L-phenylalanine 동질이상체 결정화에 대한 첨가제의 영향

우상훈, 구기갑

서강대학교 공과대학 화학공학과

Influence of additives on polymorphic crystallization of l-phenylalanine

Sang-Hoon Woo, Kee-Kahb Koo

Dept. of Chem. Eng., Sogang Univ.

Introduction

Many organic material, pharmaceutical and food substanceshave polymorphism. When a material is crystallized in different crystal structures, their physical properties are very different , even though identical in chemical speicies.  For that reason, polymorphism phenomena reveals important information of crystal structure property relationships, molecular level control of crystallization and the prediction and design of crystal structure.  Therefore, phase transformation study is essential to control the final products in industrial process.

L-phenylalanine, one of the 20 standard amino acids, is a starting material of asparatame, artificial sweetner.  It has two polymorphic forms, a hydrate and an anhydrate.  Hydrate form is stable below transition temperature and has needlelike morphology.  On the contrary anhydrate form, which has flakelike morphology, exist stably above the transition temperature.  In general anhydrate form is preferable in industry, because it has good characteristics in handling and storage.  Recently, many studys to obtain the specific form of the l-phenylalanine polymorphs are carried out and interesting results are reported.[1]

In the present study, we focus on influences of additives to the phase transformation rate to investigate selectivity of the preferable polymorph.

Experiments

Solubility measurement

The solubility of l-phenylalanine with/without additives in water is measured by the gravimetric method. 50g water, in the double-jacketed cell, is heated to the desired temperature with a excessive l-phenylalanine and mixed using magnetic bar.  The temperature is controlled by thermostat (Polyscience) with water circulation. After 24 hours, the remaining l-phenylalanine is filtered, dried in vacuum oven and weighed in microbalance.  The solubility is determined by substration of remaining l-phenylalanine weight from the original l-phenylalanine weight.  The filtered l-phenylalanine is confirmed by using the X-Ray diffraction pattern.(Rigaku, Minniflex)

Transformation rate studies

The phase trasformation experiments of anhydrate l-phenylalanine to monohydrate form is carried out at 5oC, 15oC and 30oC.  Firstly, excess amount of hydrate l-phenylalanine is put into the water to prepare a excess hydrate l-phenylalnine solution, which is maintained at desired temperature.  After 24 hours, undissolved hydrate l-phenylalanine is removed by filtration, and remained solution is kept at higher temperature than desired temperature for 3 hours. The solution is maintained at a desired temperature again to prepare a hydrate saturated solution for 3 hours.  The prepared saturated solution is distributed to 10 glass test tubes by 40g with anhydrate l-phenylalanine of 0.2g and additive.  The amount of additives is 0.2g and 2g for ammonium sulfate and dextrose, respectively.  All the prepared samples are placed in the desired temperature maintained bath simultaneously and stired by magnetic bar in fixed speed.  Fig.1. shows a experimental setup.  Each samples taken  with some time interval are separated from water by filtration.  The filtered crystals are dried for 24 hours at room temperature.  Finally dried crystals are analyzed by X-Ray diffraction.  The degree of transformation is determined by x-ray diffraction pattern area

Fig. 1 Experimental setup for transformation rate measurement.

Results and discussion

Solubility measurements

The solubility of l-phenylalanine polymorphs for water is plotted in Fig. 2, which is regressed using least square method in the temperature range from 15oC to 50oC.  The regressed curve is presented by following equations

S= 2.043+2.39410-2T+4.21110-4T2  for anhydrate

S= 1.949+5.16910-3T+9.90010-4T2  for hydrate

where S is the solubility(g l-phenylalanine/100g water) and T is the temperature(oC)  The hydrate curve meets anydrate curve at 38oC, which is the transition point.  Since the transition point exists in the temperature range, l-phenylalanine has an enatiotropic polymorph.[3]

Fig. 3 shows the solubility of anhydrate l-phenylalanine with various additives  selected on the basis of electrolyte and nonelectrolyte. Some electrolytes such as ammonium sulfate and sodium chloride lower the solubility of anhyrate.  On the other hand, aluminum sulfate and aluminum postssium sulfate dodecahydrate strikingly increase the solubility.  Nonelectrolytes, for instance, dextrose and sucrose, scarcely influence the solubility change. In aqueous solutions, amino acid bears positive and negative charge in a molecule, which is called zwitterions.  Therefore, the solubility of amino acid is affected by pH of solution.[2]  In this study, electrolytes as a additive may play a role in changing the pH of a solution and this may affect the solubility of l-phenylalanine.

Fig. 2 Solubility of l-phenylalanine     Fig. 3 Effect of additives on solubility

Transformation rate measurement

The transformation rate of anhydrate form to hydrate form was measured at 5, 15 and 30oC. At 15oC. It was observed that it takes longer to start and complete transformation as the temperature is close to the transition point. This result can be explained by the fact that the free energy difference by between two crystal structures given the solubility difference plays a major role in phase transformation kinetics rather than the kinetic effect activated by increasing temperature. But the induction time difference was formed to be small between 5oC and 15oC, which is explained by mass transport phenomena.  Mass transport efficiency is decrease, as the temperature lower. Therefore, the induction time difference of 5oC and 15oC is not so large as 15oC and 30oC.

With a ammonium sulfate, the phase transformation was proceeded very slowly.  It takes around 11 hours for complete conversion. This can be understood that effect of ammonium sulfate on the kinetics of the phase transformation is due to inhibition of molecular interactions among l-phenylalanine and water molecules by reducing the solubility.	Similarly with the ammonium sulfate, the transformaton rate was found to slow down with dextrose.  In this case, transformation may be explained with dextrose's molecular structure. Long-chained molecules of dextrose inhibit the mass transfer of the phenylalanine molecules in the solution or seem to at the interface of monohydrate crystals.

Conclusions

The solubility of l-phenylalanine in water is presented by  the following equations :

S= 2.043+2.39410-2T+4.21110-4T2  for anhydrate

S= 1.949+5.16910-3T+9.90010-4T2  for hydrate.

Transition point is 38oC and has enatiotropic polymorph.  With the electrolyte additives such as ammonium sulfate and sodium chloride, the solubility of anhydrate l-phenylalanine increase, whereas aluminum sulfate and aluminum postssium sulfate dodecahydrate decrease the solubility. Nonelectrolyte additives have no effect on the solubility.

Because the dirving force of transformation is the solubility difference between polymorphs, the transformation of anhydrate to hydrate is slow as the temperature is closer to transition temperature. But in the low temperature, transformation is slow due to the mass transfer effect. The additives, ammonium sulfate and dextrose, were found to make the transformation rate slow. However, the mechanisms of their effect on the transformation kinetics seem to be not the same.

References.

1. European Patent 0703214A2

2. □Handbook of Industrial Crystallization□, A. S. Myerson(ed.), Butterworth-Heinemann, London, 1993.

3. □Polymorphism in Pharmaceutical Solids□ H. G. Brittain(ed.), Marcel Dekker, New York, 1999.