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

Received Apr. 25, 2000; Supported by the National Natural Science Foundation of China (No. 29871023)

Abstract  The thermokinetics of the formation reaction of zinc histidine are studied by a microcalorimeter. On the basis of experiment and calculated results, three thermodynamic parameters (the activation enthalpy, the activation entropy and the activation free energy), the rate constant and three kinetic parameters (the activation energy, the pre-exponential constant and the reaction order) have been obtained. The influence of temperature and the synthetic conditions of the complex have been discussed.
Keywords Histidine, Zinc acetate, Microcalorimeter, Thermokinetics

1 INTRODUCTION
Zinc is one of the essential trace elements for human body. L-α-amino acids are the basic structural units of protein. The complexes of zinc with histidine residues are widely existed in zinc enzymes and zinc figure proteins. Zinc L-α-histidine complex as additives have widely applied prospects in medicines, foodstuff and cosmetics ^[1,2].Our research group have investigated the solubility properties of Zn(OAc)₂-His-H₂O systems at 25°C in the whole concentration range by phase equilibrium method ^[3]. The result indicates that there is a region of Zn(His)(OAc)₂^.1/2H₂O in the diagram and the complex is a congruity soluble one. While the thermokinetics study of this complex has not been reported in the literature. In this paper, the reaction thermokinetics of Zn(OAc)₂^.2H₂O with solution of histidine has been investigated by a microcalorimeter.

2 EXPERIMENTAL
2.1 Materials 
Zn(OAc)₂^.2H₂O is AR grade (Xi'an Chemical Company). L-α-histidine is BR grade (Shanghai Kangda Company) with the >99.5% purity. The conductivity of the deionized water is 5.48×10^-8S^.cm^-1.
2.2 Experimental equipment and conditions    
The reaction thermokinetics was studied by a microcalorimeter, type RD496-III (China, Southwest Institute of Electronic Engineering), which was equipped with two 16 mL vessels (Fig. 1, where 1 is the calorimetric cell, 2 is the solid sample, 3 is the spacer, 4 is the solution.). After reaching equilibrium, the spacers of the sample and reference vessels were pushed down simultaneously and the samples were mixed. The microcalorimeter was calibrated by Joule effect and its sensitivity were 63.994±0.042 mV^.mW^-1, 64.308±0.027 mV^.mW^-1 and 64.499±0.040 mV^.mW^-1 at the experimental temperature of 298.15±0.005K, 303.15±0.005K and 308.15±0.005K, respectively. The experimental precision and accuracy were checked by measuring the enthalpy of special purity crystalline KCl in deionized water at 298.15K. The experimental value of D_sol H^q_m of 17.238±0.048 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 ^[4].This indicates that the device used in this work was reliable.

Fig. 1 Device used for the study   1.calorimetric cell; 2. solid sample, 3. Spacer, 4. solution	Fig. 2 Typical thermokinetic curve of the reaction

3 RESULTS AND DISCUSSION
The reaction could be represented by the following equation (1):
Zn(OAc)₂^.2H₂O (s) + His (aq)      Zn(His)²⁺ (aq) + 2OAc^- (aq) + 2H₂O (l)                          (1)
Within the range of the experimental temperature, the reaction is an exothermic one. The typical thermokinetic (TK) curve of the reaction was shown in Fig.2. The original data obtained from the TK curve were shown in Table 1. These experimental data were used in equation (2) and the reaction order and rate constant were obtained.
ln [(dH_t/dt)/H₀] = ln k + n ln [1-(H_t/H₀) ]                         (2)
Where H₀ was the total reaction heat (corresponding to the global area under the TK curve); H_t, the reaction heat in a certain time (corresponding to the partial area under the curve); dH_t/dt, the exothermic rate at time t; k, the rate constant of reaction; n, the reaction order .
    The values of k and n obtained by Eq. (2), the values of E and A obtained by Eq. (3), the value of DG⁰ obtained by Eq. (4) and the values of DH⁰ and DS⁰ obtained by Eq. (5), which were all listed in Table 2.
ln k = ln A - [E/RT]                        (3)
DG⁰ = RT ln [RT/Nhk]                        (4)
ln [k/T] = -(DH⁰/RT)+ (DS⁰/R)+ ln [k_B/h]                         (5)
    where A was the pre-exponential constant; E, the apparent activation energy; R, the gas constant; T, the absolute temperature; DG⁰, the activation free-energy; N, Avogadro number; h, Planck's constant; DH⁰, the activation enthalpy; DS⁰, the activation entropy; k_B, Boltzmann's constant.

Table 1 Thermographic data of the reaction

H₀ = 1.55, 1.28 and 1.29 J.

Table 2 Thermographic data of the reaction

T/K	Eq.(2)			Eq.(3)			Eq.(4)	Eq.(5)
	k/s-1	n	r#	E /kJ.mol-1	logA /s-1	r#	DG0 /kJ.mol-1	DH0 /kJ.mol-1	DS0 /J.mol-1K-1	r#
298.15	31.81	0.96	0.999	22.49	5.45	0.956	64.45	19.96	-149.0	0.945
303.15	39.92	1.02	0.993				65.00
308.15	42.69	1.14	0.992				65.94

^# r, the correlation coefficient.

    The results in Table 2 clearly indicate that the higher the temperature of the reaction, the faster the rate of the reaction and the titled reaction is of the first order. While the values of E and DH⁰ were very low and DS⁰ is high. These facts showed that the titled reaction easily took place in the temperature range of 298.15K-308.15K.
    The final solution collected together from each experiment and the solution with the same mole rate as the reaction were concentrated on a 70-80°Cwater bath till crystal membrane formed on the surface, and then, this was put into a containing P₄O₁₀ desiccator to remove trace water. The analytic results indicated that they had the same composition of Zn(His)(OAc)₂^.1/2H₂O which is consistent with the result of the literature ^[3]. The titled complex can be prepared easily at mild conditions and that is why the complex of Zn(His)(OAc)₂^.1/2H₂O has a high enthalpy of formation (-1665.02±3.28 kJ^. mol^-1) ^[5].

REFERENCES  
[1] Taguchi M, Inokuchi M, Nakajima N et al. WO Patent 10 178, 1992-06-25.
[2] Jean-Noel T. FR Patent 2 649 692, 1992-08-03.
[3] Liu Jianrui, Hou Yudong, Gao Shengli et al. Acta Chimica Sinica, 1999, 57: 485-490.
[4] Marthala V K. J. Res. Nat. Bur. Stand., 1980, 85 (6), 467-481.
[5] Liu Jianrui, Yang Xuwu, Hou Yudong et al. Thermochimica Acta, 1999, 329: 123-127.

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