Temperature cycling induced deracemization (TCID) is an attractive method to provide a pure enantiomer from a racemic solid phase. In this work, we performed deracemization of the nonessential amino acid asparagine using TCID of DL-asparagine monohydrate crystals (DL-ASN•H 2 O) and an immobilized amino acid racemase (imAAR) as a racemization agent. Experiments were performed with varying initial suspension densities and dosages of imAAR to better understand the process and included measurements of both solid-phase and solution-phase enantiomeric excesses. Furthermore, we demonstrate how to improve the productivity and yield of the TCID process by adding a temperature hold at the end of the process to ensure the maximum possible yield and compare these results to a process using only standard temperature cycles. Finally, we also discuss the effect of both temperature programs on the performances of the TCID process to be a strategy to further improve this process for industrial application.

Conventional enantioselective preparative chromatographic separation using columns packed with chiral stationary phase is characterized by a 50% yield constraint. Racemization of the undesired enantiomer and recycling the formed mixture is an attractive option to tackle this limit. To implement this concept, potential is seen in particular in applying enzymes immobilized in a second fixed bed. However, the identification of suitable operating conditions and the direct connection of a chromatographic column and an enzymatic reactor is not trivial. The paper presents results of an experimental study applying jointly a batch-wise operated chiral Chirobiotic T column to resolve the two enantiomers of mandelic acid (MA) and a mandelate racemase immobilized on Eupergit CM. The general concept could be successfully demonstrated over several cycles focusing on the provision of (S)-MA. A mathematical model was developed in order to illustrate essential process features and to quantitatively describe the coupled separation and racemization processes. The key ingredients of this model, namely, the adsorption isotherms of the two enantiomers on the chiral column and the rate of racemization in the enzymatic reactor, were determined experimentally. The potential of applying the model for further process optimization and generalization is indicated.

Enantioselective chromatography is nowadays a reliable tool for single enantiomer production from a racemate. The recovery of the distomer by racemization and recycling is a promising method to tackle the 50% yield constraint and to increase the productivity. In this paper three process configurations are compared. The production of enantiopure mandelic acid and methionine enantiomers exploiting different enzymes for racemization are evaluated as part of different chromatographic process configurations. First, the benefits of conventional simulated moving bed (SMB) chromatography in contrast to a single column batch separation unit are assessed in integrated configurations. Then, a concept of coupling the racemization with a simpler three-zone SMB unit, where one regeneration zone is removed, is evaluated.

Enantioselective resolution processes can be improved by integration of racemization. Applying environmentally friendly enzymatic racemization under mild conditions is in particular attractive. Owing to the variety of enzymes and the progress in enzyme engineering, suitable racemases can be found for many chiral systems. An amino acid racemase (AAR) from P. putida KT2440 is capable of processing a broad spectrum of amino acids at fast conversion rates. The focus of this study is the evaluation of the potential of integrating ARR immobilized on Purolite ECR 8309 to racemize L- or D-methionine (Met) within an enantioselective chromatographic resolution process. Racemization rates were studied for different temperatures, pH values, and fractions of organic co-solvents. The long-term stability of the immobilized enzyme at operating and storage conditions was found to be excellent and recyclability using water with up to 5 vol% ethanol at 20 °C could be demonstrated. Packed as an enzymatic fixed bed reactor, the immobilized AAR can be coupled with different resolution processes; for instance, with chromatography or with preferential crystallization. The performance of coupling it with enantioselective chromatography is estimated quantitatively, exploiting parametrized sub-models. To indicate the large potential of the AAR, racemization rates are finally given for lysine, arginine, serine, glutamine, and asparagine.