Carbon dioxide substrate utilizers

It has already been mentioned that utilization of the permethylated ligand (L19)2 in place of (L23)2 drastically alters the ease of substitution reactions of the [M2(L19)(C1)]+ complexes (Section III.D). Further studies revealed a remarkable influence of the hydrophobic pocket on the rate and course of several substrate transformations, as for instance the fixation of carbon dioxide (239) (Scheme 8), the cis-bromination of a,/)-unsaturated carboxylate ligands (256), and some Diels-Alder reactions (215). Of these the latter two reactions will now be discussed. 

In laboratory-scale homogeneous catalysis applications, in the last decade further investigations have been carried out in which a less soluble organo-metallic catalyst system was utilized for metathesis reactions [46]. Under RCM-conditions, it was possible to convert substrates with functional groups that were problematic due to their potential to inactivate the rutheniiun catalyst here, the conversion in supercritical carbon dioxide avoids the protection of critical amino groups as an additional synthetic step. Consequently, it was possible to synthesize a number of carbo- and heterocyclic products with varying ring size (C4 to Cie). 

This enzyme [EC 1.13.12.1] catalyzes the reaction of arginine with dioxygen to generate 4-guanidobutana-mide, carbon dioxide, and water. The enzyme can also utilize canavanine and homoarginine as substrates. 

This enzyme complex [EC 1.2.4.4], also known as 3-methyl-2-oxobutanoate dehydrogenase (lipoamide) and 2-oxoisovalerate dehydrogenase, catalyzes the reaction of 3-methyl-2-oxobutanoate with lipoamide to produce S-(2-methylpropanoyl)dihydrolipoamide and carbon dioxide. Thiamin pyrophosphate is a required cofactor. The complex also can utilize (5)-3-methyl-2-oxopenta-noate and 4-methyl-2-oxopentanoate as substrates. The complex contains branched-cham a-keto acid decarboxylase, dihydrolipoyl acyltransferase, and dihydrolipoa-mide dehydrogenase [EC 1.8.1.4]. 

Mannitol has often been used as an osmotic regulator in the external solutions, and has been presumed to be inert. It was found to be a respiratory substrate in 15 of 26 species representing 17 families of higher plants, some of which were capable of utilization of mannitol that was equal to that of n-glucose and D-fructose. Oat (Avena sativa), most often used for the cell-wall studies, showed only a slight output of carbon dioxide from labeled mannitol. About 10% of the carbon in the mannitol was converted, with time, into the hemicellulose and cellulose fractions. Only the glucose, and, perhaps, the cellobiose, was labeled.4 ... 

It has been shown that, in supercritical carbon dioxide, increases in water concentration result in increases in enzyme activity. The amount of added water needed for this increase varies and can depend on many factors, such as reaction type, enzyme utilized, and initial water content of the system. This is true until an optimal level is reached. For hydrolysis reactions, activity will either continue to increase or maintain its value. For esterification or transesterification reactions, once the optimal level of hydration has been reached, additional water will promote only side reactions such as hydrolysis. Dumont et al. (1992) suggests that additional water beyond the optimal level needed for enzyme hydration may also act as a barrier between the enzyme and the reaction medium and thereby reduce enzyme activity. Mensah et al. (1998) also observed that water above a concentration of 0.5 mmol/g enzyme led to lower catalytic activity and that the correlation between water content of the enzyme and reaction rate was independent of the substrate concentrations. 

By initializing the cell concentration, X(0), either from an estimation at some point of the oxygen uptake or carbon dioxide evolution rate or by knowing the inoculum size and then integrating the estimated growth rate over time, both cell biomass and growth rate can be estimated on-line utilizing a computer. The substrate (ethanol) demand, AS is then estimated by a similar equation, i.e. 

Animal and bacterial enzymes that utilize or synthesize carbamyl phosphate have activity with acetyl phosphate. Acyl phosphatase hydrolyzes both substrates, and maybe involved in the specific dynamic action of proteins. Ornithine and aspartic transcarbamylases also synthesize acetylornithine and acetyl aspartate. Finally, bacterial carbamate kinase and animal carbamyl phosphate synthetase utilize acetyl phosphate as well as carbamyl phosphate in the synthesis of adenosine triphosphate. The synthesis of acetyl phosphate and of formyl phosphate by carbamyl phosphate synthetases is described. The mechanism of carbon dioxide activation by animal carbamyl phosphate synthetase is reviewed on the basis of the findings concerning acetate and formate activation. 

The utility of urca.s and thioureas as substrates for making imidazoles is limited by the fact that the imidazole 2-substituent can only be an oxygen or sulfur function. Synthetic methods involving ureas and thioureas will also be discussed in Section 4.1, but some cyclizations of suitably functionalized species fall under the present heading. Appropriately substituted ureas and thioureas can be made from isocyanates and primary amines [36-38], from isocyanates and hydrazines [39] or thiocyanates and hydrazines [40], from or-aminonitrilcs and carbon dioxide [41] and by heating l,3,4-oxadiazol-2-oncs with amino acids [42]. Some of the substrates prepared in these ways, though, lead ultimately to reduced imidazoles such as hydantoins. Cyclizations arc usually acid catalysed, but they can also be thermal [43]. 

The simulations presented are for the conversion of acetic acid to microorganisms, methane, and carbon dioxide. Yields will be diflFerent for other volatile acids, especially noticeable being the increased ratio of methane to carbon dioxide produced as the length of the carbon chain increases. For field digesters utilizing complex substrates it would also be necessary to include the carbon dioxide generated by the acid-producing bacteria. 

These observations were later confirmed with pure cultures. It was found that methane is formed by the reduction of carbon dioxide by hydrogen supplied by the various substrates utilized by tbe bacteria or, in the case of Methanobacillus omelianskii, by uncombined hydrogen itself (Barker, 1936,1940,1941,1943). M. omelianskii turned out to be a mixed culture and will be discussed later. 

The possibility of using C02 for the synthesis of fine chemicals that are now derived from petroleum has prompted efforts to obtain a broader understanding of the coordination chemistry of CO2 during the past 20 years.1-21 Carbon dioxide utilization will inevitably center on metal complexes and their ability to bind C02. In the past decade, many C02—metal complexes have been prepared and the ligand has demonstrated a remarkable variety of coordination modes in its complexes. The sections below outline the synthesis, characterization by X-ray crystallography and IR spectroscopy, and some characteristic reactions of these compounds. Also discussed are C02 insertion reactions into M—X bonds and oxidative coupling reactions between C02 and unsaturated substrates which occur at some metal centers. Finally, a profile of the research on catalytic reductions of C02 is provided. Where possible, references are made to reviews rather than to the primary literature. 

Currently, carbon dioxide is used as a chemical feedstock for the production of carboxylic acids, carbonates, carbon monoxide, and urea (14—16). Despite the fact that numerous chemical reactions utilizing carbon dioxide are thermodynamically advantageous, there is often a substantial kinetic barrier to their occurrence. Transition metal compounds can serve to catalyze reactions of carbon dioxide, i.e., in the utilization of carbon dioxide in synthetic organic chemistry, transition metal complexes can simultaneously activate both carbon dioxide and other substrate molecules such as hydrogen or olefins. 

The methanotrophic bacteria have one known pathway for aerobic methane oxidation to CO2 31, 42,123). MMO catalyzes the first energetically difficult step in the formation of methanol from methane. The second step is catalyzed by methanol dehydrogenase (with a PQQ cofactor) and results in formation of formaldehyde, which is then converted by formaldehyde dehydrogenase (with no known cofactor) to formate. Finally, carbon dioxide is produced by formate dehydrogenase (with five different iron-sulfur clusters, a Mo-pterin cofactor, and an unusual flavin) 31, 42, 123). MMOs have a unique ability to oxidize a broad range 31,42,128,129) of hydrocarbons in addition to methane. One other system with a similar broad substrate utilization is the monoheme cytochrome P450 family, but in this case different isozymes show different specific activities (31). For soluble MMO, one single... 

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