Alkanes enzymatic

Remarkably, alkanes are oxidatively transformed by biological organisms at benign temperatures and pressures. Clearly, enzymatic transformations of alkanes and their well studied mechanisms (e.g., for cytochrome P450) are beyond the... 

The fact that enzymes also work in organic solvents [42], ranging from apolar alkanes up to the very polar N,N-dimethylformamide [65], has (Fig. 13.10) and will continue to open up new cascade opportunities for the integration of enzymatic conversions with chemocatalytic methods that require organic solvents. Notably, however, some enzyme classes, for instance the carbohydrate-converting enzymes, do not show activity in non-aqueous media [40, 41]. 

The preparation and application of SAM systems patterned by STM and their use in catalysis was demonstrated by Wittstock and Schuhmann [123]. The patterning (local desorption) of SAMs from alkane thiols on gold was performed by scanning electrochemical microscopy (SECM), followed by the assembly of an amino-deriva-tized disulfide and coupling of glucose oxidase to form a catalytically active pattern of the enzyme. The enzymatic activity could be monitored/imaged by SECM. 

Berkessel A, Thauer RK (1995) On the mechanism of catalysis by a metal-free hydrogenase from methanogenic archaea enzymatic transformation of H2 without a metal and its analogy to the chemistry of alkanes in superacidic solution. Angew Chem Int Ed 34 2247... 

Surface lipids of plants. The thick cuticle (Fig. 1-6) that covers the outer surfaces of green plants consists largely of waxes and other lipids but also contains a complex polymeric matrix of cutin (stems and leaves) or suberin (roots and wound surfaces).135/135a Plant waxes commonly have C10 - C30 chains in both acid and alcohol components. Methyl branches are frequently present. A major function of the waxes is to inhibit evaporation of water and to protect the outer cell layer. In addition, the methyl branched components may inhibit enzymatic breakdown by microbes. Free fatty acids, free alcohols, aldehydes, ketones, 13-dike tones, and alkanes are also present in plant surface waxes. Chain lengths are usually C20 - C35.136 Hydrocarbon formation can occur in other parts of a plant as well as in the cuticle. Thus, normal heptane constitutes up to 98% of the volatile portion of the turpentine of Pin us jeffreyi.81... 

Organic solvents used must be compatible with the enzymatic activity, not take place in the reaction, be good solvent of the substrates, have a low cost, and be the more harmless possible. The solvents commonly used in the literature are aliphatic alkanes, or similar compounds of low polarity. Aliphatic ethers that were used in this work, show less hydrophoblcity than alkanes, and thus allow introduction of water for hydrolysis reactions. For interesterification experiments, trichloro-trifluoro-ethane was used, as water is not a substrate for the reaction (Fig. 5,). 

Valuable organic acids of industrial interest can be recovered from aqueous waste solutions, for example, from an enzymatic resolution process. Mass-transfer data for MBSE and MBSS in HF contactors for dimethylcydopropanecarboxylic acid (DMCCA) [97] and 5-methyl-2-pyrazinecarboxylic add (MPCA) [95, 96] have been estimated. Processes for recovery of DMCCA from a highly addic waste solution with pH below 2 containing about 19kgm 3 of DMCCA by MBSE and MBSS have been suggested [98], A solvent with 0.4 kmol m 3 of TOA in n-alkanes (dodecane fraction) was used. A recovery of more than 90% of DMCCA and a concentrate with about 200 kg m-3 of DMCCA can be achieved. 

For an overview of the scope of enzymatic alkane hydroxylation FI. L. Holland,... 

Aliphatic hydrocarbons such as n-alkanes and n-alkenes have been successfully used to distinguish between algal, bacterial, and terrestrial sources of carbon in estuarine/coastal systems (Yunker et al., 1991, 1993, 1995 Canuel et al., 1997). Saturated aliphatic hydrocarbons are considered to be alkanes (or paraffins) and nonsaturated hydrocarbons which exhibit one or more double bonds are called alkenes (or olefins)—as indicated in the simple structures of hexadecane and 1,3-butadiene, respectively (figure 9.7). It should also be noted that, n-alkanes tend to be odd-numbered as they result from enzymatic decarboxylation of fatty acids. Long-chain n-alkanes (LCH) (e.g., C27, C29, and C31) are generally considered to be terrestrially derived, originating from epicuticular waxes... 

In spite of a long-time paradigm that enzymes can be active only in their natural aqueous media and other solvents cause deactivation and denaturation of proteins, at present a growing number of investigations are devoted to enzymatic reactions in organic solvents (Klibanov, 2001 Ke et al., 1996 Koskinen and Klibanov, 1996 and references therein). Such enzymes as a-chymotrypsin, subtilisin ribonuclease, pancreatuc lipase, and horse radish peroxidase have been found to be markedly active in organic solvents (alcohols, amines, tiols,anhydrous alkanes, acetonitril, dichloromethane, methyl acetate, etc.). 

Scheme 18.3b illustrates a recent mechanistic proposal. This bears similarities to the oxygen-rebound mechanism, proposed for the hydroxylation of alkanes by high-valent transition metal species in biomimetic and enzymatic systems [31, 33, 34]. The generation of the active species and other implications of the mechanism are discussed in detail in Section 18.11.3. 

One of the most smdied examples is the mimic of the enzyme cytochrome P-450 in the pores of a faujasite zeolite [196,204,225], The iron-phthalocyanine complex was encapsulated in the FAU supercage and is used as oxidation catalyst for the conversion of cyclohexane and cyclohexanone to adipic acid, an important intermediate in the nylon production. In this case the two step process using homogeneous catalysts could be replaced by a one step process using a heterogeneous catalyst [196]. This allowed better control of the selectivity and inhibited the auto oxidation of the active compound. In order to simulate a catalyst and the reaction conditions which are close to the enzymatic process, the so obtained catalyst was embedded in a polydimethylsiloxane membrane (mimics the phospholipid membrane in the living body) and the membrane was used to limit oxygen availability. With this catalyst alkanes were oxidized at room temperature with rates comparable to those of the enzyme [205]. 

Suzuki, T. 1978. Enzymatic methylation of pentachlorophenol and its related compounds by cell-free extracts of Mycobacterium sp. isolated from soil. /. Pestic. Sci. 3 441—443. Suzuki, T., K. Tanaka, I. Matsubara, and S. Kinoshita. 1969. Trehalose lipid and alpha-branched-beta-hydroxy fatty acid formed by bacteria grown on n-alkanes. Agric. Biol. Chem. 33 1619-1627. 

Waxes mainly function as protective coatings, such as those found on leaf cuticles. They are mixtures of many constituents with high melting points, important members being esters of fatty acids with straight-chain saturated alcohols (fatty alcohols). The fatty acids and alcohols in these wax esters have similar chain lengths, mainly in the range C24 to C28. They have predominantly an even number of carbon atoms because the alcohols are biosynthesized from fatty acids by enzymatic reduction (Eqn 2.7). Lesser amounts of ketones, branched alkanes and aldehydes are present. 

Plant waxes also contain hydrocarbons, mainly long-chain /i-alkanes. In contrast to the fatty acids and alcohols, the //-alkanes contain chiefly odd numbers of C atoms, mostly in the range C23 to C35, with C27, C29 and C31 generally predominating. This results from biosynthesis of the alkanes from acids by enzymatic decarboxylation ... 

Howdle et al. was the first to report on the enzymatic emulsion polymerization of water-soluble acrylamide monomers in a w/c microemulsion [75]. Acrylamide is one of the few monomers that show limited solubility in supercritical C02, and hence was particularly suited to inverse emulsion polymerization [61]. In this report, Howdle et al. showed that the yields and molecular weights obtained via polymerization in C02 are comparable to those generated in a conventional polymerization in an alkane/water medium. Indeed, polymer molecular weights up to 900 kDa were observed when a surfactant was not used and molecular weights around 300 kDa when a perfhiropolyether was used. The generally accepted mechanism for this reaction is shown in Figure 13.5. 

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