Enzymatic oxygenation process

The enzymatic oxygenation process is of particular value as there is a significant difference in the formation rates of sulfoxides and sulfones. The initial conversion of sulfide to the optically active sulfoxide by an MO is usually very fast compared to the subsequent oxidation step to sulfone, upon which chirality is lost (Scheme 9.26). In many cases, over-oxidation to sulfone is not observed at all when employing MOs. 

It is interesting that NADH is also required as a stoichiometric co-factor in enzymatic oxygenation processes. In a detailed study of styrene monooxygenase (StyA), Andreas Schmid of the ETH/Zurich showed (J. Am. Chem. Soc. 125 8209,2003) that Cp Rh(bpy)(H 0)fc in combination with sodium formate served effectively to regenerate the NADH. Using this combination, epoxidation of aryl alkenes such as 6, 8 and 10 proceeded in high enantiomeric... 

In enzymatic hydroxylation processes the entering oxygen atom is provided by gaseous oxygen (usually from the air), and not by water or any other oxygen-containing compound... 

In an attempt to mimic an enzymatic demethylation process, nitroxide photolysis has been used as a method for oxidizing the C-4 jS-methyl group of 4,4-dimethylcholestanone.158 Further applications have been published of the photolysis of jV-nitrosoamides in the presence of oxygen to the synthesis of nitrate esters having functionalized methyl and methylene groups.180... 

These compounds develop spontaneously in the presence of oxygen, light, and high temperature. H2O2, which is partly added to the enzymatic decarbohydration process before drying, also represents a potential type of oxidation, so that mainly epoxides develop. 

The conventional electrochemical reduction of carbon dioxide tends to give formic acid as the major product, which can be obtained with a 90% current efficiency using, for example, indium, tin, or mercury cathodes. Being able to convert CO2 initially to formates or formaldehyde is in itself significant. In our direct oxidation liquid feed fuel cell, varied oxygenates such as formaldehyde, formic acid and methyl formate, dimethoxymethane, trimethoxymethane, trioxane, and dimethyl carbonate are all useful fuels. At the same time, they can also be readily reduced further to methyl alcohol by varied chemical or enzymatic processes. 

The dopamine is then concentrated in storage vesicles via an ATP-dependent process. Here the rate-limiting step appears not to be precursor uptake, under normal conditions, but tyrosine hydroxylase activity. This is regulated by protein phosphorylation and by de novo enzyme synthesis. The enzyme requites oxygen, ferrous iron, and tetrahydrobiopterin (BH. The enzymatic conversion of the precursor to the active agent and its subsequent storage in a vesicle are energy-dependent processes. 

A more detailed study of the biological oxidation of sulphoxides to sulphones has been reported165. In this study cytochrome P-450 was obtained in a purified form from rabbit cells and was found to promote the oxidation of a series of sulphoxides to sulphones by NADPH and oxygen (equation 56). Kinetic measurements showed that the process proceeds by a one-electron transfer to the activated enzymatic intermediate [an oxenoid represented by (FeO)3+] according to equation (57). 

For bio-transformation processes, immobilised enzymes are often used because their activity persists over a longer period of time than that of free enzymes. The reduction of enzyme activity in enzymatic reactors is a consequence of energy dissipation by sparging and stirring, which is required for instance for oxygen transport or realisation of constant reaction conditions as regards temperature and pH. In the other hand low and high pH-values leads also to a decrease of enzyme activity and increase the stress sensitivity. 

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