Efficient reduction of aliphatic nitro groups using Peptostreptococcus productus

The main aim of this project was to characterise the potential of Peptostreptococcus productus as a biocatalyst for asymmetric hydrogenation of C=C bonds, which cannot be accomplished by chemical catalysts, and to optimise this activity. These reductions are useful for the synthesis of novel enantiomerically pure chiral building blocks used in the pharmaceutical, agrochemical and food industries. Due to reproducibility problems in batch culture, an anaerobic fructose-limited chemostat was developed and optimised. An anaerobic cell harvesting method was developed and the cells harvested from the chemostat were used in biotransformations. An anaerobic biotransformation system was developed. Due to toxicity of the target substrates, an anaerobic 2-liquid phase system was developed using tetradecane as the organic solvent phase. The biotransformation of 2-nitro- 1-phenyl-1-propene and 2-nitro-1-phenyl-1-butene was studied using chiral HPLC method and GCMS analysis. The biotransformation of 2-nitro-1-phenyl-1-propene was optimised with respect to concentration of the buffer, the electron donor, the substrate and the biomass. It was shown that fructose is as efficient as H2 as the electron donor and 2-liquid phase system was not toxic for the biocatalyst. During the biotransformation, the fermentation was shifted to an incomplete oxidation of fructose coupled to reduction of the biotransformation substrate as an electron acceptor. P. productus reduced the C=C bond and NO2 group of 2-nitro-1-phenyl-1- propene and produced a mixture of 2-nitro-1-phenylpropane (53% yield, 38% e.e.) and 2-amino- 1-phenylpropane (amphetamine, 41% yield, 19% e.e.). When 2-nitro-1-phenylpropane was used as a substrate, the microorganism transformed it to amphetamine (41% yield). P. productus also transformed 2-nitro-1-phenyl-1-butene to the nitroalkane (38.6% yield, 15.4% e.e.) and the aminoalkane (7.5% yield). The yield of 2-nitro-1-phenyl-1-propane on the biocatalyst was 286 times higher than the best value for Baker’s yeast. 2-Nitro-1-phenyl-1-propene was toxic and the product yield decreased as the initial substrate concentration was increased. The best e.e. was obtained in the reaction mixture containing 5 mM 2-nitro-1-phenyl-1-propene. The optimum pH regarding production yield and e.e. was 7. On the whole, the enantiomeric excess of 2-nitro-1-phenyl-1-propane formed by P. productus was poor. However, the novel reduction of aliphatic nitro groups to amine seems to be useful in synthesizing pharmaceuticals, which is difficult by chemical methods.

Keywords:

Biotransformation , Peptostreptococcus productus , reduction , aliphatic nitro groups