Enzymatic Carboxylation in Ionic Liquids (ECIL)

Although CO2 is a waste gas, it that has huge potential as a carbon source for the synthesis of valuable organic compounds. Consequently, much research effort has been addressed to chemical CO2-fixation; however, the high oxidation state of CO2 and its low energy level pose a substantial challenge, requiring harsh reaction conditions, such as high pressure and temperature (e.g. in the Kolbe-Schmitt reaction) for the production of aromatic carboxylic acids. Biocatalytic processes that operate under ambient conditions, such as the recently demonstrated enzyme-catalysed carboxylation of aromatics, offer a promising biosynthetic tool as a `green` alternative using bicarbonate as CO2 source. Based on preliminary results, we could show that the enzymatic carboxylation proceeds in a highly regio- complementary fashion: Benzoic acid decarboxylases (BDCs) selectively catalyse the ortho- carboxylation of phenols yielding the corresponding o-hydroxybenzoic acid derivatives, whereas phenolic acid decarboxylases (PADs) exclusively act at the ß-carbon atom of the side-chain of styrene derivatives forming (E)-cinnamic acid derivatives. This `side-chain` carboxylation is especially remarkable since no chemical counterpart exists in traditional chemistry. Despite perfect regio-selectivity, the direction towards desired biosynthetic carboxylation is thermodynamically disfavored and therefore it is impeded by incomplete conversion, which represents one of the most demanding challenges within this project. Second, engineering of the biocatalytic process using novel tools for in-situ product removal by reactive extraction as well as the use of ionic liquids will be investigated. Third, a cascade system consisting of two consecutive enzymatic reactions in which product I is the substrate of enzyme II, will be evaluated. Overall, these strategies aim at the goal to shift the equilibrium towards full conversion. An additional research focus will be the engineering of the biocatalysts using mutagenesis as well as the evaluation of the substrate scope of the enzyme candidates since the latter is of crucial importance for a biocatalytic approach particularly in view of a broad applicability. This research proposal brings together two groups of complementary expertise and synergistically combines process engineering systems (Udo Kragl, University of Rostock, Germany) with synthetic bio-catalysis and bio-organic chemistry (Kurt Faber, University of Graz, Austria) on a high international level. Both groups will contribute in a cross-disciplinary approach to the development of strategies to improve the overall outcome of biocatalytic carboxylation reactions in ionic-liquid systems.

Novel environmentally friendly synthetic concepts, which utilize the greenhouse-gas CO2 as alternative carbon source for the production of well-defined molecules have been established. Carbon constitutes the most essential component of the vast majority of chemical products, which are still predominantly derived from non-renewable fossil feedstocks, such as petroleum, coal or natural gas. The global supplies of dwindling fossil fuels heavily increased the demand for new strategies and technologies in order to maintain the current standard of living and to ensure quality of life for future generations. Great efforts are currently being made in academia and the chemical industry in order to develop novel production processes which facilitate the use of alternative carbon sources. The utilization of the problematic waste gas CO2 which is still released in enormous amounts in the atmosphere (35 Gt/a) as inexpensive and abundantly available C1 building block for the production of value-added organic molecules would greatly fulfil such needs. However, due to the high-energy input which is required for carbon dioxide functionalization, only a few examples of CO2-fixation reactions are running on industrial scale. Most prominent, the Kolbe-Schmitt reaction for the production of salicylic acid requires harsh reaction conditions (high temperature, high pressure) and often suffers from incomplete regioselectivities. In recent years, biocatalysis has been explored as very promising and sustainable alternative to chemical methods. The key feature in this innovative technology displays the application of enzymes (biocatalysts) which are able to catalyze carboxylation reactions under ambient reaction conditions. The corresponding carboxylated products are very important building blocks in many areas of chemistry such as the pharmaceutical, polymer, cosmetic, flavour and food industry. During the course of this project, the successful extension of the biocatalytic (de)carboxylation portfolio applicable to a broad variety of structurally diverse compounds was established. The Austrian part of the project particularly focused on the chemistry of novel (de)carboxylation concepts, which was excellently complemented by the reaction engineering know-how of the project partners from the University of Rostock in order to improve the efficiency of our concepts. Further national and international collaborations were also remarkably fruitful in this interdisciplinary biocatalytic approach leading to numerous high impact publications.