Biocatalysis homogenous

Special attention is given to the integration of biocatalysis with chemocatalysis, i.e., the combined use of enzymatic with homogeneous and/or heterogeneous catalysis in cascade conversions. The complementary strength of these forms of catalysis offers novel opportunities for multi-step conversions in concert for the production of speciality chemicals and food ingredients. In particular, multi-catalytic process options for the conversion of renewable feedstock into chemicals will be discussed on the basis of several carbohydrate cascade processes that are beneficial for the environment. 

The objective of this NoE is to strengthen research in catalysis by the creation of a coherent framework of research, know-how and training between the various disciplinary catalysis communities (heterogeneous, homogeneous, and biocatalysis) with the objective of achieving a lasting integration between the main European Institutions in this area. IDECAT will create the virtual European Research Institute on Catalysis (ERIC) that is intended to be the main reference point for catalysis in Europe. 

For some recent reviews on the use of enzymes in nonconventional media, see (a) Dreyer, S., Lembrecht, J., Schumacher, J. and Kragl, U., Enzyme catalysis in nonaqueous media past, present, and future in biocatalysis in the pharmaceutical and biotechnology industries, 2007, CRC Press, pp. 791-827 . (b) Torres, S. and Castro, G.R., Non-aqueous biocatalysis in homogeneous solvent systems. Food Technol. BiotechnoL, 2004, 42, 271-277 (c) Carrea, G. and Riva, S., Properties and synthetic applications of enzymes in organic solvent. Angew. Chem. Int. Ed., 2000, 39, 2226-2254. 

The renaissance of biocatalysis, supported by the advent of recombinant DNA, is only about 20 years old. Recently several publications have appeared which deal specifically with the attitudes listed above (Rozzell, 1999 Bommarius, 2001 Rasor, 2001). Most of the points above can either be refuted or they can be levied against any novel catalytic technology the situation with some competing technologies such as chiral homogeneous catalysts is similar to that with enzymes (Chapters 18 and 20). 

The turnover number is not used frequently in biocatalysis, possibly as the molar mass of the biocatalyst has to be known and taken into account to obtain a dimensionless number, but it is the decisive criterion, besides turnover frequency and selectivity, for evaluation of a catalyst in homogeneous (chemical) catalysis and is thus quoted in almost every pertinent article. Another reason for the low popularity of the turnover number in biocatalysis, apart from the challenge of dimensionality, is the focus on reusability of biocatalysts and the corresponding greater emphasis on performance over the catalyst lifetime instead of in one batch reaction as is common in homogeneous catalysis (Blaser, 2001). For biocatalyst lifetime evaluation, see Section 2.3.2.3. 

Turnover numbers (TONs) and substrate/catalyst ratios ([S]/[C] ratios) seem the preferred quantities in homogeneous catalysis, in contrast to biocatalyst loading (units L-1) and TTNs in biocatalysis. In the case of slow homogeneous chemical catalysts, the [S]/[C] ratio can approach unity (stoichiometric conditions). In the limit of no recycle, the values for TTN and TON are identical upon re-use of catalyst, TTN increases correspondingly. Whereas recycling is very important in biocatalysis, it does not seem to be common practice in homogeneous chemical asymmetric catalysis. 

This chapter outlines the principles of green chemistry, and explains the connection between catalysis and sustainable development. It covers the concepts of environmental impact, atom economy, and life-cycle analysis, with hands-on examples. Then it introduces the reader to heterogeneous catalysis, homogeneous catalysis, and biocatalysis, explaining what catalysis is and why it is important. The last two sections give an overview of the tools used in catalysis research, and a list of recommended books on specialized subjects in catalysis. 

Biocatalysis is a rather special case, somewhere between homogeneous and heterogeneous catalysis. In most cases, the biocatalyst is an enzyme - a complex protein that catalyzes the reactions in living cells. Enzymes are extremely effident catalysts. An enzyme typically completes 1000 catalytic cycles in one second. Compared to this, conventional homogeneous and heterogeneous catalysts are slow and inefficient (100-10000 cydes per hour). Speed, however, is not the only advantage enzymes specialize in converting one specific reactant into another... 

Castro, G.R. and Knubovets, T. (2003) Homogeneous biocatalysis in organic solvents and water-organic mixtures. Crit. Rev. Biotechnol., 23, 195. 

Interestingly, this is also a good description of many (but not all) homogeneous catalysis and biocatalysis reactions. Here the reactant or the substrate first coordinates to the metal complex or to the enzyme, then a reaction occurs. Finally, the product dissociates from the catalyst and diffuses back into the solution. 

Before any catalysis can occur, at least one of the substrates must coordinate to the catalyst. This means that the catalyst must have a vacant active site. In homogeneous metal complex catalysis and biocatalysis, this will be a vacant coordination site at the metal atom. In heterogeneous catalysis, the vacant site could be a metal crystallite or an ion on the surface. For the latter, we speak of desorption and adsorption instead of dissociation and coordination. Remember that our reactions are not in vacuum, so there is no vacant site . Thus, before any chemical species can coordinate to the metal complex (or to the active site in heterogeneous catalysis or biocatalysis) the species already occupying this space must first vacate it. This happens constantly, as the system is dynamic (Figure 3.3) [15]. At any given moment... 

An interesting alternative that combines the advantages of both classical and quantum mechanics is to use hybrid QM/MM models, first introduced by Arieh Warshel for modeling enzymatic reactions [7]. Here, the chemical species at the active site are treated using high-level (and therefore expensive) QM models, which are coupled to a force field that describes the reaction environment. Hybrid models can thus take into account solvent effects in homogeneous catalysis, support structure and interface effects in heterogeneous catalysis, and enzyme structure effects in biocatalysis. 

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