Rate enhancement, electrostatic interactions

In many reactions, transfer of the anion across the interface and subsequent diffusion into the bulk of the organic phase will not be the rate-determining step when lipophilic catalysts are used, but the effect of less lipophilic catalysts may be influenced more by the anion and the mechanism of the transfer process. Thus, for example, the reactive anion is frequently produced in base-initiated reactions by proton extraction from the substrate at the two-phase interface and diffusion of the ion-pair contributes to the overall kinetics of the reaction. Additionally, the reactivity of the anion depends on its degree of hydration and on its association with the quaternary ammonium cation. In most situations, the activity of the transferred anion is enhanced, compared with its reactivity in aqueous media, as its degree of hydration is reduced, whereas a relatively weak electrostatic interaction between the two ions resulting from the bulkiness of the cation enhances the reactivity of the anion by making it more available for reaction and will be a major factor in the ratedetermining step. 

In the area of catalysis, the esterolysis reactions of imidazole-containing polymers have been investigated in detail as possible models for histidine-containing hydrolytic enzymes such as a-chymotrypsin (77MI11104). Accelerations are observed in the rate of hydrolysis of esters such as 4-nitrophenyl acetate catalyzed by poly(4(5)-vinylimidazole) when compared with that found in the presence of imidazole itself. These results have been explained in terms of a cooperative or bifunctional interaction between neighboring imidazole functions (Scheme 19), although hydrophobic and electrostatic interactions may also contribute to the rate enhancements. Recently these interpretations, particularly that depicted in Scheme 19, have been seriously questioned (see Section 1.11.4.2.2). 

Polymer catalysts showing interactions with the substrate, similar to enzymes, were prepared and their catalytic activities on hydrolysis of polysaccharides were investigated. Kinetical analyses showed that hydrogen bonding and electrostatic interactions played important roles for enhancement of the reactions and that the hydrolysis rates of polysaccharides followed the Michaelis-Menten type kinetics, whereas the hydrolysis of low-molecular-weight analogs proceeded according to second-order kinetics. From thermodynamic analyses, the process of the complex formation in the reaction was characterized by remarkable decreases in enthalpy and entropy. The maximum rate enhancement obtained in the present experiment was fivefold on the basis of the reaction in the presence of sulfuric acid. 

From these results, it can be concluded that the rate enhancement of polysaccharide hydrolysis obtained with the present copolymer catalyst was attributable to the hydrogen bonding interactions between the substrate and the catalyst and to the electrostatic interactions between the catalyst polyanions and protons. A drawing of this concept is shown in Figure 10. A polymer molecule is surrounded by a proton atmosphere. The substrate molecules are pulled into the atmosphere by hydrogen bonding interactions and hydrolyzed in the presence of a high concentration of proton. 

The amazing rate enhancement observed in enzymatic catalysis results from stabilization of the transition states and/or destabilization of the substrates. These effects are achieved by interactions of the reactants with the protein residues. Depending on the particular reaction some interactions may play a dominant role, or many different types, such as electrostatics, hydrophobic interactions, geometric distortion, or hydrogen bonds, may concurrently contribute to catalysis. Understanding these interactions is the key factor in exploiting enzymatic reactions for the purpose... 

Temperature the results compiled in Tables 4.1-4.6 were obtained at different temperatures, and in some studies the temperature was not controlled. The results reported in Table 3.11 and Fig. 3.104 indicate that the PZC of oxides and related materials shifts to low pH when the temperature increases (with a few exceptions). Most surfaces carry more negative charge at elevated temperature (at given pH), and this creates favorable conditions for adsorption of cations and unfavorable conditions for adsorption of anions. Therefore elevated temperature would enhance uptake of cations, and low temperature would enhance uptake of anions at constant pH, if the electrostatic interaction was the only factor. On the other hand, the rate of chemical reactions and diffusion is enhanced at elevated temperatures. Thus, the kinetic and electrostatic effect on cation adsorption add up and the uptake increases with temperature. With anions these effects act in opposite directions the uptake increases with temperature when the kinetic factor prevails the uptake decreases with temperature when the electrostatic factor prevails, finally the both effects can completely cancel out. 

The rate enhancement due to the electrostatic interactions in this system is over 6 fold. For the long side-chain ester NDBS we can see the effect of both the electrostatic and hydrophobic interactions. 

From the sphere-dimer studies, two major conclusions emerge. The first is that the trajectory method can be extended to structured reactants with anisotropic reactivity and anisotropic direct forces and hydrodynamic interactions. The second major conclusion is that complicated electrostatic interactions between species with anisotropic reactivity can "steer" the approaching particles into favorable orientations and enhance the reaction rate. For these model studies, rate enhancements up to 20% have been obtained. The second conclusion is likely to be of considerable relevance to molecular biology. In the third and final series of simulations, the Brownian dynamics trajectory method is applied to a particular biological system. 

Table III summarizes some calculations carried out to explore what effects contribute to the high reactivity of SOD. For the native-llke model with a monopole charge of A, inclusion of the (non-centrosymmetric) quadrupole increases the reaction rate by 40%. The quadrupole evidently helps to steer O2 into the active site. Parallel simulations were also carried out in which monopole charges of 0 and +4 were used. Although increasing the monopole charge from -4 to 0 to +4 increased the rates by factors of 2.5 and 5, respectively, the steering effect is present in each case. This suggests that the enhancement in rate due to steering by local electrostatic interactions will persist in the presence of added salt, which will suppress the effects of the monopole field more strongly than those of the shorter-ranged quadrupole field.

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