Reactions reversible, unidirection

Unidirectional arrows are also used to describe reactions in living cells where the products of reaction (2) are immediately consumed by a subsequent enzyme-catalyzed reaction. The rapid removal of product P or Q therefore precludes occurrence of the reverse reaction, rendering equation (2) functionally irreversible under physiologic conditions. 

Theoretically, all enzyme reactions are reversible but the overall flux (flow) of substrate in a pathway is unidirectional. To extend our road map analogy, this type of reaction acts as a control point in a pathway, rather like a one-way street, allowing substrates to flow in only one direction. 

So far the discussion has assumed that the reactions under study are unidirectional r —> p. The majority of biochemical reactions are however reversible under typical cellular conditions in metabolism, we are therefore dealing with chemical equilibria. 

In the above radioactive decays, a parent nuclide shakes itself to become another nuclide or two nuclides. A unidirectional arrow indicates that there is no reverse reaction or if there is any reverse reaction, it is not considered. He produced by the homogeneous reaction (radioactive decay) may subsequently escape into another phase, which would be another kinetic process. 

Remember that these are reversible reactions even though unidirectional arrows are used. The general expression for the zth stepwise formation constant is given by Eq. 7-7. 

Although for clarity the reactions of Fig. 7.13 are shown to be unidirectional, all the reactions of the catalytic cycle are in fact reversible. This is an important aspect of the first stage of the hydrocyanation process. It provides for a mechanism for the isomerization of the unwanted 2M3BN to the desired 3PN. The isomerization reaction of 2M3BN to 3PN has been studied by deuteriumlabeling experiments. The results are consistent with a mechanism where butadiene is formed in one of the intermediate steps. This means that the reversibility of all the steps allows isomerization to follow the path 7.51 — 7.50 — ... 

In response to a more fundamental question, a qualitative understanding of the origins of the treble diastereoselectivity exhibited in each cycloaddition has been developed. Arguments based on discussions in the literature have been combined with synthetic observations made in our own laboratories to assess the factors which control the reactivities and selectivities of exocyclic r-m-butadiene units, and endocyclic bisdienophilic units incorporating the (2.2. l]bicycloheptane skeleton. The syn/endo-H stereochemistry generated across each newly-formed cyclohexene ring in the cycloadditions between bisdiene and bisdienophilic building blocks is, we believe, a consequence of transition state considerations, and therefore a result of kinetic control. In spite of the sometimes tacitly assumed reversibility of the Diels-Alder reaction, evidence is provided in this review that confirms here at least the unidirectional nature of the cycloadditions under discussion. 

In principle, all reactions of Scheme 3 (except kbet) may be reversible. A detailed kinetic analysis is usually not practical under such circumstances. In polar solvents, it is commonly assumed that solvation processes (ksol and ksep) are unidirectional, i.e. solvation leads to a more stable system. In nonpolar solvents contact ion pairs dominate. As discussed previously (Sect. 2.3) BET rates depend strongly on the solvation status. The fragmentation rates are usually assumed to be independent of ion pairing, although quantitative information on that respect is not available. 

For a simple reversible chemical reaction, if one path is preferred for the backward reaction, the same path must also be preferred for the reverse reaction. This is called the principle of microscopic reversibility. Time can be measured by reversible, periodic phenomena, such as the oscillations of a pendulum. However, the direction of time cannot be determined by such phenomena it is related to the unidirectional increase of entropy in all natural processes. Some ideal processes may be reversible and proceed in forward and backward directions. 

A TPD run on a supported metal by using a carrier gas at atmospheric pressure furnishes data for the mixture of surface sites present. Usually, such studies give only a qualitative idea of the different species present on the various sites. However, simultaneous measurement of the IR spectra of the adsorbed species may furnish valuable information (88). In addition, with a carrier gas it is extremely difficult to get kinetic data that are not altered by transport effects. Furthermore, the adsorption-desorption step is not unidirectional the reverse reaction (adsorption) may occur during the TPD. This effect is called rcadsorption in surface science studies in which it is unusual, but it is usually present in a TPD obtained with a carrier gas. 

Chemical, physical, and biological processes can be viewed as either reversible equilibrium reactions or irreversible unidirectional kinetic reactions. In systems out of chemical and isotopic equilibrium, forward and backward reaction rates are not identical, and isotope reactions may, in... 

Although it is conventional to represent the reaction of hexokinase, and of many other enzymes, using a unidirectional arrow, this convention is not scientifically accurate. The unidirectional arrow is used only to make it convenient to remember that the equilibrium lies far in the direction of product formation (glucose-6-P formation), In general, most ATP-utilizing reactions are driven in the direction of product formation, and arc therefore said not to be freely reversible. 

The flexibility of this phase of the pathway allows for interconversion of a number of sugars and glycolytic intermediates, in part because of the ready reversibility of the reactions and regulation of the enzymes by substrate availability. Thus, the pathway adjusts the concentrations of a number of sugars rather than act as a unidirectional anabolic or catabolic route for carbohydrates. 

Desorbed species are not removed and are allowed to accumulate in the inherently closed system of the batch reactor. Thus, unless a unidirectional reaction is being studied, reverse reactions must be taken into account in Ihe data analysis. The accumulation of desorbed... 

Application of chemical theory to heterogeneous systems such as soils almost always comes in conflict with system complexity. Commonly used kinetic techniques are based on the assumption that the reactions are either unidirectional or discrete, but soil sorption reactions are often both reversible and multiple. The combination of multiple reversible reactions makes evaluation tedious and tenuous. It is seldom possible to be definitive in calculating rate coefficients attributed to a specific reaction. These difficulties are compounded by the difficulty of measuring reactants and products in a colloidal system and by the probability that reaction energy varies as the reaction proceeds. 

R) distomer. These compounds undergo an intriguing metabolic reaction such that the (i )-enantiomer is converted to the (S)-enan-tiomer, whereas the reverse reaction is negligible. This unidirectional chiral inversion is thus a reaction of bioactivation, and its mechanism is now reasonably well understood (Fig. 13.29) (75). 

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