Aromatic feedstock chemicals from degradation of lignin

One of the biggest problems facing society today is our dependence on dwindling fossil fuels, and the contribution to global warming of power stations, car transport and domestic heating that make use of fossil fuels. Not only is oil used to prepare fuel, but also an essential by-product of oil refining is the production of raw materials for chemical and pharmaceutical synthesis. In 50-100 years time, all of these chemicals will need to be produced from renewable, non-petroleum sources, but at present we have very limited methods to do this, so a lot of new technology needs to be developed in a relatively short space of time. The carbon content of plant lignocellulose, found in plant cell walls, represents an abundant source of renewable carbon. One component of lignocellulose is an aromatic polymer called lignin, that binds the cellulose cell walls together. Lignin is very hard to break down, so at present lignocellulose is broken down via a 'pre-treatment' step, usually involving heating with acid and steam to 200 oC, which consumes a lot of energy. Therefore, if we could use Nature to break down the lignin, we would improve the efficiency of lignocellulose breakdown, and liberate useful aromatic by-products. We have recently developed a new method for identifying strains of bacteria that are able to break down lignin, and we have already found several strains of bacteria that are able to do this. We will isolate the lignin-degrading enzymes from these strains, and use molecular genetics to produce large quantities of these enzymes for further studies. We will then use these strains to try to produce useful aromatic chemicals from breakdown of lignocellulose. One example is vanillin, which is used for food flavouring in products such as vanilla ice cream; other examples are phenols that are used in the manufacture of plastics.

Lignin is an aromatic polymer that is a major component of plant lignocellulose. Lignin is extremely hard to degrade, but represents an important source of aromatic carbon compounds, that could potentially be converted into useful, high value aromatic feedstocks. We have recently developed novel methods for the identification of bacteria that are able to degrade lignin, and we have already identified two strains of bacteria with this capability. The aim of this programme is to find novel lignin-degrading bacterial strains and enzymes, and to use them to convert the lignin component of lignocellulose into useful aromatic chemicals. In Part A, we will clone the genes encoding lignin-degrading enzymes from lignin-degrading strains, and the encoded enzymes will be expressed, purified, and characterised biochemically. Lignin-degrading strains, and recombinant lignin-degrading enzymes, will be tested for their ability to break down lignin and lignocellulose in small- and large-scale trials. In Part B, we will apply the lignin-degrading enzymes to convert plant lignocelluose into renewable feedstocks. Lignin-degrading strains and recombinant enzymes will be tested for their ability to produce the following high-value aromatic chemicals, by monitoring their production using HPLC and LC-MS methods: i.) vanillin is a high value chemical that can be produced from oxidative cleavage of the beta-aryl ether component of lignin; ii.) phenol and guaicol could be produced via breakdown of lignin containing di- and mono-substituted aryl units respectively, followed by oxidation and enzymatic decarboxylation; iii.) ferulic and caffeic acid are valuable anti-oxidants that are potential lignin breakdown products; iv.) lignin-derived bio-materials can be generated from partial lignin breakdown, and substituted styrenes can be generated enzymatically, that are feedstocks for material synthesis.

The development of novel routes to high-value aromatic chemicals from lignocellulose will be of considerable interest to industry. Aromatic chemicals are used widely for plastics manufacture (e.g. phenol). Vanillin is used widely in the food industry, and ferulic and caffeic acid are used in the personal care industry. There is a growing market for renewable plastics, that could be made from styrene bio-products or lignocellulose breakdown products. Generation of high-value chemicals from lignin waste would also add considerable value to 2nd generation biofuel production, and the paper/pulp industry. Choice of suitable plant feedstock for chemicals production will also be of interest to plant breeders and seed manufacturers. In the long term, this work could also be of significant commercial benefit to the UK, since alternative routes to feedstock chemicals from non-petroleum sources must be found over the next 50-100 years.