The chemical industry has been highly criticised for using toxic reagents and extreme reaction conditions. This has led to the perception of chemists as dirty scientists presiding over large factories billowing out nasty fumes. In the 21st century ‘Green’ chemists are trying to change all this by introducing chemistry that is in balance with the environment. One of the key tools driving this approach is biocatalysis. This article explores the potential of combining leading edge biocatalysis and chemocatalysis in order to increase the efficiency and scope of the transformations that can be performed.
by Dr A. C. Marr
Engineering natural catalysts to assist in the manufacture of chemical products is a considerable challenge; after all, biocatalysts were designed with the chemistry of life, not the chemical industry, in mind. Biocatalytic approaches to chemical synthesis can be sub-divided into whole cell and isolated enzyme methods. In whole cell methods, a microbe digests the substrate in a fermentation broth to yield the chemical product. As the broth is complex, isolating the product can be difficult. Isolated enzyme methods are closer to traditional chemical methods and separations are easier. Many enzymes can be used as highly selective chemical reagents in ‘conventional’ chemical solvents. They can, however, be expensive, co-factor dependent and fragile.
Recently, chemists have been exploring the ability of chemical catalysts to assist and expand the chemistry of biocatalysts. The incorporation of biocatalytic and chemocatalytic steps into a reaction cascade mimics biochemical pathways and enables value added products to be prepared in a one-pot processes.
The integration of isolated enzyme and chemical catalysts is exemplified by chemoenzymic Dynamic Kinetic Resolution (DKR). Kinetic Resolution is a biocatalytic chiral separation technique that exploits the ability of enzymes to react rapidly with one enantiomer and very slowly with another. The result is that one stereoisomer is transformed by the biocatalyst into the product and the other remains unchanged; this assists the separation of the two chiral forms, a process particularly valuable in the preparation of pharmaceuticals. The preparation of a chiral ester from a racemic alcohol is presented as an illustration. The disadvantage of Kinetic Resolution is that the maximum yield of the product is only 50%. Green chemists are trying to avoid the production of large quantities of chemical waste and the addition of a chemical catalyst can assist with this. The addition of a catalyst that can racemise the substrate will make all of the substrate available to the biocatalyst and give a theoretical yield of 100%.
Efficient DKR racemisation catalysts have been discovered based on iridium [1]
and ruthenium [2].
The integration of whole cell and chemocatalytic methods is a considerable challenge due to the stark differences in operation conditions and process requirements. This combination is worth pursuing as whole cell catalysts have the unique ability to cope with starting materials derived from biomass. Currently the majority of chemicals are derived from petroleum, and as this resource becomes more difficult to obtain, prices will rise sharply. It is therefore wise to investigate alternative sources of chemicals. The integration of whole cell fermentations into chemical processes opens up the opportunity of utilising a wide variety of crude biomass-derived chemicals for synthesis and creating a more sustainable chemicals industry. Transforming these into something useful is difficult as the chemistry performed by whole cells is limited. The addition of a chemical catalyst can extend the chemical transformations that can be performed. A biphasic approach was introduced by Marr and co-workers [3] to separate the bio and chemocatalytic steps and remove the chemical product from the fermentation broth. The process employs a lower aqueous phase containing the microbes, and an upper phase containing a chemical catalyst. The substrate is introduced into the fermentation broth where the biocatalyst transforms it into an intermediate chemical. The intermediate partitions across the biphasic solvent and, in the upper phase, comes in to contact with the chemical catalyst, which transforms it into a value added product. In this way glycerol from biodiesel production was converted by Clostridium butyricum into 1,3-propanediol. The diol was extracted into a second phase of toluene or an ionic liquid containing a dissolved iridium complex and was aminated to produce secondary amines.
Combining biocatalysis and chemocatalysis into one-pot methods has been shown to expand the applications of biocatalysis and lead to a wider variety of ‘greener’ more sustainable chemical processes. The ever increasing variety of bio and chemo-catalysts available can give rise to a virtually limitless variety of combinations.
References
1. Liu S, Rebros M, Stephens G, Marr AC, Chem Commun 2009: 2308-2310.
2. Martín-Matute B, Edin M, Bogá Kr, Bäckvall J-E, Angew Chem Int Ed 2004; 43: 6535–6539.
3. Marr AC, Pollock CL, Saunders GC. Organometallics 2007; 26: 3283-3285.
The author
Dr Andrew Craig Marr,
Queen’s University Belfast
email: a.marr@qub.ac.uk
Bioremediation with bacteria that metabolise petrochemicals
Bioremediation of industrial sites and petrochemical spillages often involves finding microbes that feed on the toxic chemicals. This leaves behind a non-toxic residue or mineralised material. Writing in the International Journal of Environment and Pollution, researchers in the College of Water Sciences, Beijing Normal University, China describe studies of a new microbe that can digest hydrocarbons. The activity of enzymes from the bacterium Bacillus cereus DQ01, which can digest the hydrocarbon n-hexadecane, has been investigated. The bacterium was initially isolated from the Daqing oil field in North East China where it had evolved the ability to metabolize this chemical.
Bioremediation of hydrocarbons usually involves the application of a cultured bacterium that has been optimised to feed on the specific contaminants, such as particular hydrocarbons. The microbes are cultured first in the presence of sugar or another standard medium in conjunction with a small amount of the pollutant material. Successive generations are fed an increasing proportion of the pollutant until their growth is optimised for digestion of that compound rather than the sugar. These optimised microbes are applied to the contamination site or spill in large but controlled volumes and digest their way through the pollutant material, multiplying and digesting until no pollutant remains. The byproducts are non-toxic carbon dioxide and water, and mineralised matter.
The team has now found the optimal conditions for the Daqing microbe to feed on hydrocarbon, which could point the way to a more effective approach to bioremediation of spill sites. The key step in the degradation of hydrocarbons normally depends on the presence of a multi-component enzyme system. Understanding exactly which components are needed for degradation and the temperature and pH of the soil best suited to the process could help researchers develop the perfect microbial cleanup culture. The team has found that enzymes within the microbial cell and in its membrane inner membrane are responsible for degradation of n-hexadecane, and that neutral pH and a temperature of 30° Celsius are optimal for the microbe to produce the main degradation enzyme. They also point out that adding a small amount of a surfactant material, rhamnolipid, can stimulate enzyme production and improve degradation efficiency.
Hong-Qi Wang et al. Degradability of n-hexadecane by Bacillus cereus DQ01 isolated from oil contaminated soil from Daqing oil field, China. Int J Environment and Pollution 2009; 38: 100-115
