Total turnover number substrate solubility

All of these oxidoreductases reduce aromatic and aliphatic a-chloropropargyhc ketones (27a-27c) with high activity. a-Bromopropargylic ketone 27d is also accepted as a substrate however, the enzymatic activity of TBADH and recLBADH decreases, probably for steric reasons. Due to the low solubility of substrate 27a about 25% of a short-chained alcohol was added. The large excess of short-chained alcohol shifted the substrate/product equilibrium toward the desired propargylic alcohol 28a, resulting in almost quantitative conversions with high total turnover numbers (TTN) of the cofactor. 

As a new option, for the bioconversion of poorly soluble substrates the classical EMR-concept can be extended to an Emulsion Membrane Reactor , comprising a separate chamber for emulsification (with a hydrophilic ultrafiltration membrane), an EMR-Ioop with a normal ultrafiltration module, and a circulation pump. This approach has been successfully demonstrated for the enzymatic reduction of poorly soluble ketones [107]. Using this device, e.g., for the enantioselective reduction of 2-octanone to (S)-2-octanol (e.e. >99.5%) with a carbonyl reductase from Candida parapsilosis under NADH-regeneration with FDH/for-mate, the total turnover number was increased by a factor 9 as compared with the classical EMR. 

The same principle has been used in so-called continuous membrane reactors [83,85,88-95]. In this case the membrane is used to retain a soluble polymer bound catalytic species. Low molecular weight substrates are transformed continuously in the reactor and ideally pure products can be collected beyond the membrane. This can lead to easy separation and an increase in the total turnover number of the catalyst [85]. However, these systems are very demanding on support and membrane, since for efficient use a retention of more than 99.9% has to be guaranteed. In addition, even the best membranes cannot prevent metal leaching. 

Figure 7 Influence of substrate solubility on the total turnover number (ttn) of the coenzyme.

Figure 15 Total turnover number (ttn) as a function of the ratio substrate/coenzyme concentration and coenzyme retention of the enzymatic reduction of poorly soluble ketones. The data points are experimental values (compare Table 5) and indicate the increase of the ttn compared to the operation without additional means for coenzyme retention. (A) Synthesis of (S)-l-pheny 1-2-propanol (B) synthesis of (iS)-2-octanol.

A similar water-soluble colloidal system has been described by James and coworkers [19]. Rhodium colloids were classically produced by reducing RhCk 3H2O with ethanol in the presence of PVP and triethylamine. The mono-phasic hydrogenation of various substrates such as benzyl acetone, propyl-phenol and benzene derivatives was performed under mild conditions (25 C and 1 bar H2). The nanoparticles were jxjorly characterized but benzyl acetone is reduced with 50 TTO (total number of turnovers) in 43 h. 

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