Transition state energy barrier

To achieve its catalytic reaction, glutamate mutase faces the problem of how to surmount the two relatively high transition-state energy barriers that lead from the 4-glutamyl radical to acrylate and the glycyl radical and then the recombination of these radicals to the 3-methylene-aspartate radical (Scheme 4). These barriers were computed as AH = -f 59.9 and - - 66.5 kJ mol (10), respectively, and the 3-methylene-aspartate radical was found to be significantly less stable than the resonance-stabilized 4-glutamyl radical (AH = 20.3 kJ mol ). Likewise, with methylmalonyl-CoA mu-... 

The catalytic efficiency of a-chymotrypsin cannot be solely attributed to the presence of the charge-relay system. X-Ray work (81) has indicated the many parameters operative in the catalytic process. Nine specific enzyme substrate interactions have been identified in making the process more efficient. For example, stabilization of the tetrahedral intermediate, and thus lowering of the transition state energy barrier, is accomplished by hydrogen bond formation of the substrate carbonyl function with the amide hydrogen... 

Table 6.6 The resultant geometries and energetics of the transition states for the reaction CCI2F2 + Fsurf CCIF3 + a surf- The transition state energy barrier (TS Energy) is relative to the reactants...

While the MP2(full)/6-311+G 7/B3LYP/6-311+G energies show typical discrepancies, the application of the IPCM- and CPCM-solvent models dramatically lowers the energy for the intermediate and the transition states. The barriers of 2.8 and 1.8kcalmol 1 corroborate the experimental findings for an efficient formation of a water coordinated Li+ complex. 

Fig. 5 Energy changes induced by varying electric fields for 02 dissociation on Pt( 111). The dashed curve (a) corresponds to the transition state energies and the solid curve (b) corresponds to the dissociation barrier. Oz coverage is j ML.83... 

Although accurate kinetics have not been obtained experimentally, the AG transition-state energy for the above dehydrogenation step has been estimated as 10 1 kcal/mol (15). Since this measurement starts at the energy level of the 1,5-bridged cation, the actual barrier for the second step should be at least 1 kcal/mol lower, e.g. 9 1 kcal/mol. 

Ab initio calculations of the transition-state energies in the epoxidation of alkenes by hydrogen peroxide catalysed by titanosilicates have been carried out. They indicate a markedly lower energy barrier for attack of the alkene by the oxygen atom of the titanium(IV) hydroperoxide intermediate that is closer to the metal centre. 

Enzymes accelerate reactions by stabilizing the transition states, the highest energy species on the reaction pathway, and thereby decreasing the activation barrier. In other words, the combination of enzyme and substrate creates a new reaction pathway whose transition-state energy is lower than it would be if the reaction were taking place without the participation of the enzyme. Enzymes have evolved to bind the transition states of substrates more strongly than the substrates themselves. Therefore, compounds that mimic the structure of the transition state are often potent inhibitors of the enzyme-catalyzed reaction. 

By examination of the stereochemical consequences of decarboxylation, Cram and Haberfield8 obtained evidence for internal return of carbon dioxide to the carbanion, affecting the stereochemical outcome of these reactions. It is reasonable to consider that the barrier for the combination of the carbanion and carbon dioxide may be comparable to or lower than that for diffusion, in which case the reverse reaction will be a kinetically significant component in the overall rate of reaction. In such a case, a catalyst cannot deal with the direction of the reaction -if it lowers the transition state energy for the forward reaction, conservation of energy demands that it also lower the barrier for the reverse reaction. The energy for addition of the carbanion to carbon dioxide is also inherent. The reaction should occur readily if the reaction partners have reduced entropy. 

Hydrogen bonds appear to be essential in all enzyme-catalyzed reactions, although why they are essential and how they promote function is an open question. In recent years a specific hypothesis for their involvement in catalysis has emerged so-called low-barrier hydrogen bonds (LBHB) have been proposed to lower the transition state energy for many enzymatic reactions, including those of serine protease, citrate... 

A similar interpretation holds for the preexponential factor of the rate constants for the dissociative adsorption, desorption, reaction between the adspecies and their migration. The CM is distinguished by the fact that the preexponential factor is dependent on the properties of the starting reagents only and is independent of the transition state whereas the rate constant depends on the activation barrier height, which is governed by the transition state energy. 

Two other studies239,240 examined the system described in equation 25, in which the reaction is exothermic from left to right, with the central transition state potential barrier lying 1-2 kcalmoT1 below the energy of the separated reactants but well above the ion-dipole minima. 

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