[Figure 5]
In doses of 0.2 g·mL-1, Amberlite IR-120(H), Dowex Monosphere 650C and Dowex Marathon MSC adsorb >90% of 2-MP and (S )-2-MPN from the test solution. In the smallest dose of 0.02 g·mL-1 a selective adsorption of about 40% of the product to the substrate is observed regarding of Amberlite IR-120(H), Dowex Monosphere 650C and Dowex Marathon MSC. The highest removal efficiency was found with Dowex Marathon MSC, which was used during the scale-up and the DSP of (S )-2-MPN. A remarkably high selectivity towards the product (S )-2-MPN over the substrate 2-MP was observed using the ion exchange resin IRN-150 making it also a perfect candidate for a potential in situ product adsorption (ISPA) process (entry 2 with 0.2 g resin per mL reaction medium). Due to this type of procedure, the ion exchange resin could be dispersed in the reaction solution while the biocatalytic reaction takes place. A resulting product would be directly adsorbed and thus the reaction equilibrium pushed to the products’ side according to the Le Chatelier’s principle. Unfortunately, all our attempts using this cation exchange resin within an imine reductase-catalyzed reaction were unsuccessful due to a full deactivation of the biocatalyst by the resin material. The reason for this behavior is unknown and will be investigated thoroughly in a future study.

Scale-up and Downstream-Processing of (S )-2-MPN

After establishing the successful adsorption procedure, the development of an efficient adsorption-desorption methodology was crucial. A 150 mL-preparative-scale IRED-catalyzed reaction was performed. The reaction was checked for a full conversion (see Figure 6) and after 24 h, 30 g Dowex Monosphere 650C cation exchange resin (0.2 g·mL-1) were added to the reaction broth and stirred for 30 min at 30 °C. The adsorption of the product was checked and confirmed to be >95% (see Figure S 5 and 6).