Enzyme-assisted physicochemical enantioseparation processes—Part II: Solid–liquid equilibria, preferential crystallization, chromatography and racemization reaction

Abstract

This contribution addresses the design and investigation of two hybrid enantioseparation processes including an enzymatic racemization step in order to enhance the overall performance. Complementary to part I where the manufacturing and the characterization of an amino acid racemase (EC 5.1.1.10) was emphasized [Würges, K., Petrusevska, K., Serci, S., Wilhelm, S., Wandrey, C., Seidel-Morgenstern, A., Elsner, M.P., Lütz, S., 2009. Enzyme-assisted physicochemical enantioseparation processes—part I: production and characterization of a recombinant amino acid racemase. J. Mol. Cat. B (in print), online available: doi:10.1016/j.molcatb.2008.10.006.], the work presented in this paper tends more towards developing a data base for potential process schemes for the manufacture of selected amino acids. The first proposed process concept (P-I) couples preferential crystallization (PC) and racemization for the production of l-asparagine ( l-Asn) using racemic mixture of dl-asparagine (conglomerate-forming system) as a starting material, while the second concept (P-II) integrates chromatography and racemization for the preparation of l-methionine ( l-Met) starting with racemic mixture of dl-methionine (compound-forming system). As mentioned in part I, a racemization unit, where the unwanted enantiomer will be converted into racemate, is incorporated into the hybrid processes for the sake of 100% yield, theoretically. Besides the basic investigation according to the solid–liquid equilibria, PC and chromatography, the focus of this paper is mainly on the kinetic studies of the racemization reaction. Initially, the solubility ternary phase diagrams of both examined systems were determined, leading into the idea of combination of the proposed process schemes. For P-I the concept of PC of l-Asn was experimentally proven and the kinetics of the racemization was examined for d- and l-Asn in water using purified lyophilizate (PL). Concerning P-II, for the chromatographic unit the impact on the separation of dl-Met on eremomycin based stationary phase using KPi buffer and MeOH as mobile phase was evaluated in terms of resolution and selectivity at three different temperatures by varying the content of methanol (MeOH) in the mobile phase and the pH. The experiments for determination of the racemization kinetics were done for a compromised parameter set using crude lyophilizate (CL). In both cases a Michaelis–Menten three-step model was used to describe the enzymatic reaction.