Coupling biocatalysis

The oxidation of heteroatoms and, in particular, the conversion of sulfides to asymmetric sulfoxides has continued to be a highly active field in biocatalysis. In particular, the diverse biotransformations at sulfur have received the majority of attention in the area of enzyme-mediated heteroatom oxidation. This is particularly due to the versatile applicability of sulfoxides as chiral auxiliaries in a variety of transformations coupled with facile protocols for the ultimate removal [187]. 

Biocatalysis in Liquid-Liquid Biphasic Media Coupled Mass Transfer and Chemical Reactions... 

A common characteristic of metabolic pathways is that the product of one enzyme in sequence is the substrate for the next enzyme and so forth. In vivo, biocatalysis takes place in compartmentalized cellular structure as highly organized particle and membrane systems. This allows control of enzyme-catalyzed reactions. Several multienzyme systems have been studied by many researchers. They consist essentially of membrane- [104] and matrix- [105,106] bound enzymes or coupled enzymes in low water media [107]. 

The stirred batch reactors are easy to operate and their configurations avoid temperature and concentration gradient (Table 5). These reactors are useful for hydrolysis reactions proceeding very slowly. After the end of the batch reaction, separation of the powdered enzyme support and the product from the reaction mixture can be accomplished by a simple centrifugation and/or filtration. Roffler et al. [114] investigated two-phase biocatalysis and described stirred-tank reactor coupled to a settler for extraction of product with direct solvent addition. This basic experimental setup can lead to a rather stable emulsion that needs a long settling time. 

So, in the final analysis, biocatalysis should not be considered in a separate sector available only to the specialist bioorganic chemist. It is one method, in the portfolio of catalytic techniques, that is available to all chemists for the solution of present and future problems in organic synthesis. To erect a Chinese wall between the natural and non-natural catalysts is totally illogical and prevents some creative thinking, particularly in the area of coupled natural/ non-natural catalysts11611 and biomimetic systems11621. 

Keywords Peroxidase, Biocatalysis, Asymmetric synthesis. Kinetic resolution. Hydroperoxide, Epoxidation, Sulfoxidation, Halogenation, Hydroxylation, Phenol coupling. 

Catalysis in reaction systems with undissolved substrates and products is not restricted to biocatalysis. High yields in sobd-state synthesis, sohd-to-sohd reactions, and solvent-free systems have also been reported for aldol condensation, Baeyer-Villiger oxidation, oxidative coupling of naphthols, and condensation of amines and aldehydes [1, 2]. 

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. 

Muller and co-workers have demonstrated the potential of coupling several spontaneous chemical steps to biocatalysis in a one-pot domino reaction to form bicyclo[2.2.2]octenes.13 A tyrosinase from mushrooms was immobilized on glass beads with the phenol substrate in a mixture of chloroform and a dienophile under air. Tyrosinase can transform a wide variety of phenols to the corresponding catechol, and the presence of air resulted in spontaneous oxidation to the ortho-quinone (Scheme 21.3). The presence of a dienophile then resulted in a Diels-Alder cycloaddition to form the bicyclo[2.2.2]octene product. Significant yields were achieved with a broad range of phenols and dienophiles. 

When, in 1832, Wohler and Liebig first discovered the cyanide-catalyzed coupling of benzaldehyde that became known as the benzoin condensation , they laid the foundations for a wide field of growing organic chemistry [1]. In 1903, Lapworth proposed a mechanistical model with an intermediate carbanion formed in a hydrogen cyanide addition to the benzaldehyde substrate and subsequent deprotonation [2]. In the intermediate active aldehyde , the former carbonyl carbon atom exhibits an inverted, nucleophilic reactivity, which exemplifies the Umpo-lung concept of Seebach [3]. In 1943, Ukai et al. reported that thiazolium salts also surprisingly catalyze the benzoin condensation [4], an observation which attracted even more attention when Mizuhara et al. found, in 1954, that the thiazolium unit of the coenzyme thiamine (vitamin Bi) (1, Fig. 9.1) is essential for its activity in enzyme biocatalysis [5]. Subsequently, the biochemistry of thiamine-dependent enzymes has been extensively studied, and this has resulted in widespread applications of the enzymes as synthetic tools [6]. 

In some cases, substrates and enzymes are not soluble in the same solvent. To achieve efficient substrate conversion, a large interface between the immiscible fluids has to be established, by the formation of microemulsions or multiple-phase flow that can be conveniently obtained in microfluidic devices. Until now only a couple of examples are published in which a two-phase flow is used for biocatalysis. Goto and coworkers [431] were first to study an enzymatic reaction in a two-phase flow in a microfluidic device, in which the oxidation ofp-chlorophenol by the enzyme laccase (lignin peroxidase) was analyzed (Scheme 4.106). The surface-active enzyme was solubilized in a succinic acid aqueous buffer and the substrate (p-chlorophenol) was dissolved in isooctane. The transformation ofp-chlorophenol occurred mainly at... 

For peroxidase biocatalysis, the relevant redox couples are Compound I and Compound II, the intermediates present during the catalytic cycle, as described in Chap. 5. However, Fe(III)/Fe(II) redox potential could still be a useful indicator of the oxidizing character of peroxidases. Millis et al. [54] suggested for the first time that the noncatalytic Fe(III)/Fe(II) redox potential could be used to predict the oxidative capacity of a heme peroxidase during turnover. In this work, it was suggested that a more positive Fe(III)/Fe(II) redox potential indicates a higher electron deficiency within the active site, and thus the existence of enzymatic... 

Due to increasing environmental problems, the use of metal catalysts will gradually be reduced in the future the reduction in the early use of lead- and chromium-based catalysts is evidence of this. With stricter regulations governing the release of metals, the cost of catalyst recovery and environmental remediation is quickly making noncatalytic processes for the production of carboxylic acids preferable. Coupled with recent advances in the field of biocatalysis, metal-mediated oxidations may give way to alternative processes as we enter a new millennium of chemistry. 

Another example of a coupled enzyme reaction demonstrates the versatility of the transaminase system in biocatalysis. Using a racemic d,L-amino acid mixture as the starting material, the enzyme D-amino acid oxidase from Trigonopsis mriabilis will convert the D-amino acid in the mixture selectively into the corresponding 2-keto acid. The L-amino acid of the d,l- pair is neither a substrate nor an inhibitor of d-amino acid oxidase. If a transaminase is present in the same reaction mixture, the 2-keto acid can be transaminated in the presence of L-aspartate to the corresponding L-amino acid. The entire reaction can be driven to completion as described previously by decarboxylation of the oxaloacetate. Thus, in a single pot, racemic d,l-amino acids can be convened directly into optically active L-amino acids (Fig. 12.7-11). 

Multicomponent, single pot cascade reactions, coupling and cychzation reactions, new reaction pathways Enzymes (biocatalysis)... 

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