction and the different experimental methods. Comparing the experimental data herein with that in the literature,23,24 as shown in Figure 6, the solubility of D-tryptophan in water was slightly higher than that of Ltryptophan at the same temperature. Substances with different configurations are consistent in solubility. The error in the experiment may be due to the different experimental operating environment, and the source of the material is also different. In the case of low solubility, these errors are allowed. The solubility of water + methanol + L-tryptophan in the literature25 shows a parabolic trend, which is quite different from that in this paper, as shown in Figure 7. In the literature, the solubility is determined by laser intensity using a laser method, and in this work, the gravity method is used. In addition, the mass fraction purity of L-tryptophan in the literature is higher than that in this work, which may be another reason. In short, these may be the cause of the error.

Table 17. Comparison of the Molar Fraction Solubility of LTryptophan in Water and Ethanol in the Literature T (K)

component

this work

literature21

literature22

298.15

water ethanol

0.137 0.02

0.1034 0.0243

1.22

36.81, respectively. This result further explains that the improved Apelblat model proves to be more precise and suitable for characterizing the solubility of L-tryptophan at different temperatures and solvents. 3.4.3. Solubility Analysis in Binary Mixed Solvents. Observing Tables 5−8 and Figures 3−5, the solubility of Ltryptophan in the mixed solvents increases with temperature increasing, but decreases with the increasing proportions of methanol, acetonitrile, and isopropanol at a constant temperature. In mixed solvents, first, water can be combined with methanol, acetonitrile, and isopropanol by hydrogen bonding, which is easier than L-tryptophan. Thus, the increase in the specific gravity of methanol, isopropanol, and acetonitrile in aqueous solution reduces the water molecules available for dissolving Ltryptophan, resulting in lower solubility of L-tryptophan. The last but not the least, the cavity interactions, dipole−dipole interactions, and dispersion interactions may exist in the mixed system. Therefore, water is proven to be the best solvent for Ltryptophan. Methanol, isopropanol, and acetonitrile showed antisolvent effect on L-tryptophan. It was observed that the diversification of solubility of L-tryptophan was the most obvious in the (water + acetonitrile) solvent system, indicating that acetonitrile is a significant antisolvent for L-tryptophan rather than methanol. 3.4.4. Thermodynamic Study in Binary Mixed Solvents. From the data listed in Tables 8, 11, 14, it can be seen that the solubility of L-tryptophan in (water + acetonitrile) binary mixed solvent is corrected by the Apelblat equation, the CNIBS/RK model, and the Jouyban−Acree model. The average MD values were 1.55, 0.5, and 1.69, respectively. The results show that the experimental data agree well with the calculation results of the three equations. The CNIBS/R−K model has lower MD. Therefore, the CNIBS/R−K model can be used to predict the solubility of different concentrations of mixed solvents at a constant temperature. When considering temperature and composition, the Jouyban−Acree model can be used with the improved Apelblat equation and the CNIBS/R−K model. The same method was used to analyze the MD values of solubility for

4. CONCLUSIONS (1) In our study, solubility of L-tryptophan in water, methanol, ethanol, acetone, isopropanol, n-butanol, acetonitrile, ethyl acetate, organic pure solvent, and in binary mixed solvents (water + acetonitrile), (water + methanol), and (water + isopropanol) at temperature T = 278.15−333.15 K was determined with the gravimetric method. The solid−liquid equilibrium data of Ltryptophan in the specified solvent increases with temperature increasing. L-tryptophan has the best solubility in water. In binary mixed solvents, the solubility decreases as the ratio of acetonitrile, methanol, and isopropanol increases, so acetonitrile, methanol, and isopropanol can be used as effective antisolvents in the crystallization process.

Figure 6. Comparison of the molar fraction solubility of water/ethyl acetate/acetonitrile + L-tryptophan/D-tryptophan versus temperature (T) in the literature. M



Article

Figure 7. Comparison of the molar fraction solubility of methanol + water + L-tryptophan versus different temperatures (T) in the literature.

solubility of L-tryptophan with increase in the contents of acetonitrile, methanol, and isopropanol at a constant temperature. Finally, the physicochemical information of L-tryptophan is provided in this report, which may be helpful for extraction/ separation, recrystallization, purification, and formulation development.

(2) The experimental solid−liquid equilibrium data for Ltryptophan are efficiently correlated by four models: the modified Apelblat model, the Buchowski−Ksiazaczak λh model, the Redlich−Kister (CNIBS/RK) model, and the Jouyban−Acree model, and correlations with RMSD, RD, and MD are excellent. The modified Apelblat model and the Redlich−Kister (CNIBS/RK) model nonlinear fitting data and experimental data are consistent in pure solvents and binary mixed solvents, respectively. (3) According to the analysis obtained from this study, the amino group in L-tryptophan has an affinity for a basic proton and can be used as a Lewis acid to form a hydrogen bond with a proton solvent such as water; the molecular structure of water is smaller than other solvents, and the high dielectric constant (78.514 at 298.15 k) makes it easier for water to overcome static electricity and enter the L-tryptophan molecular gap. In mixed solvents, water can be combined with methanol, acetonitrile, and isopropanol by hydrogen bonding, which is easier than L-tryptophan; there may be cavity interaction, dipole−dipole interaction, dispersion interaction, and other related factors in the mixed system, which result in reduction in the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-25-58139928. Fax: +86-25-58139928. ORCID

Yonghong Hu: 0000-0002-8268-8763 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research work was financially supported by Jiangsu Agricultural Science and Technology Innovation Fund (CX (17)3025), Jiangsu Province Science and Technology Support N



Article

(18) Hong, M.; Xu, L.; Ren, G.; Chen, J.; Qi, M. Solubility of Lansoprazole in different solvents. Fluid Phase Equilib. 2012, 331, 18− 25. (19) Jouyban-Gharamaleki, A.; Acree, W. E., Jr. Comparison of models for describing multiple peaks in solubility profiles. Int. J. Pharm. 1998, 167, 177−182. (20) Wyman, J. The dielectric constant of mixtures of ethyl alcohol and water from −5 to 40°. J. Am. Chem. Soc. 1931, 53, 3292−3301. (21) Bowden, N. A.; Sanders, J. P.; Bruins, M. E. Solubility of the Proteinogenic a-Amino Acids in Water, Ethanol, and Ethanol-Water Mixtures. J. Chem. Eng. Data 2018, 63, 488−497. (22) Nozaki, Y.; Tanford, C. The Solubility of Amino Acids and Two Glycine Peptides in Aqueous Ethanol and Dioxane Solutions. Establishment of a Hydrophobicity Scale. J. Biol. Chem. 1971, 246, 2211−2217. (23) Feng, X.; Farajtabar, A.; Lin, H.; Chen, G.; Wang, Z.; Li, X.; Zhao, H. Experimental solubility evaluation and thermodynamic analysis of biologically active D-tryptophan in aqueous mixtures of N,Ndimethylformamide and several alcohols. J. Chem. Thermodyn. 2019, 128, 34−44. (24) Li, X.; Li, K.; Farajtabar, A.; He, Y.; Chen, G.; Zhao, H. Solubility of D-Tryptophan and L-Tyrosine in Several Organic Solvents: Determination and Solvent Effect. J. Chem. Eng. Data 2019, 64, 3164−3169. (25) Chen, Q.; Wang, J.; Bao, Y. Determination of the crystallization thermodynamics and kinetics of L-tryptophan in alcohols - water system. Fluid Phase Equilib. 2012, 313, 182−189.

Plan (BE2018396), The Jiangsu Synergetic Innovation Center for Advanced Bio-Manufacture (XTB1805), and Jiangsu Fishery Science and Technology Project (Y2018-21).

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Saturated Solubility and Thermodynamic Evaluation of l-Tryptophan in

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