Barrier thermodynamic

Thermodynamic Barriers Prevent a Simple Reversal of Glycolysis... 

Figure 3 A hydrophobic permeant must negotiate through a complex series of diffu-sional and thermodynamic barriers as it penetrates into a cell. The lipid and protein compositions and charge distribution of the inner and outer leaflets of the membrane lipid bilayer can play limiting roles, particularly at the tight junction. Depending upon the permeant s characteristics, it may remain within the plasma membrane or enter the cytoplasm, possibly in association with cytosolic proteins, and partition into cytoplasmic membranes.

This may find application in biological and other systems. One way in which the effective thermodynamic barrier can be modified is through the movement of a charged group near one of the reactants since the charge distribution following electron... 

In the following sections, we shall explore the applicability of such relationships to experimental data for some simple outer-sphere reactions involving transition-metal complexes. In keeping with the distinction between intrinsic and thermodynamic barriers [eq 7], exchange reactions will be considered first, followed by a comparison of driving force effects for related electrochemical and homogeneous reactions. 

Another effective way of staying clear of the thermodynamic barriers of C-H activation/substitution is the use of the c-bond metathesis reaction as the crucial elementary step. This mechanism avoids intermediacy of reactive metal species that undergo oxidative additions of alkanes, but instead the alkyl intermediate does a o-bond metathesis reaction with a new substrate molecule. Figure 19.13 illustrates the basic sequence [20],... 

Having described the equilibrium structure and thermodynamics of the vapor condensate we then re-examine homogeneous nucleation theory. This combination of thermodynamics and rate kinetics, in which the free energy of formation is treated as an activation energy in a monomer addition reaction, contains the assumption that equilibrium thermodynamic functions can be applied to a continuum of non-equilibrium states. For the purpose of elucidating the effects of the removal of the usual approximations, we retain this assumption and calculate a radially dependent free energy of formation. Ve find, that by removing the conventional assumptions, the presumed thermodynamic barrier to nucleation is absent. 

Ve see in Figure 7 that Tolman s representation of the radially dependent surface tension also leads to a vanishing thermodynamic barrier, at high but metastable supersaturations, when a value of 6 computed from solutions of the YBG equation on the planar interface is used. This value of the Tolman parameter is consistent with values obtained from simulation studies of the planar Lennard-Jones surface (28,29). It is apparent that the physical picture of nucleation is highly dependent upon the assumed radial dependence of the surface tension. 

Another class of thermodynamic barrier theories focuses on the large increases in the elastic constants that accompany glass formation. (These theoretical approaches seem especially appropriate to polymer fluids below the crossover temperature Fj.) In particular, the barrier height governing particle displacement in the shoving model [57] is taken to be on the order of the elastic energy GqoVo required to displace a particle on a scale comparable to the interparticle distance,... 

In the unstable region, the concentration fluctuations are delocalized and there is no thermodynamic barrier to phase growth. Thus, separations that take place spontaneously lead to long range phase separation. This process is called spinodal decomposition (SD). In this mechanism, decomposition starts with a co-continuous structure and gradually shifts to a droplet morphology because of the breakdown of the continuous structure [41]. 

The difference in pKa values between the proton donor and the proton acceptor in Eq. 9-97 can be expressed as the Gibbs energy change which at 25°C is equal to 5.71 x ApKa. This is often referred to as the thermodynamic barrier AG 0 to a reaction and AG can be expressed as the sum of the thermodynamic barrier AG0 plus an intrinsic barrier AG in. . For the proton transfer of Eq. 9-97 the intrinsic barrier (for step b) is thought to be near zero so that AG 5.71 ApfCa. 

From this we can conclude that two pKa values can be as much as eight units apart and AG will still be less than 50 kj / mol, low enough to permit rapid enzymatic reactions. However, for transfer of a proton from a C-H bond to a catalytic group, for example, to form an enolate ion for an aldol condensation (Chapter 13), the intrinsic barrier is known to be about 50 kj / mol.141 In this case, to allow rapid enzymatic reaction either the thermodynamic barrier must be very low, as a result of closely matching pKa values, or the enzyme must lower the intrinsic barrier. It may do both. 

Thermodynamic barrier 492 Thermodynamic temperature scale 284, 285 Thermolysin 625... 

There are three chemical problems associated with the assembly of a protein, nucleic acid, or other biopolymer. The first is to overcome thermodynamic barriers. The second is to control the rate of synthesis, and the third is to establish the pattern or sequence in which the monomer units are linked together. Let us look briefly at how these three problems are dealt with by living cells. 

In gluconeogenesis, the thermodynamic barrier imposed by pyruvate kinase is overcome by coupling two separate reactions for the synthesis of PEP from pyruvate. 

The thermodynamic barrier encountered in charge separation in the forward electron transfer (A + D A + D" ") between a donor-acceptor pair can be overcome easily with the activation afforded by ultraviolet excitation (50-120 kcal/mole). The challenge confronted in elaborating this area of chemistry therefore lies in controlling the rate of the deactivating back reaction (A7 + D" - A + D). If the importance of the reverse electron transfer can be diminished, observable selective chemistry can ensue. 

Miller and Wolfenden6 compared the rates of decarboxylation of the substrate of orotidine-5 -monophosphate decarboxylase (OMPDC) in quantitative detail, on and off the enzyme. They showed that the apparent unimolecular rate constant of decarboxylation of the substrate when bound to the enzyme is about 1015 times greater than the decarboxylation process in the absence of the enzyme. Further studies confirm that the enzyme-promoted reaction does not involve additional intermediates or covalent alterations of the substrate. The reaction consists of carbon dioxide being formed and the resulting carbanion becoming protonated. Since thermodynamic barriers are not altered by catalysis, the energy of the carbanion must be a component that reflects the energy of the environment in which it is created, one in which the carbanion that is formed is selectively stabilized. 

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