Methane as a substrate

From the above discussion, what do you think could be used instead of methane as a substrate ... 

Shaik and co-workers have carried out a number studies using density functional theory based quantum chemical and QM/MM techniques to examine various aspects of the mechanism of alkane hydroxylation by cytochrome P450.178 181 These studies included, for example, calculation of the potential energy surface for the so-called rebound mechanism with methane as a substrate for two spin states, the high spin (HS) quartet state and low spin (LS) doublet state. In the rebound mechanism, Compound I initially abstracts a... 

Methanol can be produced relatively cheaply as a bulk chemical by the oxidation of methane. Several processes have been developed to produce feed-grade SCP using methanol as a substrate. We will now examine one such process in depth, to show how a process is developed, from inception to production scale, and how the many problems encountered can be tackled and overcome. 

In the late 1960s, Imperial Chemical Industries (IQ) in the UK were interested in developing an SCP process using abundant and cheap methane from newly developed sources in the North Sea. However, it soon became apparent that methane was unsuitable as a substrate for fermentation. 

The desulfinase enzyme was reported to have narrow substrate specificity. In addition to HBPSi, only 2-phenyl benzene sulfinate was reported to serve as a substrate [164], It was found to be inactive against benzene sulfinate, cysteine sulfinate, benzene sulfonate, /7-toluene sulfonate, 1-octane sulfonate, methane sulfonate, and taurine. This enzyme was found to be inhibited by HBP beginning at 0.5 mM with complete loss of activity at 9 mM HBP, but was not affected by sulfite. 

In some applications, it is necessary to inject nutrients or other chemicals into the aquifer to effect a more efficient restoration. Most of the time, additives are injected into separate wells. These additives may include surfactants, nutrients, pH adjustment chemicals, or additional carbon sources. Some success has been achieved with injected heated air to improve volatility of the chemicals. Where a small quantity of methane (as a primary substrate) is required, it can be added with the injection air. The lower explosive limit (LEL) of methane in air is 5% thus, extreme care must be used to control the mixture and the methane content of the vapor that reaches the surface. 

This mechanism was tested by use of C-labeled carbon dioxide (Barker, 1943 Buswell and Sollo, 1948 Stadtman and Barker, 1949, 1951 Pine and Barker, 1956 Baresi et al, 1978). Essentially none of the methane was found to be derived from carbon dioxide. Methane is derived entirely from the methyl carbon atoms and carbon dioxide is derived exclusively from carboxyl carbon atoms. Van Neil s mechanism is clearly not valid because the methyl carbon atom is not oxidized to carbon dioxide. Other work has been done to ascertain whether hydrogen atoms are removed during the fermentation of acetic acid, and whether the methyl group is incorporated intact into methane (Pine and Barker, 1954). Water and heavy water were used with deuterated and nondeuterated acetic acid. Acetic acid labeled in the methyl group, when used as the substrate, showed that the isotopic content of acetic acid and methane are the same. Unlabeled acetic acid fermented in the presence of heavy water indicated that about one atom of deuterium per molecule of methane formed is derived from heavy water. It was concluded that the methyl group is transferred from acetic acid to methane as a unit without the loss of attached hydrogen or deuterium atoms. 

Photosynthetic bacteria can rapidly assimilate volatile fatty acids and grow. Because it assimilates organic compounds, hydrogen production from various fatty acids has been investigated. As for volatile fatty acids, lactate obtained after lactate fermentation of carbohydrates and acetate contained in processed solution after methane fermentation can be used as a substrate. 

Internal Sources and Atmospheric Exchange of Methane. Methane is produced by specialized groups of obligate anaerobic bacteria (22, 23). The formation of methane as a metabolic product results either from the microbial reduction of CO2 with molecular H2, or via the fermentation of acetic acid. More structurally complex substrates may also serve as electron acceptors/donors, but the end result of methanogenesis is to produce methane and CO2 as end products (23). 

Another approach to preparing model catalysts is the preparation of inverse supported catalysts . In this approach, the catalytically active metal (usually single crystal) is used as a substrate upon which an oxide is deposited, presumably leaving patches of exposed metal. This approach has been used to study reduction of ceria, and methanation kinetics on Rh as promoted by deposited ceria, and chemisorption of various molecules. As stated above, it is generally assumed that thick enough ceria layers will continuously cover the metal substrate, placing a limit on the thickness of the ceria islands that can be achieved for an inverse supported catalyst. The different procedures used for the inverse and metal particle on bulk oxide model catalysts is expected to produce differences in thermal stability, morphology and surface structure which may have consequences for the reactivity of the model catalyst. 

Today there are several other ways of diamond synthesis besides the HPHT method. For example, it is possible to utilize the pressure of a shock-wave generated in an explosion. This process mostly yields powdery products with particle sizes in the range of micrometers (1 mm at max.) that may be employed for industrial purposes as well. Moreover, very small diamonds (5-20 nm) can be made by reacting explosives in confined containers. Diamond films are produced on various substrates by chemical vapor deposition (CVD method using methane as a carbon source. Detonation synthesis and vapor deposition will be described in detail in Chapters 5 and 6. 

The presence of halogens in reaction systems has been found to lead to an increased activation of the deposition surface, especially at low temperatures. " The rate coefficient of the abstraction reaction of surface H atoms by gaseous F atoms is more than two orders of magmtude greater than that of the abstraction reaction by H at 900°C or lower. " The deposition of diamond films at substrate temperatures as low as 250°C and relatively mild gas activation has been achieved in which fluorine was added to the gas mixture. " In addition, the sp carbon components may be preferentially etched by F atoms. " The use of chlorine-permuted methane as a carbon source also fevors high-purity diamond deposition at low temperatures and high methane concentrations. ... 

Methylcobalamin also participates as a substrate for an enzymatic reaction in the formation of methane in certain methanogenic bacteria. In this reaction, which can be considered as a model for the second half-reaction of methionine synthetase, the cobalt-bound methyl group is transferred to 2-mercaptoethanesulfonic acid (coenzyme M) [39], the 5-methyl sulfide of which is subsequently reduced to yield methane. Additionally, methylcobalamin participates in the rather obscure process of acetate synthesis in certain bacteria in which it can be shown that the cobalt-bound methyl group becomes the acetate methyl carbon. The details of the involvement of methylcobalamin in this process are, however, unknown. 

The kinetics of the formation and decay of the observed intermediates through the formation of Q show no concentration dependence on either O2 or methane [15, 26] (O formation is presumably dependent on O2 concentration, but this has not been directly confirmed). In contrast, Q decay is linearly dependent on methane concentration indicating that this is the step where substrates react with the enzyme. Other substrates also accelerate Q decay, and the second order rate constant for the process depends on the specific substrate used. When nitrobenzene is used as a substrate, the decay of Q leads to the formation of another chromophoric intermediate, which we term compound T (T) for the terminal complex [15]. Chemical quench experiments showed that T is the nitrophenol (product) complex of... 

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