Enthalpies of solution of zinc and phenylalanine complexes  in water

Ji Mian, Guo Lijuan, Gao Shengli, Hu Rongzu, Shi Qizhen
(Department of Chemistry, Northwest University, Xi'an, Shaanxi 710069, China)

Abstract The enthalpies of solution in water of L-α-phenylalanine and its zinc complexes Zn(Phe)Cl₂^.1/2H₂O, Zn(Phe)₂Cl₂^.H₂O, Zn(Phe)(NO₃)₂^.H₂O, Zn(Phe)₃(NO₃)₂^.H₂O and Zn(Phe)SO₄^.H₂O have been measured at 298.15 K. The standard enthalpy of formation of Phe(aq) has been calculated. The influence of different anions and molar ratio of zinc with L-α-phenylalanine has been discussed.
Keywords complexes, enthalpies of solution, L-α-phenylalanine, zinc salts

Received Jan. 22, 2000; Supported mainly by the National Natural Scientific Foundation of China. (No.29871023)

1 INTRODUCTION    
The complexes of zinc salts with L-α-phenylalanine as additives have been widely used in medicine, foodstuff and cosmetic. Using phase equilibrium method, Gao Shengli and Hou Yudong et al ^[1-3] studied the coordination behavior of zinc salts with L-α-phenylalanine. According to the results of phase equilibrium, they prepared a series of complexes of zinc and L-α-phenylalanine.
    In this paper, the enthalpies of solution in water of L-α-phenylalanine and its five zinc complexes have been determined at 298.15 K by heat conduction microcalorimeter. The standard enthalpy of formation of Phe(aq) has been calculated. The influence of different anions and molar ratio of zinc with L-α-phenylalanine has been discussed.

2 EXPERIMENTAL
2.1 Preparation and composition of the compounds
Referred to the literature [4], L-α-phenylalanine (B. R., purity >99.9%, made in Shanghai Kanda) was recrystallized and analyzed. Its purity, density and melting point were 99.99%, 1.357 g^.cm^-3 and 319-320 °C, respectively.
    According to the literature [1-3], five solid complexes of zinc salts with L-α-phenylalanine were prepared and analyzed. Zn²⁺ was determined complexometrically with EDTA. L-α-phenylalanine was analyzed by the formalin method. Before the titration, Zn²⁺ was removed by precipitating with K₂C₂O₄. Carbon, hydrogen and nitrogen analyses were performed on a 1106 type elemental analyzer. The analytical results are summarized in Table 1. The conductivity of the deionized water used in the experiments was 5.48×10^-8 S^.cm^-1.

Table 1. Analytical results of chemical composition for the complexes (%)

* The data in brackets are calculated values  

2.2 Experimental equipment and conditions
All measurements were performed on a heat conduction microcalorimeter (type RD496- from Southwest Electronic Engineering Institute of China), which was equipped with two 15 mL calorimetric vessels, and operated at 298.15±0.005 K. The microcalorimeter was calibrated by Joule effect and its sensitivity was 63.994±0.042 mV^.mW^-1. The experimental precision and accuracy were checked by the measurement of the enthalpy of solution of special purity crystalline KCl in deionized water at 298.15 K. The results are shown in Table 2. The experimental value of D_sol H^q_m of 17.24±0.05 kJ^.mol^-1 (t inspection, 99% believability) is excellent agreement with that of D_sol H^q_m of 17.241±0.018 kJ^.mol^-1 reported in the literature [5].This indicates that the device for the  enthalpy measurements is reliable.

Table 2. Enthalpy of solution of KCl in water at 298.15 K

Table 3. Enthalpies of solution in water of L-α-phenylalanine and its zinc complexes at 298.15 K

Results for the enthalpies of solution of L-a-phenylalanine and its five complexes with zinc salts in deionized water at 298.15 K are given in Table 3. Where r is the molar ratio n(H₂O)/n(compound). No solid residues were observed in the solutions after calorimetry. The greater values of r (4845-39071) were used and the D_sol H^q_m was nearly unchanged with the mass of the compounds. Therefore, the means of D_sol H^q_m in Table 3 can be considered at infinite dilution.

4.1 At 298.15 K, the dissolving process of L-α-phenylalanine is an endothermic reaction:
L-α-phenylalanine (s) -> L-α-phenylalanine (aq)
According to the process, the standard enthalpy of formation of L-α-phenylalanine (aq) can be calculated as follows:
D_{f, aq} H^q_m = D_sol H^q_m +D_{f, s} H^q_m
                 = [(8.311±0.080) + (-476.21±3.51)] kJ^.mol^-1
                 =-467.90±3.51 kJ^.mol^-1
where the value of D_{f, s} H^q_m is from the literature [4].

4.2 For the complexes with mole ratio 1:1, the process of solution in water are all the exothermic reactions. The value of D_sol H^q_m of the complexes decreases in the order:

An attempt at calculating the standard enthalpy of formation of Zn(Phe)²⁺_aq was not successful. Maybe because of the difference of the structures of the complexes, Zn(Phe)²⁺_aq was not always formed during the solution processes.

4.3 For the complexes of with the same anion, the larger the molar ratio of zinc with L-α-phenylalanine is, the smaller the exothermic effect is. The process of solution in water of the complex of Zn(Phe)₃(NO₃)₂^.H₂O even is an endothermic reaction. As the enthalpy of solution is connected with the formation enthalpies and the hydrated enthalpy of the solid complex and the products, it is necessary to further study the crystal structures of the solid complexes.

REFERENCES
[1] Gao Shengli, Hou Yudong, Liu Jianrui et al. Journal of Northwest University (Natural Science Edition), 1999, 29 (6): 517-522.
[2] Hou Yudong, Gao Shengli, Guo Lijuan et al. Chemical Journal of Chinese Universities, 1999, 20 (9): 1346-1348.
[3] Gao Shengli, Hou Yudong, Liu Jianrui et al. Acta Chimica Sinica, 2000, 58 (1): 65-70.
[4] Xuwu Yang, Jianrui Liu, Shengli Gao et al. Thermochim. Acta, 1999, 329: 109-115.
[5] Marthada V K. Journal of Research of the National Bureau of Standards, 1980, 85 (6): 467-481.

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