Electrostatic catalysis effects

Effective concentration 65-72 entropy and 68-72 in general-acid-base catalysis 66 in nucleophilic catalysis 66 Elastase 26-30, 40 acylenzyme 27, 40 binding energies of subsites 356, 357 binding site 26-30 kinetic constants for peptide hydrolysis 357 specificity 27 Electrophiles 276 Electrophilic catalysis 61 metal ions 74-77 pyridoxal phosphate 79-82 Schiff bases 77-82 thiamine pyrophosphate 82-84 Electrostatic catalysis 61, 73, 74,498 Electrostatic effects on enzyme-substrate association rates 159-161... 

Highly Increased Number of Effective Collisions Active Site Preorganization of Solvation and General Acid/Base Catalysts Avoiding High-Energy Intermediates Electrostatic Catalysis by Metal Ions Covalent Catalysis by Enzyme-Bound Electrophiles and Nucleophiles Coupling ATP Hydrolysis to Drive Equilibria... 

These unusual results raise the question how LiC104 influences the course of these and further reactions. The similarity to the accelerating effect of water as solvent for carbo- and hetero Diels-Alder reactions suggests that an internal pressure might be operative, [5,13] but also an electrostatic catalysis by ion pairs [14] might be involved if polar transition states are passed. Finally, the lithium cation may serve as Lewis acid in an essentially non-acidic medium, a notion... 

This phenomenon is denoted as electrostatic catalysis and was coined as Circe-effect by WP Jencks. 

J. P. Guthrie and Y. Ueda (1974), Electrostatic catalysis and inhibition in aqueous solution. Rate effects on the reactions of charged esters with a cationic steroid bearing an imidazolyl substituent. Chem. Comm. 111-112. 

One of the simplest examples of electrostatic catalysis is the acceleration of the 1,5-hydride shift in cyclopentadiene by the influence of Li" " cations. The reaction proceeds via an asymmetric transition state that is 34 kJ mol more stable than the symmetric ground state. It could be shown that this extra stabilization (catalytic rate acceleration) is completely due to the effect of the electrostatically bound cation. Difference in cation complexation energies could be adequately illustrated by molecular electrostatic potential (MEP) maps for the ground and transition states (see Electrostatic Potentials Chemical Applications). The maps are symmetric and asymmetric for the ground and transition states, respectively, and indicate larger cation attraction for the latter. 

There is evidence that electrostatic catalysis works for a number of pericyclic reactions (Table 1) and we may speculate that potentially similar effects arise for rearrangements in water or other polar solvents where the initial state of the reaction involves less polar species than the transition state. In such cases metal or other small ions may stabilize the transition state by electrostatic interactions which may or... 

In this section the influence of micelles of cetyltrimethylammonium bromide (CTAB), sodium dodecylsulfate (SDS) and dodecyl heptaoxyethylene ether (C12E7) on the Diels-Alder reaction of 5.1a-g with 5.2 in the absence of Lewis-add catalysts is described (see Scheme 5.1). Note that the dienophiles can be divided into nonionic (5.1a-e), anionic (5.If) and cationic (5.1g) species. A comparison of the effect of nonionic (C12E7), anionic (SDS) and cationic (CTAB) micelles on the rates of their reaction with 5.2 will assess of the importance of electrostatic interactions in micellar catalysis or inhibition. 

Honk et al. concluded that this FMO model imply increased asynchronicity in the bond-making processes, and if first-order effects (electrostatic interactions) were also considered, a two-step mechanisms, with cationic intermediates become possible in some cases. It was stated that the model proposed here shows that the phenomena generally observed on catalysis can be explained by the concerted mechanism, and allows predictions of the effect of Lewis acid on the rates, regioselectivity, and stereoselectivity of all concerted cycloadditions, including those of ketenes, 1,3-dipoles, and Diels-Alder reactions with inverse electron-demand [2],... 

In order to really assess the magnitude of the electrostatic effect in lysozyme on a microscopic level it is important to simulate the actual assumed chemical process. This can be done by describing the general acid catalysis reaction in terms of the following resonance structures ... 

In view of the arguments presented in this chapter, as well as in previous chapters, it seems that electrostatic effects are the most important factors in enzyme catalysis. Entropic factors might also be important in some cases but cannot contribute to the increase of kcJKM. Furthermore, as much as the correlation between structure and catalysis is concerned, it seems that the complimentarity between the electrostatic potential of the enzyme and the change in charges during the reaction will remain the best correlator. Finally, even in cases where the source of the catalytic activity of a given enzyme is hard to elucidate, it is expected that the methods presented in this book will provide the crucial ability to examine different hypothesis in a reliable way. 

Theoretically, the problem has been attacked by various approaches and on different levels. Simple derivations are connected with the theory of extrathermodynamic relationships and consider a single and simple mechanism of interaction to be a sufficient condition (2, 120). Alternative simple derivations depend on a plurality of mechanisms (4, 121, 122) or a complex mechanism of so called cooperative processes (113), or a particular form of temperature dependence (123). Fundamental studies in the framework of statistical mechanics have been done by Riietschi (96), Ritchie and Sager (124), and Thorn (125). Theories of more limited range of application have been advanced for heterogeneous catalysis (4, 5, 46-48, 122) and for solution enthalpies and entropies (126). However, most theories are concerned with reactions in the condensed phase (6, 127) and assume the controlling factors to be solvent effects (13, 21, 56, 109, 116, 128-130), hydrogen bonding (131), steric (13, 116, 132) and electrostatic (37, 133) effects, and the tunnel effect (4,... 

Clearly, then, the chemical and physical properties of liquid interfaces represent a significant interdisciplinary research area for a broad range of investigators, such as those who have contributed to this book. The chapters are organized into three parts. The first deals with the chemical and physical structure of oil-water interfaces and membrane surfaces. Eighteen chapters present discussion of interfacial potentials, ion solvation, electrostatic instabilities in double layers, theory of adsorption, nonlinear optics, interfacial kinetics, microstructure effects, ultramicroelectrode techniques, catalysis, and extraction. 

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