Asymmetric synthesis using hydrolytic enzymes in supercritical carbon dioxide

Influence of the hydrostatic pressure and pH on the asymmetric 2-hydroxyketone formation catalyzed by Pseudomonas putida benzoylformate decarboxylase and variants thereof

Benzoylformate decarboxylase (BFD) from Pseudomonas putida is a thiamine diphosphate-dependent (ThDP) enzyme that catalyzes the asymmetric C--C bond formation to (S)-2-hydroxypropiophenone [(S)-HPP] starting from benzaldehyde and acetaldehyde. The enantioselectivity of BFD was shown to be a function of temperature and substrate concentration. It can additionally be changed by site-directed mutagenesis on hot spot positions in the active site. In this article, we present the effect of hydrostatic pressure up to 250 MPa on the enantioselectivity for the recombinant wtBFD as well as for the variants BFD F464I, BFD A460I, and BFD A460I-F464I. A general tendency toward lower amounts of (S)-HPP could be observed at increasing pressures. For two of these variants an increase in pressure even caused an inversion in the enantioselectivity and thus increasing enantiomeric excesses, respectively. A pressure-induced increase in enantioselectivity could therefore be observed for the first time in biocatalysis to the best of our knowledge. Furthermore, the pH is shown to be a parameter that also significantly influences the enantioselectivity of the reaction mentioned above.

Effects of pressure and temperature on enzymatic reactions in supercritical fluids

Supercritical fluids (SCFs) are receiving increasing attention as reaction media because they permit higher reaction rates compared with the conventional solvents. The ease of manipulating the physical properties of the SCFs enables easier control of the reaction conditions and easier solvent removal after the reaction. This review focuses on effects of pressure, temperature and the properties of the SCFs, on enzymatic reactions. Phase behavior, reaction rate and activation volume in SCFs are discussed. Within the ranges of pressure (10-40 MPa) and temperature (35-60 degrees C) that typically characterize the supercritical region, an increase in pressure and/or a decrease in temperature lead to a decrease in the enzyme turnover because the diffusion coefficients of the substrates migrating to the active sites of enzymes are affected.

Optical resolution methods

Despite the large number of elaborate enantioselective syntheses for the preparation of a single enantiomer to achieve industrial and scientific goals, the separation and purification of enantiomers (components of racemic compounds) is also necessary. Hence, we present the most often used thought-provoking modern methods based on momentous recognitions (e.g. spontaneous resolution, induced crystallization, resolution by formation of diastereomers, resolution by formation of non-covalent diastereomers, resolution by diastereomeric salt formation, resolution by diastereomeric complex formation, "half equivalent" methods of resolution, separation by crystallization, separation by distillation, separation by supercritical fluid extraction, resolution with mixtures of resolving agents, resolution with a derivative of the target compound, enantioselective chromatography, resolution by formation of covalent diastereomers, resolution by substrate selective reaction, kinetic resolution without enzymes, kinetic resolution by enzyme catalysis, hydrolytic and redox enzymes, kinetic and thermodynamic control, resolutions combined with 2nd order asymmetric transformations, enrichment of partially resolved mixtures, role of the solvent and methods of optimization in the separation of diastereoisomers, non-linear effects and selected examples of resolution on an industrial scale).