Optimized enantioselective (S)-2-hydroxypropiophenone synthesis by free- and encapsulated-resting Pseudomonas putida cells

Background: Aromatic α-hydroxy ketones such as S-2-hydroxypropiophenine (S-2-HPP) are highly valuable chiral building blocks useful for the synthesis of various pharmaceuticals and natural products. In the present study, enantioselective synthesis of S-2-HPP was investigated by free and immobilized whole cells of Pseudomonas putida ATCC 12633 starting from readily-available aldehyde substrates. Whole resting cells of P. putida grown in a culture medium containing ammonium mandelate are a source of native benzoylformate decarboxylase (BFD) activity. BFD synthesized by induced P. putida resting cells is a highly active biocatalyst without any further treatment in comparison with partially purified preparation. These cells can convert benzaldehyde and acetaldehyde into the acyloin compound 2-HPP by BFD catalyzed enantioselective cross-coupling reaction. Results: The reaction was carried out in the presence of exogenous benzaldehyde (20 mM) and acetaldehyde (600 mM) as substrates in 6 ml of 200 mM phosphate buffer (pH 7) for 3 hours. The optimal biomass concentration was assessed to be 0.006 g DCW/ml. 2-HPP titer, yield and productivity using the free cells were 1.2 g L -1 , 0.56 g 2-HPP/g benzaldehyde (0.4 mol 2-HPP/mol benzaldehyde), 0.067 g 2-HPP/g DCW h, respectively, under optimized biotransformation conditions (30 ⁰C, 200 rpm). Calcium alginate (CA)–polyvinyl alcohol (PVA)-boric acid (BA)-beads were used for the cell entrapment. Encapsulated whole-cells were successfully employed in four consecutive cycles for 2-HPP production under aerobic conditions without any noticeable degradation of the beads. Moreover, there was no production of benzyl alcohol as an unwanted by-product. Conclusion: Thus, bioconversion by P. putida whole resting cells is an efficient strategy for the production of 2-HPP and other α-hydroxyketones.