Chemical reactions decarboxylation

First, these copolymers undergo decarboxylation more readily than any of the homopolymers. Second, decarboxylation involves the units of acrylic add at temperatures which do not affect homopolymers of acrylic acid. In our view, the first phenomenon is accounted for by the effect of separation of conjugation blocks exemplified by this particular chemical reaction. As to the second observation, we believe that decarboxylation under relatively mild conditions (160—170 °C) affects, apparently, the fragments of acrylic acid located at the junctions of the blocks. 

Alternatively one can make use of No Barrier Theory (NBT), which allows calculation of the free energy of activation for such reactions with no need for an empirical intrinsic barrier. This approach treats a real chemical reaction as a result of several simple processes for each of which the energy would be a quadratic function of a suitable reaction coordinate. This allows interpolation of the reaction hypersurface a search for the lowest saddle point gives the free energy of activation. This method has been applied to enolate formation, ketene hydration, carbonyl hydration, decarboxylation, and the addition of water to carbocations. ... 

Other chemical reactions, such as hydration, decarboxylation, or pyrolysis, are also potential routes for drug degradation. For example, cyanocobalamin may absorb about 12% of water when exposed to air, and p-aminosalicyclic acid decomposes with evolution... 

B) Several PPO substrates such as 3,4-dihydroxymandelate can be decarboxylated by PPO. The product of the enzyme action is an o-quinone, which, owing to its instability, evolves to the Anal product, 3,4-dihydroxybenzaldehyde, by a chemical reaction of oxidative decarboxylation. 

Interpretation of KIEs on enzymatic processes (see Chapter 11) has been frequently based on the assumption that the intrinsic value of the kinetic isotope effect is known. Chemical reactions have long been used as models for catalytic events occurring in enzyme active sites and in some cases this analogy has worked quite well. One example is the decarboxylation of 4-pyridylacetic acid presented in Fig. 10.9. Depending on the solvent, either the zwitterionic or the neutral form dominates in the solution. Since the reaction rates in D20/H20 solvent mixtures are the same (see Section 11.4 for a discussion of aqueous D/H solvent isotope effects), as are the carbon KIEs for the carboxylic carbon, it is safe to assume that this is a single step reaction. The isotope effects on pKa are expected to be close to the value of 1.0014 determined for benzoic acid. This in mind, changes in the isotope effects have been attributed to changes in solvation. 

A chemical reaction in which the carboxyl group of an a-amino acid is replaced by an acyl group Le., an acyl-decarboxylation). In this reaction, an a-amino acid is reacted with an anhydride in the presence of pyridine and the resulting product is an A-acylated ketone. In certain instances, A-substituted amino acids will also act as reactants. 

In the case of protein amino acid-derived alkaloids, the second obligatory intermedia is synthesized from the obligatory intermedia by chemical reactions. In the pelletierine synthesis pathway started with L-lysine, the second obligatory intermedia is A -piperidinium cation. It is formed by a Maimich reaction from A -piperidine (obligatory intermedia) and COSCoA. The second obligatory intermedia, by hydrolysis decarboxylation, produces pelletierine. 

The examination of the total reagent usage in the two processes (last row in Table 8.3) clearly shows that the new enzymatic route (with recycling of 4) utilizes 7 times less input of chemicals. This includes 12 times less input of solvents as compared to the first-generation route. Moreover, in the optimized process, every chemical reaction is run in water with minimal solvents used for work-up. Some of the process water can be sent directly to the wastewater treatment plant, and the solvent from the hydrolysis/decarboxylation process is recovered. Further improvements from pilot plant and production scale runs have been demonstrated and will be implemented in the future. 

The term ion pump, synonymous with active ion-transport system, is used to refer to a protein that translocates ions across a membrane, uphill against an electrochemical potential gradient. The primary pumps do so by utilization of energy derived from various types of chemical reactions such as ATP hydrolysis, electron transfers (redox processes), and decarboxylations, or from the absorption of light (Table 1). Secondary pumps are symport and antiport systems that derive the energy for uphill movement of one species from a coupled downhill movement of another species. The electrochemical gradient driving the latter movement is often created by a primary pump. 

Enzymes or enzyme complexes are biological catalysts. Recall that a catalyst facilitates a chemical reaction without the catalyst itself being permanently altered. Oxaloacetate can be thought of as a catalyst because it binds to an acetyl group, leads to the oxidative decarboxylation of the two carbon atoms, and is regenerated at the completion of a cycle. In essence, oxaloacetate (and any cycle intermediate) acts as a catalyst. 

The altered environment of the dmg molecules leads to changes in stability. The cyclodextrins catalyse a number of chemical reactions such as hydrolysis, oxidation and decarboxylation. Although interactions between dmgs and cyclodextrins have been mostly the result of deliberate attempts to modify the behaviour or properties of the dmg, they are included here to illustrate an additional mode of interaction with other... 

The keto intermediate decarboxylates spontaneously to 7-/ -(4-carboxybutanami-do)-cephalosporanic acid (glutaryl-7-ACA) and carbon dioxide by chemical reaction with the hydrogen peroxide formed during the enzymatic reaction. The occurrence of the byproduct 7-ACA-sulfoxide is possible. In general a reaction mixture is produced, caused by the catalase also produced by the yeast. However, the a-ke-toacid intermediate reacts poorly with the deacylation enzyme in the second step. 

Cyclative cleavage strategies (e.g., metathesis and heterocycle ring formation), traceless linkers (e.g., Si, Ge linkers or via chemical reactions such as desulfurization, decarboxylation and cycloreversion) and cleavage steps designed to liberate an increasing diversity of functional groups are illustrated in Section 2.4.4. 

Decarboxylation is any chemical reaction that removes a carboxyl (-COOH) group from a compound, producing CO2 in the process. Decarboxylation will convert a carboxylic acid into a hydrocarbon ... 

The general pyrolysis mechanisms of polysaccharides have been determined from model studies on cellulose and involve the splitting of the polysaccharide structure by three basic chemical reaction mechanisms dehydration, retroaldolization, and decarboxylation. Using these basic pyrolysis mechanisms, it is possible to explain the pyrolysis of polysaccharides and evolved pyrolysis products. The hexose degradation pathway for cellulose results in formation of furan- and pyran-type fragments and smaller acyclic aldehyde and ketone fragments. ... 

One of the great tragedies about being human is that it is far too easy to gain weight and far too difficult to lose it. If we had to analyze the specific chemical reactions that make this a reality, we would look very carefully at the citric acid cycle, especially the decarboxylation reactions. 

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