Biochemical Microbial Oxidations

The oxidations accomplished by microorganisms or enzymes excel in regiospecificity, stereospecificity, and enantioselectivity. Although the yields of such oxidations are sometimes fair and even low, optical purity (enantiomeric excess) is usually very high and frequently 100%. The spectrum of microbial and enzymatic oxidations is unbelievably broad many different positions in steroidal rings can be hydroxylated by different microorganisms, and usually, only one diastereomer is formed. From achiral molecules, optically active compounds are generated. 

Microbial oxidations occur under very mild conditions, usually around 37 °C, and in dilute solutions, for example, 1 g of a substance in 6 L of water. They are very slow and often take days. The isolation of products from the reaction media is done by extraction with proper solvents. Out of countless examples described in the literature, only a very narrow selection is presented in this chapter to show the versatility of microbial and enzymatic oxidations. 

A single enzyme is sometimes capable of many various oxidations. In the presence of NADH (reduced nicotinamide adenine dinucleotide), cyclohexanone oxygenase from Acinetobacter NCIB9871 converts aldehydes into acids, formates of alcohols, and alcohols ketones into esters (Baeyer-Villiger reaction), phenylboronic acids into phenols sulfides into optically active sulfoxides and selenides into selenoxides [1034], Horse liver alcohol dehydrogenase oxidizes primary alcohols to acids (esters) [1035] and secondary alcohols to ketones [1036]. Horseradish peroxidase accomplishes the dehydrogenative coupling [1037] and oxidation of phenols to quinones [1038]. Mushroom polyphenol oxidase hydroxylates phenols and oxidizes them to quinones [1039]. 

Dehydrogenations, which involve the elimination of hydrogen Ifom organic molecules, lead to compounds containing double bonds, multiple bonds, or aromatic rings. For practical reasons, only the formation of carbon-carbon double bonds, of carbon-nitrogen double bonds in cyclic amines, and of aromatic rings (both carbocyclic and heterocyclic) will be discussed in this chapter. The conversion of alcohols into aldehydes and ketones and of amines into imines and nitriles will be discussed in the chapter Oxidations (Chapter 3). 

33-Acetoxylanost-8-ene is converted into 3p-acetoxylanosta-7,9(ll)-diene in 92% yield on treatment with acetic acid and 30% hydrogen peroxide for 45 h at 22 °C [275]. 

---

I aradve microbial oxidations have long been practiced in organic synthesis, perhq>s most prominently in the steroidal field, and a number of comprehensive and specialized reviews have appeared. The most recent review, publi ed in 1981, covers most aspects of biochemical oxidations, and gives an ex-... 

Anthony, C., and Zatman, L. J., 1967, The microbial oxidation of methanol The prosthetic group of alcohol dehydrogenase of Pseudomonas sp. M27 A new oxidoreductase prosthetic group. Biochem. J. 104 9609969. 

Haywood, G. W., and Large, P. J., 1981, Microbial oxidation of amines dish ibution, purification and properties of two primary amine oxidases from die yeast Candida boidinii grown on amines as sole nitrogen source, Biochem. J. 199 187n201. 

A second newly recognized group of prokaryotes are the methane oxidizing archea. Nearly 90% of the methane produced in anoxic marine sediments is recycled through anaerobic microbial oxidation processes (Cicerone and Oremland, 1988 Reeburgh et al., 1991). However, the organisms and biochemical processes responsible for the anaerobic oxidation of methane (AMO)... 

The amount of oxygen used up in microbial oxidation is - the biochemical oxygen demand (BOD) - another important water-quality indicator. The BOD is taken as a realistic measure of water quality - a clean river would have a BOD value of less than 5 ppm, whereas a very polluted river could have a BOD value of 17 ppm or more. A BOD determination takes a few days, so another parameter called the chemical oxygen demand (COD) is sometimes measured. In a COD determination, acid dichromate is used to oxidize the organic matter in a sample of water. The measurement takes only a couple of hours. 

Intensive technologies are derived from the processes used for the treatment of potable water. Chemical methods include chlorination, peracetic acid, ozonation. Ultra-violet irradiation is becoming a popular photo-biochemical process. Membrane filtration processes, particularly the combination microfiltration/ultrafiltra-tion are rapidly developing (Fig. 3). Membrane bioreactors, a relatively new technology, look very promising as they combine the oxidation of the organic matter with microbial decontamination. Each intensive technique is used alone or in combination with another intensive technique or an extensive one. Extensive... 

Modem layered microbial communities provide a view into biochemical redox cycling. Oxidation of Fe(ll) through high O2 contents generated by cyanobacteria generally occurs in the top most (photic) portions of microbial mats. The upper, near-surface layers of microbial mats that are rich in cyanobacteria are commonly underlain by purple and green anoxygenic photosynthetic bacteria that thrive in the IR photic spectra (Stahl et al. 1985 Nicholson... 

Figure l. Microbial transformations of methionine which generate methylated sulfides. 1) methionine 7-lyase 2) and 3) possible demethiolations 4) chemical and probably biochemical oxidation 5) thiol S-methyltransferase. 

---