The development of novel biocatalysts for the asymmetric hydroamination of alkenes

Abstract

The research presented in this thesis describes the application of phenylalanine ammonia lyase from the bacteria Anabaena variabilis (AvPAL), as a biocatalyst for the asymmetric hydroamination of cinnamic acid derivatives. PALs from eukaryotic sources such as the plant Petroselinum crispum (PcPAL) and yeast Rhodotorula glutinis (RgPAL) have been widely used as biocatalysts for the synthesis of non-natural amino acids. For example the PAL catalyzed hydroamination of 2'-chlorocinnamic acid has been implemented by DSM Pharma Chemicals on a tonne scale. However, there are very few examples of prokaryotic PALs and to our knowledge their activity towards unnatural substrates has not been investigated. Herein we explore the activity of AvPAL towards a panel of cinnamic acid analogues. For comparison, the activity of the commonly studied eukaryotic PcPAL and RgPAL towards the same substrate panel was also investigated. Although the difference in substrate conversions between the three PALs was fairly unremarkable, a significant reduction in product e.e was observed following prolonged reaction times with all three PALs towards substrates bearing electron deficient aromatic rings. A time dependence on e.e. has not been previously reported for ammonia lyases and all previously described biotransformations have been reported to proceed with excellent e.e. in favour of the L-enantiomer. The mechanism leading to the formation of D-phenylalanine derivatives was explored through mutagenesis of key active site residues and isotopic labeling studies. The results obtained demonstrate that D-amino acid formation occurs via a previously unobserved competing MIO-independent pathway which proceeds in a non-stereoselective manner. In addition, the observations are consistent with amino acid deamination occurring via a stepwise E1cB elimination mechanism. In order to develop a more general biocatalytic method for asymmetric hydroamination reactions, the activity of PAL towards substrates lacking the carboxylic acid functionality was investigated. The synthesis of a panel of substrates and subsequent screening with AvPAL and RgPAL is described. Unfortunately, the wild-type enzymes demonstrated no activity towards any of the substrates screened. These enzymes were also screened for their promiscuity towards the nucleophilic amine partner and although deamination activity towards N-methyl-L-phenylalanine was observed, no hydroamination activity was detected using primary amines as nucleophiles. In order to broaden the substrate specificity of PAL enzymes, a number of screening methods have been developed. Herein we present both liquid phase and colony based colorimetric screens for the detection of PAL catalyzed hydroamination activity. Furthermore these screens have been used to screen libraries of variants for increased D-selectivity and hydroamination activity towards β-methylstyrene and cinnamyl alcohol derivatives.

Date of Award	1 Aug 2014
Original language	English
Awarding Institution	The University of Manchester