This title appears in the Scientific Report : 2011 

Enzyme supported crystallization of chiral amino acids
Würges, Kerstin (Corresponding author)
Biotechnologie 2; IBT-2
Jülich Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag 2011
Universität Düsseldorf, Diss., 2011
Dissertation / PhD Thesis
Schriften des Forschungszentrums Jülich. Reihe Gesundheit / Health 40
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Chiral molecules are versatile building blocks for the synthesis of pharmaceuticals and fine chemicals. α-amino acids (natural and non-natural ones) represent one major group among these chiral molecules having, with the exception of glycine, at least one chiral center. Besides fermentative production methods, amino acids are also synthesized chemically as racemates. Thus, for chiral applications, the enantiomers have to be separated. To overcome yield limitations of simple enantioseparation processes, which are generally limited to 50 %, the aim of this thesis was the development of new processes for the efficient production and separation of chiral amino acids by the combination of enzymatic racemization/isomerization and crystallization. To achieve this goal, two different approaches have been investigated starting from either racemic or enantiopure substrates: 1) $\textit{Enzyme-assisted preferential crystallization}$ combines the crystallization of a single enantiomer from a racemic solution with enzymatic racemization of the remaining enantiomer. This concept is only applicable to the relatively small group of conglomerate forming racemates. Asparagine was identified as a suitable model substrate for preferential crystallization and enzymatic racemization using the purified amino acid racemase AArac2440 from $\textit{Pseudomonas putida}$ KT2440. A process for the dynamic resolution of Lasparagine from a supersaturated racemic solution was developed in a 20 mL scale, and the product ee could be increased significantly by enzymatic racemization of the remaining Dasparagine during crystallization. This process is a modification of a classical dynamic kinetic resolution, where product separation occurs via enantioselective crystallization instead of asymmetric transformation. 2) The second approach focuses on the $\textit{production of chiral allo-threonine from threonine by enzymatic isomerization and crystallization}$. Both enantiomers of the valuable $\textit{allo}$-threonine have been produced with good yields from low-prized threonine using purified amino acid racemase AArac12996 from $\textit{Pseudomonas putida}$ NBRC12996. Product separation was performed by simple crystallization from the reaction solution, which was possible due to the lower solubility limit of $\textit{allo}$-threonine compared to threonine. The presented approaches are samples of new promising methods for the production and separation of chiral amino acids. Further investigations on improved catalysts may expand the general application scope of these methods to even more versatile substance groups.