Characterization and crystallization of chiral aromatic amino acids

Li, Yan (2014) Characterization and crystallization of chiral aromatic amino acids. Masters thesis, Memorial University of Newfoundland.

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Abstract

The aromatic amino acids play a vital role in both biology and pharmacology areas. On one hand, they are the essential building blocks of protein, which can carry out many crucial bodily functions. On the other hand, these substances are recognized as chiral drugs containing a racemate and two enantiomers. In addition, the special aromatic ring adds a complex structure to this kind of amino acids. As a result, the investigation of the aromatic amino acids gains great significance and can provide people some practical ideas in the researching works about protein. This thesis presents a comprehensive study on the characterization and crystallization of aromatic amino acids. In the first part, diverse recognition methods were applied to characterize the L-tryptophan (L-Try), such as X-ray single crystal and powder diffraction analyses, differential scanning calorimetry, as well as measurement of solid-liquid equilibrium in water/isopropanol solution. The absence of eutectic point was demonstrated by the binary melting point diagram and the ternary phase diagram, which indicated the existence of a pseudoracemate (i.e., solid solution). The racemate, enantiomers, and the mixtures of them gave the identical powder X-ray diffraction patterns, and the X-ray single crystal analysis showed that the racemate and enantiomers of tryptophan gained very similar solid-state packing geometries, which confirmed the conclusion above. As a result, all the thermodynamic and crystallographic analyses have supported the conclusion that the tryptophan crystallized as a pseudoracemate. The second part of this thesis mainly focused on the study of the effect of tailor-made impurity on the crystallization process. The influences of L-tyrosine (L-Tyr) on the solubility, crystallization kinetics, and polymorphism of L-Phenylalanine (L-Phe) were respectively investigated. The results indicated that the presence of L-Tyr could increase the solubility and the crystal nucleation rate of L-Phe in water, however, the growth of L-Phe on a certain direction would be suppressed and thus the formation of polymorphs became unavoidable. Specifically, the addition of L-Tyr could make the anhydrate form of L-Phe more stable at 24ﾟC.

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