[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).